Atoms and elements: every substance is built from atoms

1. All substances are made of atoms, whether they are solids, liquids or gases.
2. An atom is the smallest part of an element that can exist.
3. An element is a substance made from only one type of atom.
4. About 100 different elements are known, and they are set out in the periodic table.
5. Each element in the periodic table is shown by its name and its chemical symbol.

Chemical symbols: each element has its own abbreviation

1. Each element is represented by a chemical symbol made of one or two letters.
2. The first letter is always a capital; the second letter, when present, is always lower-case.
3. The symbol $O$ means one oxygen atom, while $Na$ means one sodium atom.
4. You are given a periodic table in the exam, so you can match element names to symbols.
5. The first 20 elements are $H,\; He,\; Li,\; Be,\; B,\; C,\; N,\; O,\; F,\; Ne$ then $Na,\; Mg,\; Al,\; Si,\; P,\; S,\; Cl,\; Ar,\; K,\; Ca$.
6. Other elements to recognise include the Group 1 metals $Rb,\; Cs$, the Group 7 elements $Br,\; I$, and transition metals such as $Fe,\; Cu,\; Zn,\; Ag,\; Au$.

Compounds: elements join in fixed proportions

1. A compound contains two or more different elements chemically combined in fixed proportions.
2. Fixed proportions means every sample of the compound has its elements in the same ratio.
3. A chemical formula uses symbols and numbers to show which atoms are present and in what ratio.
4. A subscript is the small lower number showing how many atoms of the element before it are present.
5. No subscript after a symbol means one atom of that element in the simplest particle.
6. $H_{2}O$ shows water has hydrogen and oxygen in a fixed ratio of $2:1$.
7. ${CO}_2$ shows carbon dioxide has carbon and oxygen in a fixed ratio of $1:2$.
8. Different proportions give different compounds, such as carbon monoxide $CO$ and carbon dioxide ${CO}_2$.

Chemical reactions: new substances form when atoms rearrange

1. A chemical reaction forms one or more new substances.
2. The starting substances are the reactants, and the substances formed are the products.
3. Elements join to form compounds through chemical reactions.
4. The products have different properties from the reactants because they are new substances.
5. Reactions often involve a detectable energy change, seen as a temperature change or the giving out of light.
6. A compound can be split into its elements only by chemical reactions, because its chemical bonds must be broken.
7. A physical method alone cannot separate a compound into its elements.

Names and formulae: read each symbol before naming the compound

1. Two-element compounds: the first element keeps its name and the second ends in -ide.
   1. $NaCl$ is sodium chloride, $MgO$ is magnesium oxide and ${CaCl}_2$ is calcium chloride.
2. Named groups: ${CaCO}_3$ is calcium carbonate, ${CuSO}_4$ is copper sulfate, ${KNO}_3$ is potassium nitrate and $NaOH$ is sodium hydroxide.
3. Common acids: $HCl$ is hydrochloric acid, $H_2{SO}_4$ is sulfuric acid and ${HNO}_3$ is nitric acid.
4. Diatomic elements exist as two-atom molecules: $H_2,N_2,O_2,F_2,{Cl}_2,{Br}_2,I_2$.
5. Other key formulae are water $H_{2}O$, carbon dioxide ${CO}_2$, methane ${CH}_4$ and ammonia ${NH}_3$.

Chemical equations: balancing keeps every atom accounted for

1. A word equation shows a reaction using the names of reactants and products.
2. The reactants go on the left of the arrow and the products on the right, separated by plus signs.
3. The word equation for magnesium burning in oxygen is:
   1. $$magnesium+oxygen\to magnesium\; oxide$$
4. A symbol equation shows the same reaction using formulae, which must be correct before balancing:
   1. $$Mg(s)+O_2(g)\to MgO(s)$$
5. A balanced equation has the same number of atoms of each element on both sides.
6. A coefficient is a large whole number before a formula that multiplies every atom in it.
7. Balance in steps: count the atoms on each side, add coefficients until they match, then check using the smallest whole numbers.
8. The balanced equation for magnesium and oxygen is:
   1. $$2Mg(s)+O_2(g)\to 2MgO(s)$$
9. This now has two magnesium atoms and two oxygen atoms on each side.
10. The state symbols $(s),(l),(g),(aq)$ mean solid, liquid, gas and dissolved in water.
11. Always balance by changing coefficients only, never by changing formulae.