Answer:
Atoms consist of a nucleus made of protons and neutrons orbited by electrons.
Isotopes are atoms that have same number of protons but they differ in the number of neutrons and in atomic mass.
Atomic number Bi = 83
electron configuration Bi : 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p3
p orbitals = 3p6, 4p6, 5p6 and 6p3
so, 6 + 6 + 6 + 3 = 21 electrons
