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LearnCambridge06202026–2028Papers 1, 3 · CoreAtoms, elements and compounds

Papers 1, 3 · Module

Atoms, elements and compounds

5 subtopics≈ 175 min in total

Start the module: Elements, compounds and mixtures Open graphing calculator

Elements, compounds and mixtures
An element contains only one type of atom. A compound contains two or more elements chemically bonded in a fixed ratio, with properties different from its elements. A mixture contains two or more substances not chemically joined; they can be separated by physical methods. A pure substance melts and boils at a sharp, fixed temperature; impurities make it melt over a range and at a lower temperature.
≈ 25 min
Atomic structure and isotopes
An atom has a tiny nucleus of protons (charge $+1$ , mass $1$ ) and neutrons (charge $0$ , mass $1$ ), surrounded by electrons (charge $-1$ , mass about $\frac1{1840}$ ) in shells. The proton number $Z$ identifies the element; the mass number $A$ = protons + neutrons. Electrons fill shells 2, 8, 8, … . Isotopes are atoms of the same element with different numbers of neutrons; relative atomic mass is the abundance-weighted mean of their masses.
≈ 40 min
Ions and ionic bonding
Metal atoms lose outer electrons to form positive ions; non-metal atoms gain electrons to form negative ions, each reaching a full outer shell. An ionic bond is the strong electrostatic attraction between oppositely charged ions. Ionic compounds form giant lattices, so they have high melting points, and they conduct electricity only when molten or dissolved, because then the ions can move.
≈ 35 min
Simple molecules and covalent bonds
When non-metal atoms bond, they share pairs of electrons to complete their outer shells. A shared pair is a covalent bond: strong. Simple molecules (like $\mathrm{H_2O}$ , $\mathrm{CO_2}$ , $\mathrm{CH_4}$ ) are held to each other only by weak intermolecular forces, so they have low melting and boiling points, and they don't conduct electricity because they have no charged particles free to move.
≈ 35 min
Giant structures and metallic bonding
In giant covalent structures every atom is covalently bonded into one huge network. Diamond (each C bonded to 4 others) is extremely hard with a very high melting point. Graphite has layers of C bonded to 3 others, with delocalised electrons, so it conducts and its layers slide. Silicon(IV) oxide is like diamond. In metals, positive ions sit in a sea of delocalised electrons: they conduct, are malleable and have high melting points.
≈ 40 min