Some materials are made of more than one element combined together. When sodium and chlorine combine together they form white crystals of sodium chloride, known as common salt. There are about 100 elements all together. Other examples of elements are hydrogen, copper, oxygen and iron. All materials, whether they contain one element or more than one element are made up of very small particles called atoms.
Atoms are 'building blocks' which can be joined together.
These blocks are so small, you can't even see them with a lab microscope. But, there are a few, very special and very expensive microscopes in the world which can look at atoms. This blurry picture, taken using one of them, is the clearest view anyone has ever had of an atom.