Three kinds of substance

Everything is built from atoms — the smallest part of an element that can exist. How those atoms are arranged tells you what kind of substance you have:

• Element — made of only one type of atom (e.g. copper, oxygen O₂).
• Compound — two or more types of atom chemically combined in fixed proportions (e.g. water H₂O, sodium chloride NaCl).
• Mixture — two or more elements or compounds not chemically bonded, so they keep their own properties and can be separated physically (e.g. air, sea water).

Key difference: a compound needs a chemical reaction to make or break it; a mixture can be split by physical means (filtering, distillation, etc.). A compound has a fixed formula; a mixture's proportions can vary.

Element, compound or mixture?

Tap a substance, then tap the box it belongs in.

Inside the atom

An atom has a tiny central nucleus of protons and neutrons, surrounded by electrons in shells (energy levels). Almost all the mass is in the nucleus; the nuclear radius is far smaller than the atomic radius.

You must know the relative charge and relative mass of each particle:

A neutral atom has equal numbers of protons and electrons, so the charges cancel.

Counting the particles

Two numbers describe any atom:

• Proton number (atomic number), Z — the number of protons in the nucleus. It defines which element it is.
• Nucleon number (mass number), A — the total number of protons + neutrons.

Your turn — count the neutrons

Isotopes

Isotopes are different atoms of the same element that have the same number of protons but different numbers of neutrons — so the same proton number but a different nucleon number.

Supplement Supplement: isotopes of an element have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration. Chemistry is decided by the electrons, not the neutrons.

Watch out: isotopes do not differ in protons or electrons — only in neutrons. A different proton number would mean a different element entirely.

What makes them isotopes?

Relative atomic mass A_r Supplement

Because an element is usually a mixture of isotopes, its relative atomic mass A_r is the weighted mean of the masses of its isotopes, taking their abundances into account.

Watch out: A_r is a weighted average, not a simple (35+37)/2 = 36. Because chlorine-35 is far more common, the answer is pulled down to 35.5.

Your turn — relative atomic mass Supplement

Arranging the electrons

Electrons fill shells from the inside out. For the first 20 elements: the 1st shell holds up to 2, the 2nd up to 8, the 3rd up to 8 (for these elements). We write the arrangement as numbers, e.g. sodium = 2,8,1.

The configuration links straight to the Periodic Table:

• The number of outer-shell electrons = the Group number (Groups I–VII). Sodium has 1 outer electron → Group I.
• The number of occupied shells = the Period number. Sodium uses 3 shells → Period 3.
• Supplement Group VIII (0) noble gases have a full outer shell (2 or 8), which makes them unreactive.

Read the configuration

Ionic bonding — transfer of electrons

An ion is a charged atom (or group of atoms). Metals lose outer electrons to form positive ions (cations); non-metals gain electrons to form negative ions (anions). Both reach a stable full outer shell.

An ionic bond is the strong electrostatic attraction between these oppositely charged ions, formed by the transfer of electrons from metal to non-metal.

Watch out: ionic bonding is transfer of electrons (metal → non-metal); covalent bonding (next) is sharing. Don't mix them up.

Ionic lattices and their properties

Ionic compounds form a giant ionic lattice — a regular 3D arrangement of alternating positive and negative ions held by strong electrostatic forces in every direction.

• High melting and boiling points — lots of energy is needed to overcome the many strong electrostatic forces.
• Conduct electricity when molten or in solution, but not when solid — the ions must be free to move to carry charge.
• Often soluble in water and brittle.

Predict the bonding

Covalent bonding — sharing electrons

A covalent bond forms when two non-metal atoms share a pair of electrons. Sharing lets both atoms reach a full outer shell (a noble-gas arrangement). A molecule is a small group of atoms held together this way.

Properties of simple molecules

Substances made of simple molecules (e.g. H₂O, CO₂, Cl₂) have:

• Low melting and boiling points — the covalent bonds inside molecules are strong, but the forces between molecules (intermolecular forces) are weak and need little energy to overcome.
• Do not conduct electricity — there are no free ions or delocalised electrons.

Common slip: melting a simple molecular substance breaks the weak intermolecular forces, not the strong covalent bonds. That's why mp/bp are low even though the bonds themselves are strong.

Explain the property

Giant covalent structures Supplement

Some covalent substances form a giant covalent (macromolecular) structure — a huge lattice of atoms all joined by strong covalent bonds. You must know diamond, graphite and silicon(IV) oxide.

• Diamond — each carbon forms 4 covalent bonds. Very hard, very high melting point; does not conduct (no free electrons). Used in cutting tools.
• Graphite — each carbon forms only 3 bonds, leaving layers of hexagons with weak forces between them. Layers slide (lubricant). One delocalised electron per carbon means it conducts electricity.
• Silicon(IV) oxide, SiO₂ — a diamond-like giant lattice; very hard, very high melting point, does not conduct.

Why high mp? Melting a giant covalent structure means breaking many strong covalent bonds throughout the lattice — so the melting points are very high (unlike simple molecules).

Why graphite conducts Supplement

Metallic bonding Supplement

Metallic bonding is the electrostatic attraction between a lattice of positive metal ions and a 'sea' of delocalised electrons. The outer electrons leave their atoms and are free to move throughout the whole structure.

• Good conductors of electricity and heat — the delocalised electrons are free to move and carry charge/energy.
• Malleable and ductile — layers of ions can slide over each other without breaking the bonding.
• High melting points — strong attraction between ions and the electron sea.

Name the bonding

Tap the bonding type that holds each substance together.

Structure → property

Tap a structure on the left, then its matching property on the right.

The big ideas of Topic 2

You've covered all of Cambridge IGCSE 0620 Topic 2 — Core plus the Supplement extension items. Press Finish to see your score.