Summary
Atoms bond to become more stable, usually by completing eight electrons in their outer shell (the octet rule). Between a metal and a non-metal, the metal gives up electrons and ions form that attract each other: an ionic bond, with hard crystal lattices of high melting point that conduct electricity when molten or dissolved. Between non-metals, pairs of electrons are shared: a covalent bond, forming molecules (water, CO₂) or networks like diamond. In metals, the cations are surrounded by a sea of free electrons, which explains why they conduct and are ductile.
When two atoms with different electronegativities share electrons, the bond is polar. The shape of the molecule, predicted by VSEPR theory, decides whether the poles cancel (CO₂, linear and non-polar) or not (water, bent and polar). Between molecules, weaker forces act — hydrogen bonds and van der Waals forces — which explain boiling points and solubility.
Key points
- Octet rule: 8 electrons in the outer shell.
- Ionic: metal + non-metal, electron transfer.
- Covalent: non-metals, shared electrons.
- Metallic: cations and a sea of free electrons.
- Ionic compounds conduct molten or dissolved, not solid.
- Electronegativity: highest in fluorine.
- Shape (VSEPR) decides whether a molecule is polar.
- Hydrogen bonds: water boils at 100 °C.