You are special.
When you look at yourself in the mirror, you might just see your face—your eyes, your hair, your smile. Next time you see your face think of this: your face you see in the mirror is made of atoms that are unimaginably old. Some of these atoms were formed at the very birth of the universe, about 13.8 billion years ago, in the Big Bang. Others were cooked in the fiery hearts of stars that exploded long before our Sun was born. That means every breath you take and every cell in your body carries pieces of ancient stardust. In short: you are literally made of the universe.
You may have worried about gaining weight while eating extra slice of pizza, but have you ever wondered why you have mass? Why is hydrogen in the water you drink the most abundant element in the universe? Why are we mostly made of carbon? What are isotopes? Why some elements are radioactive? Why are some things metal, some non-metal, and others behave like a Buddhist monk and not react to any temptations?
The Mass Matter
You can think of mass as the resistance that you feel when you push against an object. By you, I mean the elementary particles that makes you.
For almost 100 years after Dalton discovered atoms, they were accepted as the fundamental particles of matter. But starting in the late 1890s with the discovery of electrons, particles smaller and simpler than atoms were identified. Within a few decades, protons and neutrons were also discovered. Ultimately, many elementary particles were discovered, clubbed together in the Standard Model.
They have tiny intrinsic masses as they interact with an invisible field called the Higgs field. Don’t worry about the name, just think of it as a field that is everywhere in the universe, and particles like electrons and quarks gain weight or mass by moving through this field, which slows them down, much like how running in water is harder than running in air. The harder it is, the heavier the particle. That’s why However, this Higgs field interaction only accounts for about 1% of the mass in ordinary matter.
Rest comes from the energy inside protons and neutrons which make up atoms. Protons and neutrons are made of fundamental particles called quarks, which are glued tightly together by another group of fundamental particles called gluons through a strong force. The quarks move at close to speed of light inside the nucleus creating kinetic energy. The gluons also hold them inside the nucleus, thus there is potential energy. The energy from this strong force and the fast movement of quarks inside protons and neutrons is converted into mass, following Einstein’s equation E = mc^2, which shows energy and mass are related. So, most of the mass we see around us, including in ourselves, comes from the enormous energy holding quarks inside protons and neutrons, while the Higgs field does a smaller but important job giving particles their basic mass. In essence, mass is really energy trapped inside particles and the forces between them.
Trivia: The Mass of an object increases with its speed because speed increases the kinetic energy. This is called relativistic mass. The mass that we generally talk about is mass at rest, which means that the frame of reference of the object measured is the same as that of the person/instrument measuring it. But this has significant impact only when the object is travelling close to the speed of light.
“If an object moves with a speed of less than one hundred miles a second the mass is constant to within one part in a million” – The Feynman Lectures
May be you can loose some weight if you manage to reduce that energy by slowing down the fundamental particles. But, as of now, only in your dreams.
An atom is very tiny. If an apple is magnified to the size of the earth, the size of the atom within the apple would roughly be the size of the original apple. An atom has a central nucleus (with protons and neutrons) and electrons whizzing around it in shells. Protons are positive charged with neutral neutrons that act as glue keeping multiple positive protons together (else the positive charges would repel each other disintegrating the atom), and negative charged electrons getting attracted by and balancing the positive charge of protons.
Trivia: A proton is made of three quarks, the elemental particles : two up quarks and one down quark (uud), which gives it a total electric charge of +1. A neutron is made of two down quarks and one up quark (udd), yielding a total charge of 0. The up quark carries +2/3 of the elementary charge, and the down quark carries −1/3 of the elementary charge; adding these three quark charges for the proton: 2(+2/3) + (−1/3) = +1, and for the neutron: 1(+2/3) + 2*(−1/3) = 0. This simple picture explains why protons are positively charged and neutrons are neutral, even though both are composite particles made from quarks.
Number of protons decide what atom it is, but number of neutrons for a fixed number of proton decide what isotope it is.
Atomic number (Z)= Number of protons
Mass number(A)= Number of protons + neutrons.
Isotopes have same atomic number but different mass number. For example, carbon-12, carbon-13, and carbon-14 are all isotopes of carbon. Each has 6 protons (atomic number 6), but 6, 7, and 8 neutrons respectively, so their mass numbers are 12, 13, and 14. Generally the neutron to proton (n/p) ratio is close to one, specially for lighter elements. That makes the atom stable. Elements with n/p ratio 1.5 and above are radioactive, i.e., they emit radiations. They are unstable elements, e.g., heavy metals like uranium.
