Atomic Structure
Orbitals, electron configurations and ionisation-energy evidence.
Before you begin
Orbitals, electron configurations and ionisation-energy evidence.
Learning goals
- Subatomic Particles, Isotopes, Ions
- Atomic Orbitals: Energies and Shapes
- Orbitals and Electron Configuration
- First Ionisation Energy Trends
- Successive Ionisation Energies
Syllabus statements covered
- identify and describe protons, neutrons and electrons in terms of their relative charges and relative masses
- deduce the behaviour of beams of protons, neutrons and electrons in an electric field
- describe the distribution of mass and charges within an atom
- deduce the numbers of protons, neutrons and electrons present in both atoms and ions given proton and nucleon numbers (and charge)
- describe the contribution of protons and neutrons to atomic nuclei in terms of proton number and nucleon number
- distinguish between isotopes on the basis of different numbers of neutrons present
- describe the number and relative energies of the s, p and d orbitals for the principal quantum numbers 1, 2 and 3 and also the 4s and 4p orbitals
- describe the shapes of s, p and d orbitals (see also Section 13) [knowledge of wave functions is not required]
- state the electronic configuration of atoms and ions given the proton number (and charge)
- explain the factors influencing the ionisation energies of elements (see the Data Booklet) (see also Section 5)
- deduce the electronic configurations of elements from successive ionisation energy data
- interpret successive ionisation energy data of an element in terms of the position of that element within the Periodic Table
Lessons
Work through them in order.
- Subatomic Particles, Isotopes, IonsUse particle properties, electric-field deflection and nuclide notation to describe atoms, isotopes and ions.
- Atomic Orbitals: Energies and ShapesCompare s, p and d orbitals by number, energy and shape, including the order of 4s and 3d.
- Orbitals and Electron ConfigurationWrite electron configurations for atoms and ions, including chromium, copper and transition-metal ions.
- First Ionisation Energy TrendsExplain first ionisation energy trends and their anomalies across periods and down groups.
- Successive Ionisation EnergiesDeduce valence electrons and group from the jumps in successive ionisation energies.
Practise and check
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Topic reference
Atomic structure links particle evidence to electron arrangements and ionisation-energy data. The lessons go from particles to orbitals and electron configurations, then to ionisation energies.
Be comfortable with:
- Atoms: subatomic particles, charges and basic structure.
- Elements and Isotopes: nuclide symbols and what an isotope is.
- Electron Configuration: shells and simple configurations.
- The Periodic Table: groups, periods and basic trend language.
Quick Reference
| Item | What to remember (exam-safe) |
|---|---|
| neutrons | A - Z |
| electrons (ion) | Xⁿ⁺: electrons = Z-n; Xⁿ⁻: electrons = Z + n (e.g. Cl⁻ has 17 + 1 = 18 electrons) |
| orbitals per subshell | s = 1, p = 3, d = 5 |
| max electrons per subshell | s = 2, p = 6, d = 10 |
| key energy order (to 4p) | 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p |
| Period 3 IE anomalies | Mg → Al: 3s vs higher-energy 3p; P → S: paired 3p electron repulsion |
| Trend driver chain | nuclear charge / shielding / distance → attraction to outer electron → ionisation energy |
What You Must Memorise
- Proton number, Z: number of protons.
- Nucleon number, A: protons + neutrons.
- Isotope: same Z, different A (different neutrons).
- First ionisation energy: energy to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.
- Successive ionisation energies: energies to remove 1 mole of electrons one-at-a-time from gaseous atoms/ions.
- “Big jump” in successive IE: start removing an inner-shell electron (after valence electrons are removed).
Common Exam Traps
- First ionisation energy definition missing per mole, gaseous, or 1+ ions.
- “Explain the trend” answers that don’t name drivers (nuclear charge, shielding, distance) and link to attraction.
- Mixing up the Period 3 anomalies: Mg → Al (3s vs 3p) vs P → S (pairing/repulsion).
- For ions, changing protons/neutrons (ions form by electron gain/loss).
- Successive IE: using the jump as “number of shells” instead of valence vs inner-shell removal.
- Electron configuration: removing electrons from the wrong subshell (4s before 3d for transition ions).
- Electron configuration: writing a neutral atom configuration when the question is for an ion (make the ion first, then write its configuration).