It is a well-established fact that everything around us is made up of atoms. Atoms combine to form molecules, and then molecules combine to form matter that we observe around us. A molecule is considered stable if the atoms composing it have more attractive forces than repulsive forces among them. In other words, atoms with higher potential energy combine to gain stability and form molecules. But how do they join up in the first place? In chemistry, this phenomenon falls under the study of chemical bonding. The subatomic particles (protons and electrons) act under the influence of electrostatic forces. When the electron clouds of two individual atoms get close enough to interact with both nuclei, they begin to pull them together with repulsion among electrons acting as a counterbalanced force. When the interplay of these attractive and repulsive forces results in a stable state, where the outermost valence electrons are shared by both the atoms, a covalent bond is formed among them. On the other hand, if the attractive force from one of the nuclei is so overwhelming that it can almost take away the shared pair of electrons, an ionic bond is formed. Therefore, the term covalent bond, in essence, means that the atoms share “valence.”
There are several aspects based on which covalent bonds can be categorized. For instance, a covalent bond can either be heteronuclear or homonuclear, i.e., either it consists of atoms of one chemical element, as with two atoms in the chlorine molecule ({Cl}_{2}), or composed of more than one element, as with methane ({CH}_{4}). Moreover, based on the polarity of the bond, a covalent bond can be categorized as a polar covalent bond or a non-polar covalent bond. Covalent bonding also includes several kinds of interactions, such as σ-bonding, π-bonding, metal-to-metal bonding, agostic interactions, bent bonds, three-center two-electron bonds (3c-2e), and three-center four-electron bonds (3c-4e). Let’s discuss a few real-life examples of covalent bonds.
1. Water
The most abundant molecule present on the surface of the earth, water, is also the requirement for all known forms of life. Since the dawn of civilization on the earth, water has remained a commodity of utmost importance to human beings, and therefore, it has been given several adjectives including “elixir of life” and “universal solvent.” In comparison to the complexity of its role in our lives, water has a much simpler molecular structure. It consists of two hydrogen atoms attached to an oxygen atom via covalent bonds. The oxygen atom is electronegative, and therefore, it gives a polar character to the bond. Consequently, the shared pair of electrons have a higher probability to be found near the oxygen atom than the hydrogen atom. Therefore, the oxygen atom at the center has a slight negative charge (from the presence of extra electron share), while the hydrogens are slightly positive (due to the extra un-neutralized protons). Also, the presence of lone pairs of electrons at oxygen distorts the perfect tetrahedral structure of {H}_{2}{O} by changing the bond angle from 109.5 ° to 104.5 °. Water is capable of dissolving a variety of different substances, which is why it is called the universal solvent. The polar covalent character of water allows its molecules to become attracted to many other different types of molecules and disrupt their pre-existing bonds. However, it is important to note that the bond formed between two individual water molecules is a hydrogen bond and not covalent.
2. Sugar
Sugar is a carbohydrate compound that is composed of 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms ({C}_{12}{H}_{22}{O}_{11}) to which a total of 136 valence electrons are distributed amongst the 45 atoms, all linked together via covalent bonding. In more technical terms, two monosaccharides, glucose ({C}_{6}{H}_{12}{O}_{6}) and fructose ({C}_{6}{H}_{12}{O}_{6}), are linked together via another covalent bond known as a glycosidic bond to form a disaccharide called sugar. For human consumption, it is mainly extracted from a plant called sugarcane. Although sucrose is a covalent compound, it breaks down into fructose and glucose, and therefore, it can dissolve in water readily. This happens because of many -OH bonds present in the molecule; however, the process is so gradual that it could sit in the solution for years with negligible change.
3. Oxygen
Originally forged in superhot and superdense stars, oxygen is the third most abundant element present in the universe. However, due to its high electronegativity, it can not be found in its elemental form and mostly occurs as a diatomic molecule ({O}_{2}). An oxygen atom has 6 electrons in its outer shell. Two oxygen atoms will each share two electrons to form two covalent bonds, otherwise known as a double bond, corresponding to each shared pair of electrons. Another reactive allotrope of oxygen i.e., ozone ({O}_{3}) in which two oxygen atoms are linked together with a double covalent bond, while the third oxygen atom is linked via a coordinate covalent bond with one of these two oxygen atoms. This makes ozone reactive as it easily decomposes to form oxygen gas. In vertebrates, {O}_{2} diffuses through membranes in the lungs and into red blood cells. Hemoglobin binds to {O}_{2}, which changes the color of the blood from bluish red to bright red.