Curious properties of electrons
Electrons are tiny and hardly have mass. They are fundamental particles and are negatively charged and relatively light compared to protons, which are positively charged and 1,840 times as “heavy” as electrons. But electrons are the ones at the surface, and those in the outermost shell decide how the atom behaves, especially in chemical reactions.
Electrons behave in interesting ways. As protons increase, more electrons gets attracted to balance the charge. But, like protons, electrons too don’t like to be cozy as the negative charge repels each other. Also, a principle called Pauli’s exclusion principle says that two electrons cannot be in exactly the same state or place, which keeps electrons spread out.
Electrons in an atom are arranged in different shells or energy levels that surround the nucleus. These shells are like layers at increasing distances from the nucleus—starting with the K shell (first shell), then L, M, N, and so on. The further the shell is from the nucleus, the longer circumstance it has, and thus can hold more electrons.
Each shell can hold a limited number of electrons because on limited area: the first (K) holds 2, the second (L) holds up to 8, the third (M) up to 18, and this follows the rule of 2n² electrons per shell, where n is the shell number.
The shells have different energy levels, with the shell closest to the nucleus having the lowest energy, and shells farther away having higher energy (else the electron would fly away). Electrons fill these shells starting from the lowest energy shell to higher ones, because atoms tend to be most stable when their electrons have as little energy as possible (which means closer to nucleus).
Why can’t we pass through walls?
About 99.99% of the atoms, which means you too, are empty. To imagine how empty we are let’s blow up an atom to the size of a football field. The size of the nucleus would be that of a tiny pea in the middle of the field. The electrons, which are incredibly small, are like tiny gnats buzzing way out near the edge of the stadium. We are all very hollow in the inside. Why can’t we pass through walls then?
Because the electrons around atoms create electric fields that repel the electrons in other atoms, when two objects come close, these electron clouds push each other away, preventing atoms from passing through each other. This repulsion, called electromagnetic force, gives matter its solidity and stops us from walking through walls. Furthermore, how light interacts with the electrons in atoms determines whether something is transparent or opaque. In solids and liquids, electrons absorb or scatter light, making materials opaque even though it is 99.99% hollow. So even though atoms seem empty, the electrons and forces between them fill space and give materials the strength and appearance we see every day.
Can you guess what that means about touching something? You can never touch anything because the electrons repel before things actually come in contact. If you are standing on the floor, sitting on a chair or lying down on the bed, you are actually not touching anything, but levitating at a tiny tiny distance above the floor, chair or the bed as your electrons and the ones of what’s below you repel each other.
If you manage to reduce the empty space inside the atoms, you can compress yourself to a tiny size… but again, only in your dreams!
Why metals shine?
Check: https://www.google.com/search?q=periodic+table
Arranging electrons in shells also explains why elements show periodic properties in the periodic table. All known elements based on the number of protons, and thus arrangement of electrons, is organised like a map in the periodic table.
The table is organized in rows (called periods) and columns (called groups or families). Elements in the same group act in similar ways because they have similar numbers of electrons in their outer shells.
The combining power of an atom is usually determined by electrons in the outer shell (valence electrons), and called valency.
If an atom has 1, 2, or 3 valence electrons, it is usually a metal. To be stable it can loose the free electrons in outer shell. The outer electrons are loosely held and can move freely, forming what is called a “sea of electrons.” When light hits the metal, these free electrons absorb the energy of the light waves and start to vibrate. As they vibrate, they quickly send out or re-emit the light waves back. This reflected light is what makes metals look shiny and gives them their characteristic metallic luster. Different metals reflect different wavelengths of light, which is why gold looks yellowish and copper looks reddish, while most metals appear silvery. This ability of free electrons to absorb and then re-emit light efficiently is the main reason metals have that brilliant shiny surface we see in jewellery, and many metal objects. Metals are usually shiny, hard, and good at conducting electricity because of the free electrons on outer shells.
