Atomic structure and the periodic table
The Catalonia PAU chemistry exam tests atomic structure through quantum-number sets, electron configurations, isotope calculations and the reasoning behind periodic trends. Questions are usually short, but every answer must be justified with the model - a bare number earns little credit.
Quantum numbers and orbitals
The four quantum numbers fix an electron unambiguously: n = 1, 2, 3… (shell); l = 0…n−1 (subshell s, p, d, f); m_l = −l…+l (orbital orientation); m_s = ±½ (spin). Pauli's exclusion principle forbids identical sets, so an orbital holds two electrons and a subshell holds 2(2l+1). Shapes: s spherical, p dumbbell with three orientations, d with five.
Electron configuration
Fill by Aufbau (1s 2s 2p 3s 3p 4s 3d 4p…) and apply Hund's rule to degenerate orbitals. Memorise the two anomalies: Cr = [Ar]3d⁵4s¹ and Cu = [Ar]3d¹⁰4s¹, because half-filled and full d subshells are extra stable. For cations, remove electrons from the highest-n orbital first: Fe is [Ar]3d⁶4s², so Fe²⁺ is [Ar]3d⁶, not 3d⁴4s².
Periodic trends
Across a period Z_eff rises, so atomic radius falls while first ionisation energy, electron affinity and electronegativity rise. Down a group n increases, so radius rises and ionisation energy falls. Note the exceptions IE(Be) > IE(B) and IE(N) > IE(O), caused by a filled 2s or half-filled 2p subshell.
Worked example
Natural chlorine is ³⁵Cl (75.77%) and ³⁷Cl (24.23%). Average atomic mass = 35(0.7577) + 37(0.2423) = 26.52 + 8.97 = 35.49 u, matching the periodic table. The average is weighted by natural abundance; it is not the simple mean of 36 u, and mass number is not atomic mass.
Traps that cost marks
- Writing 3d before 4s when filling, or forgetting that 4s empties first when transition metals ionise.
- Quoting a trend without cause - always name Z_eff or shielding.
- Confusing mass number (protons + neutrons) with relative atomic mass.
- Giving only three quantum numbers, or allowing m_l outside −l…+l.