Covalent Bonding

What a covalent bond is

• A covalent bond forms when two non-metal atoms share a pair of electrons
• Each atom contributes one electron to the shared pair
• The shared pair counts toward each atom's outer shell, so both atoms end up with a full (noble-gas) outer shell
• Covalent bonds are very strong

Bonding and non-bonding electrons

• The two shared electrons in a bond are the bonding electrons (or a bonding pair)
• Outer-shell electrons that are not part of any bond are non-bonding electrons or lone pairs

What holds a covalent bond together

• The shared pair of electrons (negative) sits between the two nuclei (positive)
• Both nuclei are electrostatically attracted to the same shared pair
• That mutual pull on the same electrons is what holds the atoms together

Notation: a line for each bond

• A single covalent bond is drawn as a short straight line between two atom symbols:
  • H–H means a single bond between two hydrogen atoms
• A double bond is drawn as two lines:
  • O=O means a double bond
• A triple bond is drawn as three lines:
  • N≡N means a triple bond

Covalent vs ionic — the key difference

• In a covalent bond, electrons are shared. No atom permanently gains or loses an electron, and no ions form
• In an ionic bond (Topic 6), electrons are transferred from one atom to another, and oppositely charged ions form

Two kinds of covalent substance

• Every covalent substance falls into one of two structural classes:
  • Simple molecular — small, discrete molecules (H₂O, O₂, CO₂); covered in section 3
  • Giant covalent (also called macromolecules) — many millions of atoms covalently bonded into one continuous lattice (diamond, graphite); covered in section 4