Every atom has a nucleus which is composed of elementary particles, protons and neutrons. The protons possess a positive charge while neutrons have no electrical charge. Orbiting the nucleus are very small negatively charged particles called electrons.
Neutral atoms have the same number of positively charged protons as negatively charged electrons.
Atoms differ from each other in several ways. Elements are distinguished by the number of protons in the nucleus. Each element possesses physical and chemical properties which are unique to that element. These elemental variations are called isotopes. They cannot be distinguished from other isotopes of the same element by chemical analysis since they react identically.