Atomic Structure & Periodicity

Protons (positive, in the nucleus), neutrons (neutral, in the nucleus), and electrons (negative, in the surrounding orbitals). Protons and neutrons supply nearly all of the atom's mass.

The number of protons in an atom's nucleus. It defines the element's identity and equals the number of electrons in a neutral atom.

The total count of protons plus neutrons in the nucleus: A = Z + N. It is always a whole number for a specific isotope.

Atoms of the same element (same Z) that differ in neutron count and therefore in mass number A. Example: carbon-12 and carbon-14 both have 6 protons but 6 and 8 neutrons.

A cation forms when an atom loses electrons (net positive charge); an anion forms when it gains electrons (net negative charge). Charge = protons − electrons.

The weighted average of an element's isotope masses, weighted by their natural abundances. This is the value shown on the periodic table (in amu or u).