Atomic Structure & Periodicity

Three subatomic particles

Protons (positive, in the nucleus), neutrons (neutral, in the nucleus), and electrons (negative, in the surrounding orbi

Atomic number (Z)

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

Mass number (A)

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

Isotopes

Atoms of the same element (same Z) that differ in neutron count and therefore in mass number A. Example: carbon-12 and c

Ions: cations and anions

A cation forms when an atom loses electrons (net positive charge); an anion forms when it gains electrons (net negative