Each element is given its own chemical symbol, like O for oxygen and Cl for chlorine. Chemical symbols are usually one or two letters long, but sometimes three letters are used.
Every chemical symbol starts with a capital letter, with the second or third letters written in lower case.
For example, Mg is the correct symbol for magnesium, but mg, mG and MG are wrong.
Everything is made from atoms, including you. Atoms are tiny particles that are far too small to see, even with a microscope. If people were the same size as atoms, the entire population of the world would fit into a box about a thousandth of a millimetre across!
We usually imagine atoms as being like tiny balls: