What a covalent bond is
A covalent bond 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
A near-every-paper question: what forces hold a covalent bond together?
Write: the (electrostatic) attraction between the shared pair of electrons and the two nuclei .
Watch out: say nuclei (plural), and name both the electrons and the nuclei. "Atoms sharing electrons" on its own scores nothing.
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:
A triple bond is drawn as three lines:
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