All substances are made of atoms. An atom is the smallest part of an element that can exist. An element is made of just one type of atom, and there are about 100 different elements, all shown in the periodic table. Each element has a chemical symbol of one or two letters, for example O stands for oxygen and Na stands for sodium.
In a chemical reaction, atoms of different elements join up to make a compound. A compound contains two or more elements chemically combined in fixed proportions. Carbon dioxide, CO2, always has 2 oxygen atoms for every carbon atom, so 2 carbon atoms are always found with 4 oxygen atoms. Its formula is written using the symbols of the elements it was made from. A compound can only be broken back down into its elements by another chemical reaction.
Every chemical reaction forms one or more new substances and often has a detectable energy change, such as the temperature rising. A reaction can be shown as a word equation, such as hydrogen + oxygen → water, or as a symbol equation, such as 2H2 + O2 → 2H2O. In the exam you are given a periodic table, but you should know the symbols of the first 20 elements and of the elements in Groups 1 and 7.