When hydrogen atoms are close enough to one another, they form pairs, with each of their single electrons moving around the two nuclei. As a result of sharing electrons, the diatomic hydrogen gas molecule is electrically balanced because it now contains two protons and two electrons. In addition, each hydrogen atom has two electrons in its outer shell, completing this shell. This relationship also results in the pairing of two free electrons. Thus, by sharing their electrons, the two hydrogen atoms form a stable molecule.
Covalent bonds can be very strong, that is, difficult to break.
Double bonds , those bonds in which two pairs of electrons are shared, are stronger than single bonds in which only one pair of electrons is shared. As you might expect, triple bonds , those bonds in which three pairs of electrons are shared, are the strongest of these three types of covalent bonds. In chemical formulas that show the structure of covalently bonded molecules, single bonds are represented by a single line between two bonded atoms, double bonds by two lines, and triple bonds by three lines. An atom can also form covalent bonds with more than one other atom. To satisfy the octet rule, it must gain 4 additional electrons by sharing its 4 outer-shell electrons with another atom or atoms, forming 4 covalent bonds.