If an atom has 4, 5, 6, or 7 valence electrons, it’s usually a non-metal. It will react in a way to gain more electrons to be stable. They are not shiny, poor conductors (like oxygen, sulfur). If it has 8 (full shell), it does not react much. They are Noble Gases (Group 18). Metalloids have properties in between, Metalloids usually have 3 to 6 electrons in their outermost shell. For example, boron has 3 valence electrons, silicon and germanium have 4, arsenic and antimony have 5, and tellurium has 6 in their outer shell. These valence electrons give metalloids the ability to behave sometimes like metals (by losing electrons) and sometimes like nonmetals (by gaining or sharing electrons), which is why they show mixed properties. The number of outer shell electrons plays a big role in how metalloids react chemically and why they are important in materials like semiconductors.
Why Hydrogen is the most abundant element?
The first element in the periodic table is Hydrogen, the simplest element, which has just one proton and one electron. Since it has only one proton there is no need of neutron to act as glue.
Near the beginning of the universe, when it was extremely hot and expanding, hydrogen was the easiest atom to form. That is why over 90% of atoms in the universe are hydrogen.
It is unique in the periodic table because it has one electron in a shell that can only hold two. So, it can both loose or gain one electron with equal ease. Due to its position and versatile chemical behaviour it can act like an alkali metal by losing its electron to form H+ or like a halogen by gaining an electron to form H−. Hydrogen exists naturally as a diatomic molecule (H2) with a very strong H–H bond, making it relatively inert at room temperature but reactive under certain conditions. Its low atomic mass gives it the highest diffusion rate and thermal conductivity among gases. Hydrogen’s chemical versatility is fundamental in forming water (H2O), organic compounds, biological molecules, and is crucial in energy-related processes such as combustion and fuel cells (hydrocarbons). Additionally, hydrogen forms important hydrogen bonds that stabilize the structures of many biological molecules like proteins and DNA.
If you manage to sneak in a neutron in a hydrogen nucleus, it gains weight. Since number of proton stays same, it is still Hydrogen, but a different isotope known as Deuterium. Water (H2O) formed by Deuterium is called heavy water. Add another neutron and you get Tritium. But so many neutron packed in the nucleus of a tiny atom is not stable, making Tritium radioactive. Their protons and neutrons feel too much tension inside, so the atom breaks apart, releasing energy and particles. This process, called radioactive decay. Tritium has a half-life of about 12.3 years, meaning it slowly decays over time by emitting weak radiation called beta particles. It occurs naturally in very small amounts in the atmosphere, created by cosmic rays, and can also be produced artificially in nuclear reactors. Tritium is important as a fuel in nuclear fusion reactions, especially when combined with deuterium, because it releases a large amount of energy. It is also used in scientific research, self-luminous devices like watch dials, and as a tracer in environmental studies.
Stars, including our Sun, are made mostly of Hydrogen, and they shine by fusing hydrogen into elements like helium (nuclear fusion). K- fills max 2 electrons, and in Helium it’s full. Helium is the first noble gas and the second element in the periodic table after hydrogen.
Noble gases have larger atomic sizes compared to other elements in the same period because their outermost shell is completely filled with electrons. This full outer shell causes the electrons to repel each other strongly, making the electron cloud expand. Unlike other elements, noble gases do not form bonds easily, so their atomic size is measured by Van der Waals radius, which includes the space the atom occupies when it is not bonded to another atom. This Van der Waals radius is generally larger than the covalent radius used for bonded atoms, so the measured atomic size of noble gases appears bigger.
Heavier elements are formed later in the core of stars because it takes increasingly higher temperatures and pressures to fuse larger atomic nuclei. In the early life of a star, lighter elements like hydrogen fuse to form helium as the core temperature reaches millions of degrees. As the star ages and its core temperature rises further, fusing helium produces heavier elements like carbon and oxygen. Even heavier elements such as neon, magnesium, silicon, and eventually iron are formed only when the core temperature reaches hundreds of millions to billions of degrees. Each stage of fusion requires more extreme conditions because larger nuclei repel each other more strongly due to their positive charges, so more energy is needed to overcome this repulsion. This layered fusion process creates shells within the star’s core, with heavier elements forming in the innermost hottest regions last, just before the star ends its life in a supernova explosion. Elements heavier than iron are generally made during such explosive events or in neutron star collisions, as fusion beyond iron consumes energy rather than releases it.
After Hydrogen (1 proton) and Helium (2 protons), the next element in the periodic table is Lithium (3 protons). It is 2 electrons in K-shell and one in it’s valence shell- L. Lithium is special because it is the lightest metal and the first element in the alkali metal group. It has a very low density—lighter than water—allowing it to float on water, which is unusual for metals. Lithium also has the highest specific heat capacity among solids, meaning it can absorb a lot of heat before its temperature rises significantly.
