A covalent bond is an inter atomic linkage between two atoms. The bonded atoms have a lower total energy than that found in widely separated atoms. The sharing of a pair of electrons creates a force of attraction between the atomic centers (positively charged) and the same two electrons.

The American chemist G.N. Lewis first proposed the idea that a pair of electrons could be shared between two atoms. He saw that this sharing would forge a link between them, and that such a bond would result in both atoms having their outermost energy levels filled with electrons.

Atoms of hydrogen have an incomplete energy level. Only one electron orbits in the 1s orbital. As two atoms of hydrogen come together the positively charged atomic centers begin to attract both electrons (their own and the one in the other atom). At a certain distance apart, the orbitals overlap and merge into a single, larger molecular orbital in which the pair of electrons distribute themselves over the pair of atomic centers.

Each atom in the newly created hydrogen molecule now has a filled outermost energy level, and, by sharing electrons in this way, achieves maximum stability and a lower level of energy.

A covalent bond is drawn or represented in atomic and molecular diagrams using a short straight line. The uniting of two atoms of hydrogen in this way would be drawn H-H. Very frequently this is written H₂.

Covalent bonds can form between similar atoms (H-H), or different atoms (H-Cl) and an atom can form more than one covalent bond at the same time (H-O-H).

In some cases, atoms may share four electrons between them, forming a double bond. (O=O) Or in some extreme cases two atoms can share 6 electrons forming a triple covalent bond between them.

Covalent bonds are directional. Atoms are bonded together in preferred orientations relative to one another. Molecules, therefore, have definite shapes such as that seen in the bent structure of a water molecule.

Electrons in the bonds between identical atoms (H-H) are shared uniformly, so the electrons spend equal amounts of time around each atomic center. These covalent bonds are non-polar.

Electrons shared between unlike atoms are not shared equally, one atom gets more of the common electrons and is thus slightly negatively charged. The other atoms gets less than a full share of the electrons and is thus slightly positively charged.