A covalent bond is a shared electron pair attracted to both bonded nuclei. A single, double or triple bond contains one, two or three shared pairs. In common octet examples carbon forms four bonds, nitrogen three and oxygen two, with non-bonding electron pairs completing outer shells. Hydrogen forms one bond and has a two-electron filled shell. Bond formation releases energy; breaking a bond requires energy.
If bonded atoms attract electrons unequally, the bond is polar. The more electronegative atom is partially negative, δ−, and the other partially positive, δ+. These are partial charges, not full ionic charges. A bond between identical atoms is non-polar. Use a named molecule, structural formula or electron-dot description to count atoms and bonds; molecular polarity also depends on three-dimensional shape, treated in the next topic.
Example 1
In H–Cl, chlorine is more electronegative. Assign partial charges.
H is δ+ and Cl is δ− because shared electron density is drawn towards chlorine.
Example 2
How many shared pairs and non-bonding pairs surround oxygen in H2O?
Oxygen forms two single bonds, giving two shared pairs, and has two non-bonding pairs.
Watch out: Bond breaking requires energy; forming the corresponding bond releases energy.