Atomic structure

Subshells, ionisation energies, mass spectrometry

Trends across period 3

• General increase: nuclear charge rises, same shell, so attraction increases.
• Drop Mg → Al (738 → 578 kJ mol⁻¹): Al's outer electron is in a 3p subshell, higher in energy than Mg's 3s — evidence for subshells.
• Drop P → S (1012 → 1000 kJ mol⁻¹): in S the 3p electrons first pair up; repulsion between the paired electrons makes one easier to remove. (Same logic as Be→B and N→O in period 2: 899→801 and 1402→1314 kJ mol⁻¹.)

Electron configuration rules

• Fill in order of increasing energy: 1s 2s 2p 3s 3p 4s before 3d.
• Exceptions: Cr is [Ar] 3d⁵ 4s¹ and Cu is [Ar] 3d¹⁰ 4s¹ — half-filled and full d-subshells are extra stable.
• When transition metals ionise, 4s electrons are lost first.

Time-of-flight mass spectrometry

• Stages: ionisation (electron impact or electrospray) → acceleration (all ions gain the same kinetic energy) → flight tube (lighter ions travel faster) → detection (ions gain electrons; current ∝ abundance).
• Relative atomic mass from spectrum: Ar = Σ(isotope mass × % abundance) ÷ 100

3.1.1.1Fundamental particles

• Mass number A = protons + neutrons; atomic number Z = protons. Isotopes: same Z, different A — identical chemistry (same electron configuration), different physical properties.
• Ions: electrons change, never protons. Ti²⁺ has 22 protons and 20 electrons.

3.1.1.3Successive ionisation energies — the evidence for shells

• Removing each successive electron costs more (same protons pulling fewer electrons).
• A large jump between the nth and (n+1)th ionisation energies means the (n+1)th electron comes from a new shell, closer to the nucleus — the element is in group n.

Extended🎓 Beyond the standard course

• Orbital shapes: s orbitals are spherical; the three p orbitals are dumbbells along x, y and z. An orbital is a region of 95% electron probability — not a track.
• Why 4s fills before 3d: the 4s orbital penetrates closer to the nucleus than 3d, so in K and Ca it is lower in energy. Once 3d is occupied, the order flips — 3d electrons shield 4s poorly — which is exactly why 4s electrons are also lost first on ionisation.
• Why Cr and Cu break Aufbau: half-filled (d⁵) and filled (d¹⁰) subshells maximise exchange energy — electrons with parallel spins in separate d orbitals repel less and are quantum-mechanically stabilised.
• Log plots of successive IEs compress the huge range and make shell breaks unmistakable — a favourite of harder data questions.
• EI vs ESI: electron-impact ionisation fragments molecules (rich fingerprint, weak M⁺); electrospray is "soft", preserving [M+H]⁺ — which is why it's used for proteins in TOF instruments.

Deep dive📚 The rest of the chapter, in full

Time-of-flight mass spectrometry, quantitatively

All four stages carry marks. Ionisation: either electron impact (a high-energy electron knocks one electron out: X(g) → X⁺(g) + e⁻ — used for low-mass samples, causes fragmentation) or electrospray (the sample gains a proton from the solvent: X(g) + H⁺ → XH⁺(g) — used for large biological molecules; remember the detected mass is M+1). Acceleration: an electric field gives every ion the same kinetic energy, so lighter ions end up faster. Flight: ions drift through a field-free tube; since KE = ½mv², the speed is v = √(2KE/m) and the flight time over distance d is t = d√(m/2KE) — time is proportional to √m. Detection: each ion gains an electron at the detector; the tiny current is proportional to abundance.

Reading mass spectra

• Aᵣ from a spectrum: Aᵣ = Σ(isotope mass × % abundance) ÷ 100. For Cl (75% ³⁵Cl, 25% ³⁷Cl): (35 × 75 + 37 × 25)/100 = 35.5.
• Diatomic traps: Cl₂ shows molecular-ion peaks at m/z 70, 72 and 74 (³⁵–³⁵, ³⁵–³⁷, ³⁷–³⁷) in a 9 : 6 : 1 ratio — a favourite multi-mark question.
• Working backwards: given Aᵣ and one abundance, set up x + y = 100 and solve the weighted mean for the unknown isotope.

