Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in energy levels (shells).
Proton: +1 charge, mass 1
Neutron: 0 charge, mass 1
Electron: −1 charge, negligible mass
Atomic number = number of protons
Mass number = protons + neutrons
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Electron configuration fills shells in order: 2, 8, 8. The periodic table arranges elements by atomic number. The group number tells you the number of outer electrons, and the period tells you the number of electron shells.
Relative atomic mass can be calculated from isotope abundances using the formula:
Ar = ((mass of isotope 1 × abundance) + (mass of isotope 2 × abundance)) / 100
For example, Chlorine has two common isotopes: Chlorine-35 (75% abundance) and Chlorine-37 (25% abundance).
Ar = ((35 × 75) + (37 × 25)) / 100 = (2625 + 925) / 100 = 3550100 = 35.5