When two atoms get close to each other, they get attached to form a covalent bond. In a bond, the atoms are not stable since their outer rings of electrons are not full. Through sharing electrons with other atoms in a bond, the atoms fill their outer rings and become stable. The covalent bond between atoms in a molecule is as strong as in ionic bonds but unlike in ionic bonds, the total number of covalent bonds that each atom can form has a limit. For example, carbon can only form four bonds and oxygen can form two bonds and once they have made all the bonds, they will no longer be able to interact with other atoms. This is the reason why two molecules cannot form a covalent bond. Light molecules are therefore gases like ethane, nitrogen and hydrogen. The heavier molecule like isooctane molecule is a liquid at room temperature and others like cholesterol are solids.
A covalent bond makes a very strong connection between atoms making it hard to break the molecules apart. On the other hand, the molecules that join to form a covalent bond are not attracted to each other that much and so they are able to move freely around each other. This means that most molecules that create a covalent bond make either gases or liquids like carbon dioxide and water. The major exception is metals that hold together through a covalent bond but are still solids. This is why metals are flexible and can be melted and made into different shapes. Non-metals form a covalent bond with other nonmetals in order to be stable and they can form between one to three covalent bonds with the other non-metals depending on the electrons that they have. Although atoms share electrons when they form a bond, they don’t normally share the electrons equally. Electrons are shared equally only when two atoms of the same element create a covalent bond. When atoms from different elements form a covalent bond, electrons are drawn more towards the atom that has a higher ability to draw electrons. This results to a polar covalent bond. A covalent bond can be in the form of liquid, gas or solid and does not conduct heat or electricity well.
In water, the oxygen atom requires two or more electrons in order to form a stable covalent bond but the hydrogen atom each needs one. When they combine, oxygen atom shares one electron which each hydrogen atom and each of the hydrogen atoms shares one electron with the oxygen atom. The atoms will now be stable and it will be difficult to take them back to being unstable. This implies that a covalent bond is strong and the molecules that form from sharing bonds are strong molecules. In methane molecule (CH4), each of the four electrons in the outer shell of the carbon is shared with one hydrogen atom and each of the hydrogen atoms shares one electron with the carbon atom. In the end, carbon contains ten electrons thus satisfying the octet rule and hydrogen contains two electrons. When studied experimentally it is found that the four electrons in hydrogen (CH4) bond spread equally around the carbon atom.
In isomers, a covalent bond is formed between two carbon atoms to create a molecule like ethane (C2H6). When two more carbon atoms and four more hydrogen atoms are added to form butane (C4H10), an interesting situation occurs. Two carbons can form a covalent bond in two different ways to form two different molecules or isomers. The first method is where one carbon atom forms a covalent bond to all the three carbon atoms. The other method is where one carbon is bonded to three atoms of hydrogen and another carbon is bonded to another two hydrogen atoms and the other remaining carbon is bonded with the other remaining carbon. Other larger compounds can also be created and they will contain more isomers. For example, isooctane has eight carbons and is a major component in petrol. Since a bond can be formed in several ways, it is easy to write down and make several different molecules. Most of them are natural compounds formed by plant or animals within their cells. One of the molecules is cholesterol (C27H46O), which contributes to heart diseases in people whose diet contains a lot of fat. In cholesterol, two carbon atoms that are next to each other form only three bonds which go against the octet rule. These atoms actually bond through sharing two electrons each, thus completing their electron shell like the other atoms. This state is known as a double bond. Other compounds are synthetic which are created by chemists. Chemists can create natural compounds from simple things like water and methane. The molecules that are formed this way are similar to the real natural compounds. Other synthetic molecules don’t exist in nature. They have desirable properties, for instance, many medicines are formed this way. A small medicine molecule example is aspirin C9H8O4. In an aspirin molecule, two bonds between oxygen and carbon are double bonds.
Solids can also form a covalent bond. An example is the diamond which is pure carbon in which every carbon atom bonds to four other carbons forming a huge molecule that contains millions of atoms. In diamond, every carbon atom shares one electron with each of their four neighboring atoms of carbon. Another form in which pure carbon can be created is graphite. Graphite is a major component of lead. In graphite, instead of every carbon having four neighboring carbon atoms, they each have only three. Every carbon atom shares one electron with two of its neighbors and two electrons with the third neighbor. In this way, one carbon bond becomes a double bond.
The atom’s ability to draw electrons (electron-negativity) can be used to determine the type of covalent bond that will be formed. In a covalent bond, there are atoms that contain a higher ability to draw electrons and there are others that have a lower ability. It is just like a tug of war game among atoms. Just like in the tug of war game in human beings, whereby when one person on the side of the rope is stronger than the other person, the strong person easily pulls the other person to their side. On the other hand, if both persons have the same strength, the rope will not move to any direction. In a similar way, if one atom is stronger (has a high ability to draw electrons), it will easily pull the electrons to itself and the electrons will be shared unequally. But if the atoms have the same ability, the electrons will be shared equally between the two atoms.
There are two types of covalent bonds formed depending on the ability of an atom to draw electrons of the elements being combined. The first type is the non-polar covalent bond. When a covalent bond is being formed between two atoms that are from the same element, the shared pair will lie at the center of the two atoms. This implies that the atoms will share the electrons equally and the molecule that will be created will be electrically symmetrical. Being electrically symmetrical means that, the middle of the positive charge coincides with the middle of the negative charge. This is the covalent bond and its examples include H2, O2, Cl2 and many others.
The second type of covalent bond is the polar covalent bond. A polar covalent bond is the bond formed between two atoms that are from two different elements. The electron affinity of the atoms that are involved in a polar covalent bond is different. The bonding pair of electrons lies more on the atom which has a higher electron affinity. The atom that has a higher affinity for electrons acquires a slightly negative charge and the atom that has a lesser affinity for electrons gets a slightly positive charge. An example of a polar covalent bond is hydrogen chloride (HCl). In hydrogen chloride, the covalent bond of chlorine and hydrogen atoms lies more towards chlorine atom because chlorine has more ability to draw electrons. Therefore, chlorine atom gets a slightly negative charge while the hydrogen atom gets a slightly positive charge.
Other types of covalent bonds include the single covalent bond. A single covalent bond is formed when there is a mutual sharing of one electron pair between two atoms. Each atom provides one electron. It is denoted by a single line and its examples include H-CI, Br-Br and C-C. There is a double covalent bond as well whereby two atoms share two electron bonds mutually. It is denoted by a double line, examples include O=O and C=C. Another type of a covalent bond is the triple covalent bond whereby atoms mutually share three electrons. It is denoted by triple line.