Atoms are basic building blocks of matter, and cannot be chemically subdivided by ordinary means. The word atom is derived from the Greek word atom which means indivisible. The Greeks concluded that matter could be broken down into particles to small to be seen. Both the protons and neutrons reside in the nucleus.
Electrons reside in orbitals around the nucleus.
The chemical properties of isotopes are the same, although the physical properties of some isotopes may be different. All matter is made up of elements which are fundamental substances which cannot be broken down by chemical means. There are 92 elements that occur naturally. The elements hydrogen, carbon, nitrogen and oxygen are the elements that make up most living organisms. By the late 1800's many elements had already been discovered.