Chapter 2. Atoms and Atomic Structure

Atoms consist of protons (+1), neutrons (0) and electrons (−1). Protons and neutrons sit in the nucleus; electrons occupy the surrounding space [1].

Atomic number (Z): number of protons; it defines the element.

Mass number (A): protons + neutrons.

Isotopes: atoms of one element with different neutron numbers.

Ions: charged atoms formed by gaining (anions) or losing (cations) electrons.

Quantum model. Electrons occupy orbitals described by four quantum numbers: n (energy level), l (subshell shape: s, p, d, f), mₗ (orbital orientation) and mₛ (spin) [2][4].

Rules for filling orbitals.

Aufbau principle: fill lowest-energy orbitals first.

Pauli exclusion principle: no two electrons share all four quantum numbers.

Hund’s rule: fill degenerate orbitals singly before pairing.

Example: oxygen (Z = 8) is 1s² 2s² 2p⁴. Chromium and copper are well-known exceptions (Cr: [Ar] 4s¹ 3d⁵) [3].

Atomic mass is the weighted average of isotope masses. Example: chlorine is about 75.8% ³⁵Cl and 24.2% ³⁷Cl, giving roughly 35.45 u.

Review: Write the electron configuration of Fe³⁺.