Every element has a chemical symbol of one or two letters. The first letter is a capital and the second, if there is one, is lower case: C for carbon, Na for sodium and Cl for chlorine. A formula uses these symbols to show which elements are in a substance and how many atoms or ions of each there are. A small number written after a symbol applies only to that symbol. If there is no number, there is one atom.
Most elements are written as just their symbol, for example Fe, Mg, C and the noble gases He, Ne and Ar. Some elements exist as diatomic molecules, with two atoms joined together. These are hydrogen H2, nitrogen N2, oxygen O2 and the Group 7 elements such as fluorine F2 and chlorine Cl2.
In a simple covalent compound the formula shows the number of atoms of each element in one molecule. Water is H2O, so a molecule has two hydrogen atoms and one oxygen atom. Carbon dioxide is CO2, with one carbon atom and two oxygen atoms. Methane is CH4, with one carbon atom and four hydrogen atoms, so 5 atoms in total.
An ionic compound is a giant lattice of ions, not separate molecules. Its formula shows the simplest whole-number ratio of ions. In NaCl there is one sodium ion for every chloride ion. In MgCl2 there are two chloride ions for every magnesium ion.