Compounds and Molecules

Many everyday substances are built from tiny particles called atoms. How these atoms join decides whether a substance is an element, a compound, or a mixture, and whether it forms molecules.

Key definitions

• Atom: the smallest particle of an element.
• Element: a substance made of only one type of atom, e.g. oxygen, carbon.
• Molecule: two or more atoms joined together. The atoms can be the same element (e.g. oxygen gas, $\mathrm{O_2}$) or different elements (e.g. water, $\mathrm{H_2O}$).
• Compound: a substance made when atoms of different elements are chemically joined. Many compounds are made of molecules (e.g. $\mathrm{CO_2}$); some (ionic compounds like $\mathrm{NaCl}$) are not molecular.
• Covalent bond: a strong link formed when atoms share pairs of electrons to make a molecule.

Molecules: element or compound?

• Molecules of elements: $\mathrm{O_2}$, $\mathrm{N_2}$, $\mathrm{Cl_2}$ (same kind of atom joined).
• Molecules of compounds: $\mathrm{H_2O}$, $\mathrm{CO_2}$, $\mathrm{CH_4}$ (different kinds of atoms joined).

How atoms stick together

In simple molecules, atoms share electrons to fill their outer shells. A dot-and-cross diagram can show this sharing. The covalent bonds inside the molecule are strong.

Properties of simple molecular substances

• Low melting and boiling points: molecules are held to each other by weak forces between molecules, so little energy is needed to separate them.
• Usually gases or liquids at room temperature; some are soft solids.
• Poor conductors: no free electrons or ions to carry charge.

Formulas you should read

• Molecular formula: shows the number and type of atoms in one molecule, e.g. glucose $\mathrm{C_6H_{12}O_6}$.
• Empirical formula: simplest whole-number ratio of atoms, e.g. glucose $\mathrm{C_6H_{12}O_6}$ simplifies to $\mathrm{CH_2O}$.