IMAT Chemistry · Periodic Table

Atomic Radius and Ionic Radius

Atoms get smaller from left to right along a period and larger down a group; positive ions are smaller than their atoms and negative ions larger.

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Why this matters

Atomic radius was asked in 2011 (the trend across period 4) and in the 2025 ministry paper (which way radius grows in both directions). The 2026 ministry paper asked whether Mg²⁺ is larger than the Mg atom.

Concept 1 of 2: Atomic radius across a period and down a group

Two things set the size of an atom: how many shells it has, and how strongly the nucleus pulls on the outer shell. Along a period the electrons go into the same shell while the nuclear charge rises by one each step, so the outer shell is pulled in. Down a group each element has one more shell, so the atom is bigger even though the nucleus has more protons.

Definition

Across a period (left to right): atomic radius decreases. Same number of shells, more protons, and the inner electrons shield about the same amount, so the effective nuclear charge on the outer electrons rises.
Down a group: atomic radius increases, because each step adds a shell.
So the largest atoms are at the bottom left (caesium, francium) and the smallest at the top right.
In the d-block the radius falls only slightly, because the new electrons enter an inner (3d) shell. Across period 4, from potassium to bromine, the radius still falls throughout.
Method for comparing atoms: compare the number of shells (the period) first; only within one period compare the atomic numbers.

Worked example

Arrange sodium (

Z = 11

), magnesium (

Z = 12

), chlorine (

Z = 17

) and potassium (

Z = 19

) in order of decreasing atomic radius.

Practice this conceptself-check · 3 quick reps

An atom gets smaller across a period even though it gains electrons

Adding electrons sounds like it should make an atom bigger, but along a period they go into the same shell while the nuclear charge rises. The stronger pull wins. Radius increases from right to left and from top to bottom.

Concept 2 of 2: Ionic radius compared with atomic radius

When a metal atom loses its outer electrons, it usually loses a whole shell, and the same nuclear charge now pulls on fewer electrons: the ion is much smaller. When a non-metal atom gains electrons, the extra electrons repel each other while the nuclear charge stays the same: the ion is larger. In a set of ions with the same number of electrons, the one with most protons pulls hardest and is smallest.

Definition

A cation is smaller than its atom: $\mathrm{Na^+} < \mathrm{Na}$ .
An anion is larger than its atom: $\mathrm{Cl^-} > \mathrm{Cl}$ .
In an isoelectronic series, radius decreases as $Z$ increases: $\mathrm{N^{3-}} > \mathrm{O^{2-}} > \mathrm{F^-} > \mathrm{Na^+} > \mathrm{Mg^{2+}} > \mathrm{Al^{3+}}$ (all 10 electrons).
For one element, the higher the positive charge, the smaller the ion: $\mathrm{Fe^{3+}} < \mathrm{Fe^{2+}}$ .
Down a group, ions of the same charge get larger: $\mathrm{Li^+} < \mathrm{Na^+} < \mathrm{K^+}$ .

Worked example

Compare the sizes of Na and

\mathrm{Na^+}

, of Cl and

\mathrm{Cl^-}

, and then of

\mathrm{Na^+}

and

\mathrm{Cl^-}

Practice this conceptself-check · 3 quick reps

A positive ion is smaller than its atom, never larger

Losing electrons removes the outer shell and leaves the full nuclear charge pulling on fewer electrons. So

\mathrm{Mg^{2+}}

is smaller than Mg. It is negative ions that are larger than their atoms.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Watch out for (2)

An atom gets smaller across a period even though it gains electrons
→ Atomic radius across a period and down a group
A positive ion is smaller than its atom, never larger
→ Ionic radius compared with atomic radius

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