4. Carbon Dioxide
Carbon dioxide ({CO}_{2}) is considered the most dangerous greenhouse gas, and at the same time, it is one of the most essential requirements for life to exist on our planet. The primary sources of carbon dioxide in our atmosphere include organism’s respiration or decomposition (decay), weathering of carbonate rocks, forest fires, volcanic eruptions, and several human activities such as the burning of fossil fuels and the production of cement. The molecular structure of {CO}_{2} is fairly simple that includes two atoms of oxygen attached to a carbon atom through covalent bonds. A carbon atom is four electrons short of a full outer shell (8 electrons) and oxygen is two electrons short of a full outer shell (8 electrons), so one carbon atom shares its four outer electrons with two outer electrons from each of the oxygen atoms, so all three atoms can have a full outer shell of 8 electrons with the formation of two double bonds (O=C=O). The molecular geometry of carbon dioxide is linear with carbon at the center and oxygen at the opposite ends. This structure corresponds to the minimum potential energy and stability of the molecule. {CO}_{2} is produced by human beings and other aerobic organisms when they metabolize organic compounds to produce energy by respiration. The process can be understood by the following reaction.
{C}_{6}{H}_{12}{O}_{6} + 6{O}_{2} → 6{CO}_{2} + 6{H}_{2}{O} + Energy
Plants, on the other hand, undergo a related process, photosynthesis in which they inhale carbon dioxide and exhale oxygen. The reaction for this process is:
6{CO}_{2} + 6{H}_{2}{O} → {C}_{6}{H}_{12}{O}_{6} + 6{O}_{2}
Although carbon dioxide gas is important for both human beings and plants, it can also cause great harm to life on earth. You have probably heard of the rising levels of {CO}_{2} in the atmosphere and the news concerning global warming. As {CO}_{2} builds up in our atmosphere from burning fossil fuels, it has a warming effect that could change the earth’s climate, and therefore, it is considered a greenhouse gas.
5. LPG
Commonly known as a cylinder gas or cooking gas, LPG (Liquified Petroleum Gas) is a vividly used fuel for cooking purposes around the world. The composition of LPG is a mixture of flammable hydrocarbon gases. Generally, a mixture of propane ({C}_{3}{H}_{8}) and butane ({C}_{4}{H}_{10}) along with several other hydrocarbon gases are compressed together to form LPG. Hydrocarbons are compounds that consist entirely of carbon and hydrogen, linked together via covalent bonding. A carbon atom has four electrons in its outermost valance shell that can be shared with other four atoms. Since hydrogen is one electron short for a stable configuration, it forms a covalent bond by mutually sharing one electron with carbon. The simplest hydrocarbon can be methane as it has 4 hydrogen atoms attached to a carbon atom via covalent bonds, thereby fulfilling the outermost shell of all the atoms. The propane and butane in our LPG make good fuels because their covalent bonds store a large amount of energy, which is released when the molecules react with oxygen to form carbon dioxide and water. Moreover, it may come as a surprise to many of us that both propane and butane are odorless gases. The smell we sense while turning the gas stove on is of another covalent compound Ethanethiol ({CH}_{3}{CH}_{2}{SH}), or commonly known as ethyl mercaptan.
6. Vinegar
There are not many food items that have multipurpose uses such as cooking, baking, and even cleaning. Most of us are familiar with the use of vinegar to provide sweet and sour, pungent flavor to our food, but not many of us are aware that we can make use of its acidic character for cleaning purposes also. Vinegar is composed of 5-8% volume by volume ethanoic acid ({CH}_{3}{COOH}), also known as acetic acid, diluted with water. In vinegar, acetic acid is generated by fermentation, which produces ethanol, and then subsequent oxidation of this ethanol. The method of acetic acid’s production by the oxidation of ethanol also helps explain why wines can begin to taste vinegary if the bottle is left open.