Chemically, lithium is highly reactive, readily losing one electron to form Li+ ions. It reacts vigorously with water to produce lithium hydroxide and hydrogen gas. Lithium’s compounds, such as lithium carbonate and lithium hydroxide, have important industrial and medicinal uses, including in batteries, ceramics, and mood-stabilizing drugs.
How Atoms React?
Reactions depend on how electrons behave. The energy needed to remove an electron from an atom, like in Lithium, is known as its Ionization energy. Electron affinity is the energy change when an atom gains an electron. How strongly an atom attracts electrons in a bond is known as Electronegativity.
Periodic Table helps us understand Redox Reaction too. Oxidation means losing electrons, and metals tend to lose electrons during reactions, forming positive ions (cations). Reduction means gaining electrons, and nonmetals usually gain electrons to form negative ions (anions). For example, when metal reacts with nonmetal, the metal atom loses electrons (oxidized), and the nonmetal atom gains electrons (reduced). However, this is a generalization mostly true for ionic reactions. There are also redox reactions involving only nonmetals or complex reactions where the roles can vary, but broadly, metals act as reducing agents by losing electrons, and nonmetals act as oxidizing agents by gaining electrons.
Since the negative electrons balance the positive charge of proton, when an atom loose electron it becomes positive, a cation. Anions are made when an atom gains electrons (so it becomes more negative). Such charged particles are called ions. Cation is always smaller than the parent atom, because, cation is formed by the loss of electrons, so, protons are more than electrons in a cation. Therefore, electrons are strongly attracted by the nucleus, hence the size decreases. Anion is larger then the parent atom, because, anion is formed by the gain of electrons, so the number of electron are more than protons. The effective positive charge in the nucleus is less so less inward pull takes place. Hence, the size of anion increases
What is Carbon the key to life?
We talked about metals and non-metals. What about atoms with inbetween number of four valence electrons, like carbon, silicon, and germanium? They have a unique ability to form a variety of bonds. Because they have four electrons in their outermost shell, they can either share, gain, or lose electrons to reach a stable arrangement of 8 electrons in their valence shell, following the octet rule. This makes them very versatile: they can form strong covalent bonds by sharing electrons with other atoms, creating complex molecules and materials.
Carbon is the first (in Group 14) in the periodic table to have this weird property, making it very special. Being first, it means it is the easiest to make. That’s why Carbon is relatively abundant on Earth and in the universe, making it readily available as a building material for life, especially in combination with Hydrogen (hydrocarbons). Its availability supports the formation of organic molecules that life depends on. The four valence electrons, allows it to form four strong covalent bonds with other atoms, including other carbon atoms. This tetravalency enables carbon to create a vast variety of complex, stable molecules with different shapes—chains, branches, and rings—which are essential for the diverse chemical functions life requires. Carbon compounds can form stable, long chains and complex structures like DNA and proteins, which are fundamental for growth, replication, and biological processes. Carbon forms strong, stable bonds compared to other elements that can also form multiple bonds (like silicon). Its bonds are shorter and more durable because its valence electrons are in the second shell, closer to the nucleus (another virtue of being first in Group 14). This means that carbon-based molecules remain intact under the conditions life experiences, allowing complex biological molecules to function properly. Thanks to this, we don’t randomly disintegrate, or melt, while walking in the streets.
Silicon, another atom with four valence electrons, is widely used in electronics because it can form stable crystal structures but also conduct electricity under certain conditions as a semiconductor. Thus, four valence electrons give atoms the flexibility to participate in many types of chemical reactions and make a wide range of substances.
Other special groups in the periodic table are Halogens (Group 17), which are very reactive and form salts, and Alkali Metals (Group 1), which are also very reactive and form soft metals.
Predicting through periodic table
So, by looking at the periodic table one knows how many protons an element has, how many electrons, and thus how it reacts. Thus, just by looking at the periodic table you can guess some tends helping you predict how elements behave.
Atomic size increases down a group because as we move down, new electron shells are added, which places the outermost electrons farther from the nucleus. Even though the nucleus has more protons (higher positive charge), the increased distance and the shielding effect of the inner electrons reduce the pull on the outer electrons, making the atom bigger. Across a period, however, atomic size decreases from left to right because the number of protons in the nucleus increases, meaning the positive charge of the nucleus is stronger. Since electrons are added to the same outer shell during this period, they don’t add much extra shielding. The stronger positive charge on the other hand pulls the electrons closer, shrinking the size of the atom.
