Covalent bonding
A covalent bond forms when two nonmetal atoms share a pair of electrons. Neither atom pulls strongly enough to take the electrons outright, so both hold on. A group of atoms joined this way is a molecule.
- A single bond is 1 shared pair (2 electrons).
- A double bond is 2 shared pairs.
- A triple bond is 3 shared pairs.
A Lewis structure shows shared pairs as lines between atoms and unshared electrons, called lone pairs, as dots.
To draw one:
- Add up the valence electrons of all the atoms.
- Connect the atoms with single bonds. The atom that can form the most bonds usually goes in the center.
- Give the outer atoms lone pairs until each has 8 electrons. Hydrogen needs only 2.
- Put any leftover electrons on the central atom.
- If the central atom has fewer than 8, turn lone pairs from outer atoms into double or triple bonds.
Worked example: water, H2O
Valence electrons: 1 + 1 + 6 = 8.
Oxygen goes in the center with a single bond to each hydrogen. That uses 4 electrons. The other 4 go on oxygen as 2 lone pairs.
Oxygen now has 2 bonds and 2 lone pairs, which is 8 electrons. Each hydrogen has 2.
Worked example: carbon dioxide, CO2
Valence electrons: 4 + 6 + 6 = 16.
With only single bonds, carbon would have just 4 electrons. Moving one lone pair from each oxygen into the bonds gives two double bonds, and every atom has 8.
Common mistake
Counting a shared pair for only one atom. Both atoms in a bond count the shared electrons toward their octet.