The smallest part of an element that can exist chemically.
Atoms consist of a small, dense nucleus of protons and neutrons surrounded by moving electrons.
A substance that can't be decomposed into simpler substances. Compound A substance formed by the combination of different elements in fixed proportions. Proton An elementry particle that is stable, bears a positive charge equal in magnitude to that of an electron, and has a mass of 1.672614 x 10 -27 kg. A neutral particle that is stable in the atomic nucleus but decays into a proton, an electron, and an antineutrino with an average half life of 6 minutes outside the nucleus. Isotope One of two or more atoms of the same element that have the same number of protons in their nucleus but different numbers of neutrons.