Atoms, Elements and Compounds

All substances are made of atoms. An atom is the smallest part of an element that can exist. You cannot see a single atom, but everything around you is built from them.

An element is a substance made of only one type of atom. There are about 100 elements, and they are all shown in the periodic table. Each element has its own chemical symbol of one or two letters, for example O for oxygen, Na for sodium and Fe for iron.

The first letter of a symbol is always a capital and any second letter is lower case. This matters: CO means one carbon atom joined to one oxygen atom, but Co is the single element cobalt.

Some of these symbols look nothing like the English name because they come from Latin. Sodium is Na (from natrium), potassium is K (from kalium) and iron is Fe (from ferrum). You are given a periodic table in the exam, so a symbol can be looked up, but you do need to read and write them correctly.

The specification requires you to use the names and symbols of the first 20 elements, together with the elements in Group 1 and Group 7. Learning these makes reading formulae and equations far faster.

The first 20 elements, in order of atomic number:

Number	Element	Symbol	Number	Element	Symbol
1	Hydrogen	H	11	Sodium	Na
2	Helium	He	12	Magnesium	Mg
3	Lithium	Li	13	Aluminium	Al
4	Beryllium	Be	14	Silicon	Si
5	Boron	B	15	Phosphorus	P
6	Carbon	C	16	Sulfur	S
7	Nitrogen	N	17	Chlorine	Cl
8	Oxygen	O	18	Argon	Ar
9	Fluorine	F	19	Potassium	K
10	Neon	Ne	20	Calcium	Ca

Group 1 (the alkali metals) runs lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs) and francium (Fr). Group 7 (the halogens) runs fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). The first three of each group appear in the table above; the rest are worth knowing by symbol.

A compound contains two or more elements chemically combined in fixed proportions. Water is always two hydrogen atoms to one oxygen atom, written H₂O, wherever the water comes from.

Compounds are represented by formulae. The small subscript number tells you how many atoms of the symbol immediately before it are present:

Compound	Formula	Atoms in one unit
Water	H₂O	2 hydrogen, 1 oxygen
Carbon dioxide	CO₂	1 carbon, 2 oxygen
Sodium chloride	NaCl	1 sodium, 1 chlorine
Calcium carbonate	CaCO₃	1 calcium, 1 carbon, 3 oxygen
Ammonia	NH₃	1 nitrogen, 3 hydrogen

A compound has different properties from the elements that make it. Sodium is a soft, reactive metal and chlorine is a poisonous green gas, yet sodium chloride is ordinary table salt. The elements in a compound can only be separated by chemical reactions, not by physical methods such as filtering.

Naming rule: a compound of two elements ending in a metal-then-non-metal usually ends in -ide (sodium chloride, magnesium oxide). If oxygen is also present as part of the group, the name often ends in -ate (calcium carbonate, copper sulfate).

A chemical reaction always forms one or more new substances, and it often involves a detectable energy change (a temperature rise or fall, light, or sound). The starting substances are the reactants and the new substances made are the products.

A word equation names the reactants and products with an arrow meaning "turns into":

$\text{reactants}\to \text{products}$

For example, when magnesium burns in oxygen:

$\text{magnesium}+\text{oxygen}\to \text{magnesium oxide}$

The + between two reactants means "reacts with", and the + between two products means "and". The arrow only ever points from reactants to products in a normal reaction; you do not write it the other way round.

Word equations are quick to write and score well when a question does not ask for symbols. They are also a sensible first step before writing a symbol equation, because they force you to identify every reactant and product first.

A symbol equation replaces the names with formulae and must be balanced: the same number of each type of atom must appear on both sides, because atoms are never created or destroyed in a reaction.

You balance by placing a large number in front of a formula (a multiplier). You must never change a subscript inside a formula, because that would change the substance itself.

Worked example — magnesium burning in oxygen. Start with the unbalanced version:

$\text{Mg}+{\text{O}}_2\to \text{MgO}$

Count the atoms:

Atom	Left	Right
Mg	1	1
O	2	1

Oxygen is unbalanced. Put a 2 in front of MgO to give 2 oxygen atoms on the right:

$\text{Mg}+{\text{O}}_2\to 2\text{MgO}$

Now magnesium is 1 on the left and 2 on the right, so put a 2 in front of Mg:

$2\text{Mg}+{\text{O}}_2\to 2\text{MgO}$

Final check: Mg is 2 and 2; O is 2 and 2. Balanced.

(Higher Tier only) Writing balanced half equations and ionic equations is assessed only at Higher Tier.

Some reactions are shown more clearly using ions (charged atoms) rather than whole compounds. An ionic equation shows only the particles that actually change, leaving out ions that appear unchanged on both sides.

A half equation shows what happens to electrons at one part of a reaction. Electrons are written as e⁻. For example, when a sodium atom becomes a sodium ion it loses one electron:

$\text{Na}\to {\text{Na}}^{+}+{\text{e}}^{-}$

A half equation must balance for both atoms and charge. On the left the charge is 0; on the right it is $(+1)+(-1)=0$, so charge balances. When chlorine gains electrons to form chloride ions:

${\text{Cl}}_2+2{\text{e}}^{-}\to 2{\text{Cl}}^{-}$

Here 2 chlorine atoms are balanced on each side, and the charge is $-2$ on both sides. Combining these ideas, the ionic equation for neutralisation (met later) is simply ${\text{H}}^{+}+{\text{OH}}^{-}\to {\text{H}}_2\text{O}$.

1. Changing a formula to balance an equation

You balance only by adding large numbers in front of formulae. Turning H₂O into H₂O₂ balances the oxygen but describes hydrogen peroxide, a different substance. Adjust multipliers, never subscripts.

2. Writing symbols with the wrong case

A second letter in a symbol is always lower case. Writing CO for cobalt or NA for sodium is wrong: cobalt is Co and sodium is Na. Case changes the meaning entirely.

3. Saying a compound "is a mixture" of its elements

A compound is chemically combined in fixed proportions and has new properties; the elements cannot be separated by physical means. A mixture is not chemically combined. Do not describe sodium chloride as "a mixture of sodium and chlorine".

4. Miscounting atoms inside brackets

In a formula such as Ca(OH)₂ the subscript 2 multiplies everything in the brackets, giving 1 calcium, 2 oxygen and 2 hydrogen. Multiply the whole bracket, not just the last symbol.

5. Forgetting the final atom count

After balancing, recount every element on both sides. Many marks are lost by balancing one element and disturbing another without checking, so a quick atom table before you write the answer down catches it.