Ionization energy is the energy needed to remove the outermost electron from an atom. It decreases down a group in the periodic table because as we move down, atoms have more electron shells, so the outer electrons are farther from the nucleus and experience less pull due to shielding by inner electrons. This makes it easier to remove an outer electron, so ionization energy is lower. On the other hand, ionization energy increases across a period from left to right because the number of protons in the nucleus increases, which means a stronger positive charge pulling electrons closer. Since electrons are added to the same shell across a period, there is little shielding change, so the stronger nuclear attraction makes it harder to remove an electron, increasing ionization energy.
Electron affinity is the energy released when a neutral atom in the gas phase gains an electron to become a negatively charged ion. It shows how easily an atom attracts an extra electron. When an electron is added, the atom usually releases energy, making this process exothermic. The more energy released, the higher the electron affinity, meaning the atom more willingly accepts an electron. Electron affinity generally decreases as you move down a group because the atoms get bigger, and the added electron feels less pull from the nucleus. Across a period from left to right, electron affinity increases because atoms get smaller and the nucleus has a stronger pull, making it easier for the atom to gain an electron. Some exceptions to these trends exist, like noble gases and certain groups, due to the stability of their electron arrangements.
Electronegativity is the ability of an atom to attract shared electrons in a chemical bond. It decreases down a group because as you move down, atoms get bigger as more electron shells are added. This makes the outer electrons farther from the nucleus, and the increased distance plus the shielding effect of inner electrons reduce the nucleus’s pull on bonding electrons, lowering electronegativity. Across a period, electronegativity increases from left to right because the number of protons in the nucleus increases (higher nuclear charge), while electrons are added to the same shell with little extra shielding. This stronger positive charge pulls bonding electrons closer, making atoms more electronegative as you go across a period. Thus, atoms at the top right of the periodic table, like fluorine, have the highest electronegativity values.
Knowing the groups of periodic table can also help you find out the likely formula of a molecule formed by two elements. To predict the formula of a molecule using the periodic table, look at the group numbers of the elements involved because they indicate how many electrons each element can gain, lose, or share to form a stable bond. For example, elements in Group 1 have a valency of 1, meaning they want to lose one electron, while Group 17 elements have a valency of 1 but want to gain one electron. When two elements combine, balance their valencies so that the total charges cancel each other out. This is often done by swapping the valency numbers to become the subscripts in the formula. For instance, sodium (Na, Group 1) combines with chlorine (Cl, Group 17) to make NaCl, and magnesium (Mg, Group 2) combines with chlorine to make MgCl₂. By following this pattern, you can easily predict chemical formulas for most compounds using only the periodic table’s group numbers and a basic understanding of how charges must balance in a molecule.
What about acids and bases? Can we predict acid-base behavior based on the position, atomic size, and electronegativity?
From the Lewis theory we know that acids are electron pair acceptors and bases are electron pair donors. The position of elements on the periodic table makes it kinda obvious whether a molecule or ion will behave as an acid or base. Patterns on the periodic table also indicate groups of strong acids (such as those involving halogens) and strong bases (like hydroxides of alkali metals and alkaline earth metals in the first two groups). Generally, acid strength increases as you move from left to right across a period and down a group in the periodic table. This happens because electronegativity and atomic size influence how easily an atom can donate or accept protons (H+) or electron pairs. For example, among hydrohalic acids, strength increases from HF to HI as you go down the group, due to increasing atomic size which makes it easier to release a proton.
The more reactive an element the more harmful it is. Search “Can I Lick it? Periodic Table” in Google. I am sure you can now guess most of the elements unsafe to lick, simply based on their position in periodic table.
Why we breathe Oxygen?
Fluorine (Group 17) is special because it is the most electronegative and reactive element in the periodic table. Its high electronegativity means it has a very strong tendency to attract electrons, making it a powerful oxidizing agent. Fluorine’s small atomic size and high effective nuclear charge cause its valence electrons to be tightly held, contributing to its intense reactivity.
Fluorine readily forms bonds with almost every element, including some noble gases and metals that are normally very unreactive. It forms very polar bonds, as seen in hydrogen fluoride (HF), where the H-F bond is highly polar, leading to strong hydrogen bonding and unusual physical properties compared to other halogens. It forms stable compounds with a wide range of elements and can rapidly oxidize materials that other elements cannot. Fluorine’s reactivity and electronegativity make it essential in many industrial and scientific applications, such as in pharmaceuticals, fluoropolymers like Teflon, water fluoridation, and nuclear fuel processing.