{CH}_{3}{CH}_{2}{OH} + {O}_{2} → {CH}_{3}{COOH} + {H}_{2}{O}
Acetic acid is the second simplest carboxylic acid (after formic acid) consisting of a methyl group attached to a carboxyl group via covalent bonding. Although acetic acid is a covalent compound, the hydrogen atom in the carboxyl group (-COOH) can go under ionization to form hydrogen cation and acetate anion.
{CH}_{3}{COOH} ⇌ {CH}_{3}{COO}^{-} + {H}^{+}
Because of this release of the proton ({H}^{+}), acetic acid has an acidic character. In households, diluted acetic acid is often used in descaling agents. In the food industry, acetic acid is controlled by the food additive code E260 as an acidity regulator and as a condiment. In biochemistry, the acetyl group, derived from acetic acid, is fundamental to all forms of life. When bound to coenzyme A, it is central to the metabolism of carbohydrates and fats.
7. Nail Polish Remover
In the modern world, nail polish is a must-have accessory to look elegant and classy. But no matter how good the manicured nails look with the nail polish, it all fades away over time. Thanks to the nail polish remover that people can get rid of their nail paint before it starts to look faded. The major ingredient of the nail polish remover is a volatile, flammable, and colorless liquid known as acetone. It is an organic compound that belongs to the ketonic group of classification with three carbon atoms, six hydrogen atoms, one oxygen atom, all linked together via covalent bonding to form {CH}_{3}{—}{CO}{—}{CH}_{3}.
The center carbon atom forms a double covalent bond with the oxygen atom and two single covalent bonds with the other two carbon atoms, whereas all the 6 hydrogen atoms form single covalent bonds with the exterior carbon atoms. Acetone removers can be used to remove most unwanted household stains. Acetone polish remover works by breaking down nail polish and removing it from the nail plate surface. Acetone is an organic solvent. Hence, the interactions between acetone molecules and polymer molecules of the nail polish are stronger than those between polymer molecules, and the polymer turns from solid to liquid. Once the nail polish becomes a liquid, it is free to be wiped off.
8. Diamonds
Diamond is an exquisite object. For centuries, people have adored the brilliance of these gems. While for some people it is a perfect object to express love to someone, others consider it the most valuable commodity to acquire. In technical terms, a diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. It is one of the naturally occurring allotropes of carbon, which has the highest hardness and thermal conductivity. Apart from being a valuable gemstone, diamond is also used industrially for cutting, grinding, sawing, and drawing wire. The hardness of a diamond can be understood by its crystal structure. Each carbon atom is attached to four other carbon atoms via covalent bonding in a regular tetrahedral structure. Each of these attached carbon atoms is then further attached to three other carbon atoms. This pattern continues to form a single, giant molecule held together by covalent bonds. this whole process takes place under intense pressure and temperature conditions that force the carbon to crystalize. To break the crystal, multiple bonds must be broken.
9. Urea
Have you ever heard of or seen Urea, a white chemical being spread on a field of the crop by farmers to fertilize the land. It may surprise you that not only it is important for the growth of a crop, but our liver also forms it to get rid of toxic ammonia from our body. It is also known as carbamide, an organic compound with the chemical formula {NH}_{2}{—}{CO}{—}{NH}_{2}. The molecular structure of urea is composed of two amino radical groups (—{NH}_{2}), attached to a functional group, carbonyl (C=O). The C=O bond is a double covalent bond while the C-N bonds are single covalent bonds. In terms of intramolecular bonding, there are hydrogen bonds between urea molecules (each carbonyl oxygen accepts 4, from N-H hydrogens). This leads to the urea’s high solubility in water.
10. Plastic
Plastic is the most versatile material in the modern world. You can bend, shape it, and use it the way you require. Almost every third to the fourth item you come across every day can be some kind of plastic. But what plastic actually is? Plastics is the term commonly used to describe a wide range of synthetic or semi-synthetic materials that includes polymers as the main ingredient. The polymer used in plastic manufacturing is a chemical compound that contains carbon and hydrogen linked together via covalent bonding. Some of the important groups in which the plastics are classified are acrylics, polyesters, silicones, polyurethanes, and halogenated plastics.
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