Successive ionisation energies — the evidence for shells

Removing electrons one by one from the same atom gives a rising staircase with giant jumps between shells. For sodium: the 1st IE is small (lone 3s electron), then a huge jump to the 2nd (now breaking into the full n = 2 shell), then eight steadily rising values, then another leap into n = 1. Count the electrons removed before each jump to read off the group: an element whose big jump comes after the 2nd ionisation energy is in Group 2.

The two classic first-IE anomalies

• Be → B (and Mg → Al): boron's outer electron is in 2p, higher in energy than beryllium's 2s and slightly shielded by it — so it needs less energy to remove despite the greater nuclear charge.
• N → O (and P → S): oxygen is the first element to pair electrons in a 2p orbital; the paired electrons repel, making one easier to remove than nitrogen's three unpaired, half-filled-subshell electrons.

Configurations of ions

Write the atom first, then remove from the highest n first — 4s before 3d for transition metals: Fe is [Ar]3d⁶4s², Fe²⁺ is [Ar]3d⁶, Fe³⁺ is [Ar]3d⁵ (extra stability of the half-filled d-subshell explains why Fe²⁺ oxidises easily). Negative ions add electrons to the next empty orbital: O²⁻ is 1s²2s²2p⁶, isoelectronic with Ne, F⁻, Na⁺ and Mg²⁺ — but their radii differ because the nuclear charge differs.

Mastery vault🏛 Every remaining spec point, banked

The evolving model of the atom (spec 3.1.1.1)

Dalton: indivisible spheres. Thomson (1897): discovery of the electron → "plum pudding" of negative electrons in positive dough. Rutherford (1911): alpha-particle scattering — most passed straight through gold foil, a tiny fraction bounced back → mass and positive charge concentrated in a minute nucleus. Bohr: electrons in fixed energy levels, explaining line spectra. Modern quantum model: orbitals as probability regions. The examinable lesson: models are accepted, tested and replaced as evidence accumulates — name the evidence when you name the model.

Orbitals, subshells and the three filling rules

• An orbital holds at most 2 electrons of opposite spin; s-orbitals are spherical, the three p-orbitals are dumbbells along x, y, z. Subshell capacities: s 2, p 6, d 10, f 14.
• Aufbau: fill lowest energy first (note 4s fills before 3d — and empties first in ions). Hund: within a subshell, occupy orbitals singly before pairing. Pauli: no two electrons in an atom share all four quantum labels — paired electrons must have opposite spins.
• Box-diagram questions test Hund directly: nitrogen's 2p is ↑ ↑ ↑, never ↑↓ ↑ –.

First ionisation energies across Period 3 — the data

General rise (increasing nuclear charge, same shell, similar shielding) with the two dips: Al (3p above 3s) and S (first 3p pairing). Sketch questions want the zigzag shape with those two elements below the trend line, labelled with the reasons.

Mass spectrometry: the corner cases

• 2+ ions appear at half the expected m/z: ⁵⁶Fe²⁺ registers at 28. If a peak sits at half-integer m/z (e.g. 43.5), suspect a 2+ ion of an odd-mass species (87).
• Electrospray vs electron impact: electrospray adds a proton, so read Mᵣ = m/z − 1; electron impact reads Mᵣ directly from M⁺ but fragments the molecule (useful for structure, annoying for mass).
• Isotopes have identical chemical properties (same electron configuration) but different masses — hence different flight times, and slightly different physical properties (density, rate of diffusion).

Applications you can quote

• ¹⁴C dating (5730-year half-life) for archaeology; ⁶⁰Co and ⁹⁹ᵐTc in radiotherapy and imaging; ²³⁵U enrichment monitored by mass spectrometry.
• Mass spectrometers on space probes (identifying elements on Mars), drug-testing labs, and airport security — the same four-stage instrument every time.