atoms do not combine with any other atoms; the substance is described as ‘monatomic’.

The forces between helium atoms are very so this substance is a at room conditions.

Chlorine atoms combine with each other to give a ‘diatomic’ . One chlorine atom links with another by overlapping and sharing one pair of in their outer ; this forms a covalent bond and covalent bonds are very . The force of attraction between one Cl₂ molecule and the next is very so chlorine exists as a at room conditions.

Magnesium atoms link to other magnesium atoms through bonding which arises from outer escaping from atomic orbits and flowing over all the neighbouring atoms; these moving electrons are described as ‘’ because they are not ‘local’ to the atom. Metallic bonding is so all metals (apart from ) are at room conditions. Metal atoms can slide over each other, so metals are and .

Diamond and graphite structures are both composed of pure atoms. In graphite each C atom is bound to others; this gives a structure of repeating in planes. Between the planes of atoms there is a layer of . In diamond each carbon atom is joined by bonds to four others in a tetrahedral arrangement to form a repeating structure.

All the above substances are . Those below are .

forms strong covalent bonds between a hydrogen atom and an oxygen atom within a ; these are not broken when water changes state (eg melting or boiling). There are much forces between one water molecule and the next which are relatively easily overcome, so water is a liquid at room conditions.

   Sodium chloride is . attraction between Na⁺ and Cl^- is very indeed. Common salt exists as a at room conditions.

(sand; formula SiO₂) has a repeating structure like diamond where every atom is linked by bonds to others. Like diamond, sand is exceptionally and has a very melting point.

Substances with repeating structures made of strong bonds will be at room conditions and are described as ‘’. Substances which have forces between particles will melt and boil easily, so most are gases at room conditions, though water is liquid and there are some low melting point solids in the ‘’ category (eg sulphur).

Substances will conduct if they have charged particles which are free to move. Metals and graphite conduct electricity through . Ionic solids cannot conduct because the ions are held in a , but always conduct when or in .