Being in Group 17 makes Fluorine not an ideal gas to be used in respiration. It is highly reactive, making it toxic, and it forms strong bonds leading to double problem. Firstly, it is not available freely as any free Fluorine will react quickly and the bonds will be difficult to break. It’s mostly locked up in minerals like Fluorite (CaF2). Secondly, for respiration you need bonds that can break free easily so that energy/electrons can be utilised. Which element is then the obvious choice?
Before Fluorine we have Oxygen, which is also highly electronegativity due to being towards the top right of the periodic table like Fluorine. Oxygen is in Group 16 (the chalcogen family) and Period 2, which means it has six valence electrons—two short of having a full outer shell of eight electrons. This makes oxygen highly reactive, and yet not as reactive as Fluorine, so it readily forms strong bonds with many other elements, including carbon, hydrogen, and metals, which are crucial for building complex biological molecules like proteins, DNA, and carbohydrates.
Unlike Fluorine, Oxygen is abundant because it forms stable O2 molecule and is freely available and abundant. It also forms other stable products like water. It is not toxic.
Oxygen is special because it’s thermodynamically unstable but kinetically stable, meaning it readily accepts electrons to release energy for life but is prevented from reacting too fast by a “kinetic barrier” under normal conditions. This combination is crucial because oxygen’s extreme reactivity allows organisms to efficiently extract energy from nutrients through aerobic respiration, forming water, a non-toxic substance. Its small atomic size and high electronegativity allow oxygen to attract electrons strongly, enabling it to efficiently accept electrons during cellular respiration—a process vital for energy production in cells. It acts as the final electron acceptor in the electron transport chain, efficiently producing ATP (energy). Oxygen’s ability to form stable, versatile compounds while also being reactive enough to support oxidation (energy release) but not so reactive as to be uncontrollable makes it ideal for sustaining complex life.
Periodic table also show diagonal relationship, which refers to the similarity in chemical and physical properties between certain pairs of elements that are positioned diagonally to each other in the second and third periods. This relationship exists because as you move down a group, atomic size increases and electronegativity decreases, while moving across a period causes atomic size to decrease and electronegativity to increase. These opposing trends cancel out when moving diagonally, leading elements like lithium (Li) and magnesium (Mg), beryllium (Be) and aluminum (Al), and boron (B) and silicon (Si) to show surprisingly similar properties despite belonging to different groups. For example, these diagonal pairs may have similar atomic sizes, electronegativity, and chemical behaviors, such as forming similar types of compounds and exhibiting comparable bonding characteristics.
Why is mercury liquid?
As we add more proton in the nucleus, and to glue, a lot more neutrons, the nucleus’s positive charge (atomic number) increases. Which means that in heavy elements like mercury or gold, the inner electrons move very fast due to strong attraction by the nucleus. When electrons move very fast near the nucleus, especially in heavy atoms, their speed causes their effective mass to increase (remember e=mc^2?), meaning they behave like heavier particles. This makes the orbitals closest to the nucleus, such as the s and p orbitals, shrink or contract. This contraction lowers the energy of these orbitals and makes the electrons more tightly bound to the nucleus, which affects the atom’s size, how reactive it is, how much energy is needed to remove an electron (ionization energy), and how strongly it attracts electrons (electronegativity). In addition, this effect indirectly changes the energy and stability of other orbitals like d and f, influencing the chemistry of heavy elements. These relativistic effects explain unusual properties of heavy elements, such as why mercury is liquid at room temperature and why gold has its characteristic color—things that simpler classical models cannot predict.
The relativistic contraction of mercury’s outer electrons (due to relativistic atomic effects) affects the electron density and bonding, contributing to its shiny, reflective surface. This is why mercury can reflect light nearly perfectly, making it ideal for mirrors and other reflective surfaces.
Bonding
Periodic table also helps us understand how atoms connect to form bonds.
