An atom is the smallest part of any element that can exist alone or combined with other particles. Accordingly, all atoms are elements, and there are more than a hundred natural elements. Accordingly, there are more than a hundred natural atoms. Atoms are the particles of matter that make matter what it is.
It is natural, then, to assume that atoms are the basic and primary units of all matter in the universe, and for more than a hundred years they have actually been called that. They were referred to as "the building blocks of matter." It was thought that it was not fissionable. But if the atom is not divided, then why do the elements differ so greatly from each other? Why is gold different from oxygen, and platinum from chlorine? Why do we find that lead is heavy and hydrogen is light? If all atoms are one, then it is certain that the elements must be one, but we know with certainty that they are not.