Non-metal atoms can share pairs of electrons. The shared negative electrons are strongly attracted to both positive nuclei: this is a covalent bond. Hydrogen needs two electrons in its outer shell; the other atoms in these examples complete eight.
One shared pair is a single bond. In H₂, each hydrogen contributes one electron. In Cl₂ and HCl, each chlorine has three unshared pairs as well as the bonding pair. Dots and crosses track origin, not different kinds of electron.
Oxygen forms two bonds in water, nitrogen three in ammonia, and carbon four in methane. Count the shared pairs and the unshared pairs. These flat drawings show electron arrangements, not the exact 3D molecular shapes.
O₂ has a double bond: two shared pairs. N₂ has a triple bond: three shared pairs. A bond line stands for one shared pair, so O=O has two lines and N≡N has three.
One covalent bond = one shared pair. H forms 1 bond, O 2, N 3 and C 4 in these molecules.
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