An atom is considered stable when it has the same electron arrangement as an inert gas, meaning it has eight electrons in its outermost shell, known as an octet (or 2 for Hydrogen). To reach this stable state, atoms tend to chemically combine by transferring, sharing, or gaining electrons in their outer shell. This process helps them achieve a full outer shell similar to the nearest noble gas. Atoms are electrically neutral because they have an equal number of positively charged protons and negatively charged electrons. By losing, gaining, or sharing electrons, atoms can form a stable electronic configuration and become more stable. The force that holds two atoms together in a molecule, maintaining this stability, is called a chemical bond. Atoms form chemical bonds to achieve the stable electron arrangement of the nearest noble gas, often releasing energy in the process.
Ionic Bonds (Electrovalent Bonds) forms when one atom gives up electrons (becomes cation) and another gains them (becomes anion). Example: Sodium chloride (common salt), where sodium gives an electron to chlorine.
Ionic compounds are hard solids because they are made up of ions held together by strong electrostatic forces of attraction, which makes it difficult to separate them. These compounds have high melting and boiling points, making them non-volatile solids since it takes a lot of energy to break the strong bond between the ions. In solid form, ionic compounds do not conduct electricity because their ions are fixed in place and cannot move freely. However, when melted or dissolved in water, the strong forces weaken or are overcome, allowing the ions to move and conduct electricity. Ionic compounds usually dissolve in water, a polar solvent with a high dielectric constant, because water molecules reduce the attraction between ions, freeing them to spread out in the solution. When an electric current passes through molten or aqueous ionic compounds, the ions separate and move towards the electrodes, enabling conduction. Additionally, ionic compounds react quickly when dissolved in water due to the mobility of the free ions.
Caesium fluoride (CsF) is considered one of the most ionic compounds because it forms from cesium, which is a very large atom and easily loses an electron, and fluorine, which is a very small atom and highly electronegative, meaning it strongly attracts electrons. The large size difference between the cesium ion (Cs⁺) and the fluorine ion (F⁻), combined with the great difference in their tendencies to lose and gain electrons, results in a nearly complete transfer of electrons from cesium to fluorine. This creates a very strong ionic bond, making CsF highly ionic in character compared to many other compounds.
A covalent bond is formed when two non-metal atoms share electrons equally or nearly equally to achieve stability, typically by completing their outermost electron shells (octet rule). For a covalent bond to form, the atoms usually have four or more electrons in their outer shell, high electronegativity, high ionization energy, and electron affinity, and their electronegativity difference should be very small or zero. Covalent bonds can be single, involving the sharing of one pair of electrons (like in H₂ or Cl₂), double, sharing two pairs of electrons (such as O₂), or triple, sharing three pairs of electrons (like N₂). There are also polar covalent bonds, where atoms share electrons unevenly due to a difference in electronegativity, causing partial charges on the bonded atoms, as seen in molecules like HCl, H₂O, and NH₃. Covalent compounds are generally liquids or gases with low melting and boiling points because their molecules are held together by weak forces. They do not conduct electricity in solid or liquid form since they lack free ions, although polar covalent compounds may conduct when dissolved in water due to ion formation. These compounds usually dissolve in organic solvents but are insoluble in water and tend to react slowly compared to ionic compounds.
In a water molecule, hydrogen atoms have a partial positive charge. This happens because oxygen is more electronegative, meaning it attracts the shared electrons in the O–H covalent bonds more strongly than hydrogen does. As a result, the electrons spend more time around the oxygen atom, giving oxygen a slight negative charge, while the hydrogen atoms become electron-deficient and acquire a slight positive charge. This unequal sharing of electrons makes water a polar molecule.
Because of this polarity, hydrogen atoms in one water molecule are attracted to the oxygen atoms in nearby water molecules, forming hydrogen bonds. These hydrogen bonds are crucial to many of water’s unique properties, like high surface tension, boiling point, and its role as an universal solvent.
A coordinate bond is a special type of covalent bond where both electrons in the shared pair come from only one of the atoms involved. The atom that provides these electrons is called the donor, while the atom or ion receiving the electrons is the acceptor. This bond has characteristics of both covalent and ionic bonds, which is why it is sometimes called a co-ionic bond. Unlike regular covalent bonds, the shared electron pair in a coordinate bond is not shared with any other atom. The bond is usually represented by an arrow pointing from the donor atom to the acceptor atom (→). For a coordinate bond to form, one atom must have at least one lone pair (an electron pair not involved in bonding), and the other atom must be electron-deficient, meaning it lacks a lone pair. Examples include the formation of the ammonium ion (NH₄⁺), where the nitrogen in ammonia donates a lone pair to a hydrogen ion (H⁺), and the hydronium ion (H₃O⁺), formed similarly by water donating a lone pair to H⁺. This type of bonding is important in many chemical reactions and complex molecules.
