General Chemistry

Atomic number Z = number of protons (defines the element); mass number A = protons + neutrons; neutrons = A − Z. Isotopes share Z but differ in A (different neutron count).

Isotopes are atoms of the same element (same number of protons, same Z) that differ in their number of neutrons, and therefore in mass number A. They have nearly identical chemical behavior but different masses.

Average atomic mass = Σ(fractional abundance × isotopic mass). = (0.7577)(34.97) + (0.2423)(36.97) = 26.50 + 8.96 = 35.45 u. This matches the periodic-table value of ~35.45 u for chlorine.

n (= 1, 2, 3, …) specifies the principal energy level (shell) and the average size/energy of the orbital. Larger n means higher energy and greater average distance from the nucleus. The maximum electrons in a shell is 2n².

l specifies the subshell/orbital shape. It ranges from 0 to n − 1. l = 0 → s, l = 1 → p, l = 2 → d, l = 3 → f. The number of orbitals in a subshell is 2l + 1.

m_l specifies the spatial orientation of an orbital within a subshell. It ranges in integer steps from −l to +l, giving 2l + 1 values (= number of orbitals in that subshell). E.g., for a p subshell (l = 1): m_l = −1, 0, +1 → three p orbitals.