Atomic or ionic radius appeared in four of five years and ionisation energy in four; electronegativity in two. The explanations are all built from the same three factors.
📖 The three factors every trend answer needs
3 terms
Nuclear charge
Number of protons. More protons pull the electrons in harder. Say "greater nuclear charge", not "bigger atom".
Shielding
Inner-shell electrons repel the outer electrons, reducing the pull they feel from the nucleus. Shielding depends on the number of inner shells, not total electrons.
Distance / number of shells
More occupied shells means the valence electrons sit further from the nucleus and are held less tightly.
The trends, and the reason to quote
Trend
Across a period →
Down a group ↓
Atomic radius
Decreases: same shells, more protons, so stronger pull
Increases: more shells, more shielding
First ionisation energy
Increases: electron held more tightly
Decreases: outer electron further out and better shielded
Electronegativity
Increases: stronger pull on a bonding pair
Decreases: bonding pair further from the nucleus
Worked example Why is Fe²⁺ larger than Fe³⁺? (2025) Fe²⁺ has radius 92 pm, Fe³⁺ has 79 pm. Explain the difference. Both ions have the same nuclear charge — the same 26 protons. Fe²⁺ has more electrons in its valence shell than Fe³⁺. More valence electrons means greater electron–electron repulsion within that shell. That repulsion spreads the shell out, so the radius is larger. ✓ Same nuclear charge, but greater electron–electron repulsion in Fe²⁺ pushes its valence shell further out. ✓
Always name what is the SAME first
Comparisons of two ions of the same element, or an atom and its ion, are marked on whether you identify the constant (same protons / same nuclear charge) before the variable. The 2025 schedule’s first bullet is literally "identifies same nuclear charge or same number of protons". ℹ
Atom vs ion
A cation is always smaller than its atom (loses a whole shell in many cases, and the remaining electrons feel more pull each). An anion is always larger (added electrons increase repulsion with no extra protons). Asked in 2022 (Cl vs Cl⁻) and 2023 (Ca vs Ca²⁺).