When ammonium chloride (NH4Cl) forms, it consists of the ammonium ion (NH4⁺) and the chloride ion (Cl⁻). Inside the ammonium ion, nitrogen forms three regular covalent bonds with hydrogen atoms and one coordinate covalent bond, where both shared electrons come from nitrogen. These positively charged ammonium ions and negatively charged chloride ions attract each other strongly due to opposite electrical charges, creating an ionic bond between them. Therefore, ammonium chloride is a great example of a compound that has all three types of chemical bonds: covalent, coordinate covalent, and ionic bonds.
Trivia: Why is glass transparent?
Let’s go back to the electrons. The electrons can be either in the inner filled bands, or the outer valence bands. There is actually a third band, called conduction band. It is the range or band of electron energy levels in a solid where electrons are free to move throughout the material, enabling electrical conductivity. When electrons gain enough energy (for example, from heat, light, or electrical voltage) to jump from the valence band into the conduction band, they become mobile charge carriers that can flow and create an electric current.
Glass is mostly made of silica (silicon dioxide, SiO₂), along with small amounts of other oxides such as sodium oxide (Na₂O) and calcium oxide (CaO). These ingredients melt together and solidify into an amorphous solid, meaning glass lacks a regular, repeating crystal structure. Instead, its atoms are arranged in a random, disordered network.
Glass is transparent because of the way its electrons and energy levels work. In solid materials, electrons exist in groups of energy levels called bands — primarily the valence band (filled with electrons) and the conduction band (where electrons can move freely). Between these two bands is an energy gap called the band gap. For glass, this band gap is very large, which means that the energy of visible light photons is too small to excite electrons from the valence band to the conduction band. Since the electrons cannot absorb the light energy to jump to a higher state, the light passes right through without being absorbed or reflected. This allows photons of visible light to travel through glass smoothly, making it transparent. In contrast, metals have no band gap because their valence and conduction bands overlap, so electrons easily absorb and reflect light, making metals opaque.
Transparent materials are generally poor electrical conductors because transparency requires that the material does not absorb visible light. This implies a large band gap between the valence and conduction bands, so photons of visible light do not have enough energy to excite electrons across the gap to the conduction band, preventing absorption.
The universe in you
According to the best model we have of the universe, there exists fundamental forces which interacts to create fundamental particles – mass and matter from energy. The fundamental particles combine, based on their structure and charge, to make atoms, molecules, and ultimately you. But there is more to the universe than what the fundamental particles tell us.
95% of the universe is made of mysterious stuff that we can’t perceive because we aren’t perfect. We evolved to comprehend just enough to increase our chance of survival. Our body has enough imperfections to prove evolution’s struggles and not intelligent design.
Your body is composed of about 7 octillion atoms (mainly oxygen, hydrogen, and carbon)- that’s a 7 followed by 27 zeros! Most hydrogen atoms were formed in the first few minutes after the Big Bang, about 13.8 billion years ago, and the heavier elements (like carbon and oxygen) were created later in the interiors of stars and scattered across the universe during supernova explosions between 7 and 12 billion years ago.
Those atoms have electrons which are indistinguishable from each other. Some believe that there is only one electron traveling back and forth in time, giving us the illusion of many electrons. When two electrons collide, it is basically same electron interacting with itself!
The photon formed in the sun travels about eight minutes and 20 seconds until they are absorbed in our eyes. At speed of light, time stops and space collapses. For the photon, the journey from emission to absorption is instantaneous and happens at the same place. In other words, for the photon my eye and the sun was at same spot…my birth and The Big Bang 13.8 billion years ago happened at the same time.
Science makes you realise that the world is not as it seems! It makes you realise that there is a greater reality than we comprehend. And still we evolved our imperfect and evolving consciousness to observe and feel that awe! It creates an appreciation of the interconnectedness and beauty of life and the cosmos, a sense of humility and wonder about the unknown aspects of existence.
So the next time you look in the mirror, remember: you are not just “you.” You are hydrogen from the Big Bang, carbon from ancient stars, calcium from exploding supernovas, and oxygen shaped in galaxies. Your brain that allows you to think and know the universe, is made of the universe. You are the living, breathing proof that the universe wanted to know itself.
