CHEM1011 · Chemistry 1a
Atomic Structure and Periodic Trends
Week 2 builds the electronic structure of atoms — the four quantum numbers, orbital shapes and ground-state electron configurations written with the Aufbau principle, Hund's rule and the Pauli exclusion principle — then uses effective nuclear charge and shielding to rationalise periodic trends. Configurations, ion configurations and trend-ranking questions are staple Threshold-quiz and in-term-test items, while the deeper 'explain the trend from Z_eff' reasoning extends into the final exam.
What this chapter covers
- 01The four quantum numbers: n (size/energy), ℓ (shape, 0…n−1 = s/p/d/f), mₗ (orientation, −ℓ…+ℓ), mₛ (spin ±½)
- 02Orbitals per level = n², maximum electrons = 2n²; number of orbitals in a subshell = 2ℓ + 1
- 03Orbital shapes: s spherical, p dumbbell (three), d cloverleaf (five); nodal surfaces increase with n
- 04Electron configurations by Aufbau order (1s 2s 2p 3s 3p 4s 3d 4p…); Hund's rule and Pauli in arrows-in-boxes
- 05Ion configurations: remove the highest-n (4s before 3d) electrons first for cations; the Cr and Cu anomalies
- 06Effective nuclear charge Z_eff = Z − S and shielding as the engine behind every trend
- 07Atomic and ionic radius, ionisation energy, electron affinity and electronegativity across periods and down groups
- 08Trend anomalies: the Mg→Al and P→S ionisation-energy dips, from subshell energy and pairing
Electron configuration of an ion and its magnetism
- +1Neutral Fe (Z = 26) by the Aufbau order, filling 4s before 3d: [Ar]4s²3d⁶ (the [Ar] core accounts for 18 electrons, leaving 8 in 4s and 3d).
- +1Form Fe³⁺ by removing 3 electrons. For cations, remove the highest principal-quantum-number electrons first: take both 4s electrons, then one 3d electron.
- +1That leaves [Ar]3d⁵ for Fe³⁺ (18 + 5 = 23 electrons, correct for a 26 − 3 = 23-electron ion).
- +1The five 3d electrons occupy the five degenerate d orbitals singly with parallel spins (Hund's rule), giving 5 unpaired electrons — so Fe³⁺ is paramagnetic.
Key terms
- Quantum numbers (n, ℓ, mₗ, mₛ)
- The four indices that label an electron: n (principal, size/energy), ℓ (angular, shape; 0…n−1 for s/p/d/f), mₗ (magnetic, orientation; −ℓ…+ℓ) and mₛ (spin, +½ or −½). No two electrons in an atom share all four (Pauli).
- Aufbau principle
- Fill orbitals from lowest energy upward (1s 2s 2p 3s 3p 4s 3d 4p…), placing electrons in the lowest available orbital first.
- Hund's rule
- Within a set of degenerate orbitals, place one electron in each with parallel spins before any pairing, maximising the number of unpaired electrons.
- Effective nuclear charge (Z_eff)
- The net positive charge felt by a valence electron, Z_eff = Z − S, where S is the shielding by inner electrons. Rising Z_eff across a period pulls electrons in and drives the periodic trends.
- Ionisation energy
- The energy to remove an electron from a gas-phase atom or ion; it rises across a period and falls down a group, with dips at Mg→Al (3p vs 3s) and P→S (pairing in 3p⁴).
- Paramagnetic / diamagnetic
- Paramagnetic species have unpaired electrons and are drawn into a magnetic field; diamagnetic species have all electrons paired and are weakly repelled.
Atomic Structure and Periodic Trends FAQ
Why do I remove 4s electrons before 3d when making a cation, even though 4s filled first?
Once the 3d orbitals are occupied, they drop below 4s in energy, so the 4s electrons become the outermost and highest-energy ones — and those leave first when the atom ionises. That is why Fe = [Ar]4s²3d⁶ becomes Fe²⁺ = [Ar]3d⁶ and Fe³⁺ = [Ar]3d⁵. Filling order and removal order are genuinely different; mixing them up is a common in-term-test slip.
What causes the dips in ionisation energy at aluminium and sulfur?
Both come from subshell structure. Mg→Al: aluminium's outer electron is in a higher-energy 3p orbital (vs magnesium's 3s), so it is easier to remove and IE dips. P→S: phosphorus has a stable half-filled 3p³, whereas sulfur's 3p⁴ has one paired electron whose electron–electron repulsion makes it easier to remove — so IE dips again. These are the standard 'anomalies' the exam asks you to explain from Z_eff and orbital occupancy.
How do effective nuclear charge and shielding explain the trends?
Valence electrons are shielded from the full nuclear charge by the core, so they feel Z_eff = Z − S. Across a period Z rises but shielding barely changes, so Z_eff climbs — atoms shrink and ionisation energy, electron affinity and electronegativity rise. Down a group a new shell is added, the valence electrons sit further out and are better shielded, so atoms grow and ionisation energy falls. Almost every trend question reduces to a Z_eff argument.
Can Sia help me with electron configurations and periodic trends?
Yes. Sia can write a configuration step by step, handle the Cr and Cu half/full-d anomalies, form the correct ion by removing the right electrons, and count unpaired electrons for a magnetism call. It can also walk a trend-ranking question through the Z_eff and shielding logic. It explains the method and checks your reasoning; it does not do graded assessment, and UNSW academic-integrity rules apply.
Exam move
Get configurations automatic first: learn the Aufbau fill order, write neutral atoms in noble-gas shorthand, and practise the two habits the exam tests — removing 4s before 3d for cations and handling the Cr ([Ar]4s¹3d⁵) and Cu ([Ar]4s¹3d¹⁰) anomalies. Pair every configuration with an arrows-in-boxes sketch so Hund's rule and unpaired-electron counts (and therefore paramagnetism) are second nature. For periodic trends, do not memorise the arrows — derive them each time from Z_eff = Z − S and shielding, because the same reasoning answers radius, ionisation energy, electron affinity and electronegativity, and it is what earns the marks in the 'explain' questions. Keep the two ionisation-energy dips (Mg→Al, P→S) ready with their one-line explanations. Rehearse a few 'which element am I?' clue chains, since these bundle several trends into one item. When a configuration or trend won't click, ask Sia to re-derive it a different way.
Working through Atomic Structure and Periodic Trends in CHEM1011? Sia is AskSia’s AI Chemistry tutor — ask any CHEM1011 Atomic Structure and Periodic Trends question and get a clear, step-by-step explanation grounded in how CHEM1011 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
