Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- An atom is a small positive nucleus (protons and neutrons) surrounded by negative electrons. Attraction between them holds the atom together.
- Electrons occupy shells (n = 1, 2, 3 …) split into subshells s, p, d, f, holding 2, 6, 10 and 14 electrons.
- The ground-state configuration fills the lowest-energy subshells first (Aufbau). The periodic table’s s, p, d and f blocks show the order.
- Core electrons are inside the noble-gas core; valence electrons are in the highest occupied shell.
- An electron is harder to remove when it is closer to the nucleus and feels a larger effective nuclear charge.
Key relationships
| Idea | Rule | Use it to |
|---|---|---|
| Coulomb’s law | F ∝ q₁q₂ / r² | compare attractions: 2 × charge → 2 × force; 2 × distance → ¼ force |
| Filling order | 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p | write configurations of atoms and anions |
| Cation rule | remove from the largest n first (4s before 3d) | write configurations of cations |
| Core charge | ≈ protons − core electrons | estimate the charge an outer electron feels |
Assumptions behind the model
- The core charge is an estimate: shielding by inner electrons is not perfect. Use it for comparisons, not exact values.
- Configurations are for the ground state (lowest energy) of an isolated atom or ion.
- Exceptions such as Cr and Cu exist; writing them is not assessed on the exam.
Mistakes to avoid
- Removing 3d before 4s. Fe²⁺ is [Ar]3d⁶, not [Ar]4s²3d⁴.
- Forgetting the d block. Arsenic is [Ar]4s²3d¹⁰4p³, not [Ar]4s²4p³.
- Superscripts that do not add up. Check the total against the electron count.
- Treating 1/r² as 1/r. Doubling the distance quarters the force.
- Changing protons in an ion. Only electrons change.
- Ranking without reasons. Name both charge and distance.
Quick self-check
- Noble-gas configuration of K? ([Ar]4s¹)
- Configuration of Zn²⁺? ([Ar]3d¹⁰)
- If the distance between two charges halves, what happens to the force? (It becomes 4 times larger.)
- Which is harder to remove: a 3s electron from Na or from Mg? (Mg: core charge about +2 against +1, at a similar distance.)
Next: practice questions.