The atom can be thought of as a system containing a positively charged nucleus and negatively charged electrons which are in orbit around the nucleus. The nucleus is the central core of the atom and is composed of two types of particles, protons which are positively charged and neutrons which have a neutral charge.
Electrons surround the nucleus in orbitals of various energies.
Electrons are very light compared to protons and neutrons. Each electron has a mass of approximately 5.5E-4 amu. The A number is equal to the total number of protons and neutrons in the nucleus. Nuclides with the same number of protons but with different numbers of neutrons are called isotopes. Radioactive nuclides can generally be described as those which have an excess or deficiency of neutrons in the nucleus.