Ionic Radius: Why Cations Shrink and Anions Grow

A sodium atom has a radius of about 186 pm. A sodium ion, Na⁺, is little more than half that size: about 102 pm. A chlorine atom, by contrast, measures about 99 pm, but a chloride ion, Cl⁻, is about 181 pm, almost twice as big. Turning atoms into ions changes their size dramatically, and in predictable ways. This article explains why, how ionic radii are measured, and why the size of ions matters so much in chemistry.

What is ionic radius?

The ionic radius is a measure of the size of an ion in an ionic crystal. Ions don’t have hard edges (their electron clouds fade gradually), so ionic radius is defined from the distances between the centres of neighbouring ions in crystals.

How it’s measured

1. X-ray crystallography measures the distance between the nuclei of a neighbouring cation and anion in a crystal, for example Na⁺ and Cl⁻ in sodium chloride. This distance is the sum of the two radii.
2. To split that distance into two separate radii, chemists need one reference value. Different schemes choose differently (for example, based on the oxide ion or fluoride ion), which is why published ionic radii vary slightly between tables.
3. The most widely used modern set was compiled by R.D. Shannon in 1976, from thousands of crystal structures.

Because of these choices, compare ionic radii only from the same table, and treat them as approximate. See also atomic radius trend.

Cations are smaller than their atoms

When a metal atom loses electrons to form a cation:

1. It often loses its entire outer shell. Sodium (2, 8, 1) becomes Na⁺ (2, 8): one fewer shell of electrons.
2. The remaining electrons feel a stronger pull. The same 11 protons now hold only 10 electrons, so each electron is attracted more strongly.
3. Less electron–electron repulsion lets the cloud contract.

Atom	Atomic radius (pm)	Ion	Ionic radius (pm)
Li	152	Li⁺	76
Na	186	Na⁺	102
K	227	K⁺	138
Mg	160	Mg²⁺	72
Al	143	Al³⁺	54

(Atomic radii here are metallic radii; values are approximate.)

Higher positive charge → smaller ion. For the same element, Fe³⁺ (about 65 pm) is smaller than Fe²⁺ (about 78 pm), because the same nucleus holds fewer electrons.

Anions are larger than their atoms

When a non-metal atom gains electrons to form an anion:

1. The number of protons stays the same, but there are more electrons.
2. Extra electron–electron repulsion pushes the electron cloud outward.
3. Each electron is held a little less tightly (lower effective attraction per electron).

Atom	Covalent radius (pm)	Ion	Ionic radius (pm)
F	71	F⁻	133
Cl	99	Cl⁻	181
O	66	O²⁻	140
S	104	S²⁻	184

Higher negative charge → larger ion (for ions of neighbouring elements with the same electron configuration).

Trends in ionic radius

Down a group: ions get larger

Each step down adds a shell of electrons:

• Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺
• F⁻ < Cl⁻ < Br⁻ < I⁻

Across a period: it depends on the charge

Across period 3:

• Cations (Na⁺, Mg²⁺, Al³⁺) get smaller: all have the neon configuration, but nuclear charge increases.
• There’s a big jump from Al³⁺ to the anions, because the anions have an extra shell (the argon configuration).
• Anions (P³⁻, S²⁻, Cl⁻) also get smaller across the period, for the same reason: same electrons, more protons.

Isoelectronic series

Ions with the same number of electrons get smaller as nuclear charge increases:

N³⁻ (146) > O²⁻ (140) > F⁻ (133) > Na⁺ (102) > Mg²⁺ (72) > Al³⁺ (54) (radii in pm)

See isoelectronic species.

Coordination number matters

The same ion appears slightly larger when surrounded by more neighbours. For example, Na⁺ is about 99 pm with four neighbours, 102 pm with six and 118 pm with eight. Tables usually quote radii for six-coordinate ions, the most common environment. This is another reason to compare values from the same source and context.

Worked examples: ranking by size

Example 1: Arrange in order of increasing radius: K⁺, Cl⁻, Ca²⁺, S²⁻.

All have 18 electrons (isoelectronic). Protons: S 16, Cl 17, K 19, Ca 20.

Ca²⁺ < K⁺ < Cl⁻ < S²⁻

Example 2: Which is larger, Mg²⁺ or Ca²⁺?

Same charge, same group; Ca²⁺ has an extra shell. Ca²⁺ is larger (100 pm vs 72 pm).

Example 3: Which is larger, Fe²⁺ or Fe³⁺?

Same nucleus, fewer electrons in Fe³⁺. Fe²⁺ is larger.

Example 4: Which is larger, Na⁺ or F⁻?

Isoelectronic (10 electrons); F⁻ has 9 protons, Na⁺ has 11. F⁻ is larger.

Why ionic size matters

Lattice energy and melting points

Smaller, more highly charged ions pack closer together and attract more strongly, giving larger lattice energies and often higher melting points. Magnesium oxide (Mg²⁺, O²⁻, both small and doubly charged) melts at about 2,850 °C, while sodium chloride melts at 801 °C. See how atoms become ions.

Crystal structures

The ratio of cation radius to anion radius helps decide how many anions fit around each cation. Small cations (like Li⁺ or Mg²⁺) are often surrounded by fewer anions than large cations (like Cs⁺). Caesium chloride and sodium chloride adopt different crystal structures for exactly this reason.

Hydration and solubility

Small, highly charged ions attract water molecules strongly, releasing a lot of hydration energy. Li⁺ is surrounded by a tightly bound shell of water, which makes its effective size in solution larger than K⁺’s. This affects conductivity, solubility and how ions move through biological channels.

Polarising power

Small, highly charged cations (Al³⁺, Be²⁺) distort nearby anions’ electron clouds, especially large, easily distorted anions such as I⁻. This gives the bond some covalent character, which is why aluminium chloride behaves more like a covalent compound than a typical ionic salt. See ionic vs covalent bonds.

Biology and geology

• Nerve signals depend on channels that let K⁺ through while excluding the smaller Na⁺, a selectivity based partly on ionic size and hydration.
• Mineral substitution: ions of similar size and charge can swap places in minerals. Fe²⁺ and Mg²⁺ (78 and 72 pm) substitute readily in olivine; this is also how toxic ions such as Cd²⁺ can replace Ca²⁺ in bone.

A quick rule for exams: fewer electrons than protons → smaller; more electrons than protons → larger. Always compare like with like: same table, same coordination number.

Key takeaways

• Ionic radius is derived from distances between ions in crystals, so values depend on the reference scheme.
• Cations are smaller than their atoms (lost shell, stronger pull); anions are larger (more electron repulsion).
• Down a group ions get larger; in an isoelectronic series, more protons means a smaller ion.
• Higher positive charge shrinks an ion; higher negative charge enlarges it.
• Ionic size controls lattice energy, crystal structure, hydration, covalent character and biological selectivity.