Covalent bonding was first used in 1919 by Irving Langmuir where he wrote ‘we shall denoted by the term covalence the number of pairs of electrons that a given atom shares with its neighbours.’
So why does bonding occur? Atoms aim to achieve the highest form of stability, the noble gas configuration (what group 8/0 have.) This means there are 8 electrons on the outer shell. To do so, atoms may join together and share electrons. A covalent bond is by definition the sharing of electrons.
Covalent bonds are the strongest type of bond (approximately 50-200 kcal/mol) which holds atoms together. For example, a DNA strand is held together by covalent bonds. An infamous example would be oxygen. Two oxygen molecules remain together, hence why it is called O2. Non-covalent interactions are weak in comparison (approximately 1.5 kcal/mol) and determine the 3-dimensional shape of macromolecules. For example, protein structure is a result of non-covalent interactions. Examples of non-covalent interactions are ionic bonds, hydrogen bonds, van der Waals, electrostatic interactions and hydrophobic interactions.
An atom is composed of a dense nucleus which consists of protons and neutrons. The protons give the atomic number and the protons added to the neutrons give the atomic mass. If you look at an element on the periodic table, you will be able to distinguish the tow values. The larger number will be the atomic mass. An easy way to remember this is, the atomic mass is the massive number. Surrounding the nucleus are shells which contain electrons. An orbital by definition is an area of space surrounding a nucleus occupied by electrons in opposite spins. Within each shell there are sub shells/ atomic orbitals. There are different types of orbitals which vary in shape.
Covalent bonds are directional, their shape is determined by the mutual repulsion of the atoms. Electrons are found in pairs, but some are not (called lone pairs.) Lone pairs of electrons cause a bigger repulsion. They are formed between non-metal atoms.
The strength of the bonds varies, and it is dependent on electronegativity. Electronegativity is a measure of tendency of an atom to attract a bonding pair of electrons. Fluorine is the most electronegative element. A double bond has a higher energy value than a single bond, and a triple bond has the highest. The higher the energy value, the more energy required to break the bond. For example, a triple carbon double bond has an energy value of approximately 195 unit kj/mol.
There are two types of covalent bonds, polar and non-polar. Non-polar covalent bonds occur when the two atoms difference in electronegativity’s are below 0.5. For example, in a single carbon bond, the two carbons have the same electronegativity value. In a non-polar covalent bond, electrons are share equally. In a polar covalent bond the two atoms have different electronegativity’s and thus electrons are shared unequally. The shared electrons are nearer one atom than the other. One atom is slightly negatively charged, and one is slightly positively charged. This creates a dipole effect (this expresses the magnitude of the polarity.
Quote reference: https://en.wikipedia.org/wiki/Covalent_bond