Covalent bonding

What covalent bonding is

• A covalent bond is a shared pair of electrons between two atoms.
• It forms between two non-metals.
• Neither atom can give electrons away easily, so instead they share — and both count the shared pair towards a full outer shell.

Why the shared pair holds the atoms together

• The shared electrons sit between the two nuclei.
• Both positive nuclei are attracted to the same negative pair.
• That shared attraction is what holds the atoms together — it is the bond.

Single, double and triple bonds

• Atoms can share more than one pair if they need more electrons to fill their shells.

Bond	Pairs shared	Electrons shared	Drawn as	Example
single	1	2	one line	$Cl–Cl$
double	2	4	two lines	$O{=}O$
triple	3	6	three lines	$N{\equiv}N$

• One line always means one shared pair, which is two electrons.
• More shared pairs make a shorter, stronger bond — a triple bond is harder to break than a single one.

Bonding pairs and lone pairs

• Bonding pair — a pair of electrons shared between two atoms.
• Lone pair — a pair of outer electrons that stays on one atom and is not shared.
• Lone pairs still take up space around the atom, which matters a lot when you work out molecular shape.

Worked through:

• In water, oxygen has two bonding pairs (one to each hydrogen) and two lone pairs.
• In ammonia, nitrogen has three bonding pairs and one lone pair.
• In methane, carbon has four bonding pairs and no lone pairs.

How many bonds an atom makes

• An atom forms enough bonds to reach a full outer shell, which usually means 8 electrons (2 for hydrogen).
• The count is predictable from the group:

Atom	Outer electrons	Bonds it forms
H	1	1
C	4	4
N	5	3
O	6	2
F, Cl, Br, I	7	1

• This table is what makes structures checkable: if your drawing gives nitrogen four bonds, it is wrong.

Molecules

• A molecule is a discrete group of atoms joined by covalent bonds.
• Unlike an ionic lattice, a molecule is a real, countable particle — you can point at one $H_2O$.
• Molecules are attracted to each other only weakly, by forces between molecules, which is a separate idea from the strong bonds inside.