Bonds and Molecules

Atoms are more stable when their valence (outer) shell is full — eight electrons for most elements (two for H and He).

Their valence shells are already full, so they have nothing to gain by giving, taking, or sharing electrons.

Electrostatic attraction between oppositely charged ions (e.g. Na⁺ and Cl⁻) — no electrons are shared.

A struck crystal shifts one layer so Na⁺ lines up with Na⁺; like charges repel and the lattice shatters itself.

Charge carriers must be free to move; ions are locked in the lattice as a solid but mobile when molten or dissolved.

An atom's 'electron greed' — how strongly it pulls shared electrons toward itself. Fluorine is the champion.

A shared pair of electrons between two electron-hungry atoms, so both effectively count a full shell.

Its triple bond is extraordinarily strong — making fertiliser manufacture energy-hungry and nitrogen explosives energy-rich when they snap back to N₂.

Oxygen has four electron pairs (two bonding, two lone) that repel each other and push the hydrogens into a bent shape.

Unequal electronegativity: shared electrons sit nearer the greedier atom, giving it δ− charge and the other δ+ (as in H–O).

Ionic compounds form extended 3D lattices with no discrete molecules; covalent bonding makes genuine, countable molecules with definite shapes.