When we pick up a rock it seems heavy for its size, how heavy an object feels has to do with how much and what kind of stuff it contains, which is called its density. — British spelling. Atoms are the smallest particles of chemical elements that can exist and are the basic building blocks of everything that we can see, touch or smell. Apart from a hydrogen atom which has only one proton at its centre and one spinning electron, all other individual atoms contain various amounts of protons, neutrons, and electrons.