Atomic Structure and the Periodic Table
This domain tests whether you can move fluently between the nuclear model, electron configuration and an element's position in the periodic table, and use that model to explain trends in ionisation energy, atomic radius and chemical behaviour.
Subatomic particles and isotopes
Atomic number Z is the number of protons and fixes the element. Mass number A = protons + neutrons. Isotopes have identical chemistry but different masses; a mass spectrum gives m/z peaks whose intensities are abundances, so Ar = Σ(abundance × isotopic mass)/Σ(abundance).
Electron configuration
Fill in order of increasing energy: 1s 2s 2p 3s 3p 4s 3d 4p. Apply the Pauli exclusion principle (maximum two electrons per orbital, opposite spins) and Hund's rule (singly occupy degenerate orbitals before pairing). Chromium is [Ar]3d5 4s1 and copper [Ar]3d10 4s1, because half-filled and filled d subshells gain extra exchange stability. When a transition metal ionises, the 4s electrons are removed first, so Fe2+ is [Ar]3d6.
Periodic trends and their evidence
Atomic radius decreases across a period (effective nuclear charge rises with similar shielding) and increases down a group (an extra shell). First ionisation energy rises across a period but dips at Group 13 (Al: 3p1 is higher in energy and better shielded than 3s) and at Group 16 (S: the 3p4 pair suffers electron–electron repulsion). Successive ionisation energies show large jumps at shell boundaries; the number of small steps before the jump identifies the group. Electronegativity rises across a period and falls down a group; metallic character follows the opposite trend.
Worked example
Magnesium consists of 79.0% 24Mg, 10.0% 25Mg and 11.0% 26Mg. Ar = (79.0 × 24 + 10.0 × 25 + 11.0 × 26)/100 = (1896 + 250 + 286)/100 = 24.32. Magnesium therefore appears as 24.3 in the periodic table, between the masses of its two main isotopes.
Traps that cost marks
- Writing 4s before 3d is correct for filling order, but 3d must be listed first when you write the final configuration, and 4s electrons are lost first on ionisation.
- Claiming ionisation energy rises smoothly across Period 3; you must name the Group 13 and Group 16 anomalies.
- Confusing mass number with relative atomic mass, which is a weighted average and not an integer.
- Describing isotopes as chemically different; their chemistry is essentially identical.