The Atomic Radius Trend: Why Atoms Shrink Across a Period

It seems backwards at first. Going across a period from sodium to argon, each atom has more electrons and more protons than the last — and yet the atoms get smaller. Understanding why is the key to half of the periodic trends.

The trend in one sentence

Atomic radius decreases from left to right across a period and increases from top to bottom down a group.

So the largest atoms are in the bottom left (cesium, francium) and the smallest in the top right (helium, neon, fluorine).

Across a period: more pull, same shell

In period 3, every element from sodium to argon has its outer electrons in the third shell. Each step to the right adds:

• one proton to the nucleus, increasing the positive charge, and
• one electron to the same outer shell.

Electrons in the same shell are poor at shielding each other from the nucleus. So the effective nuclear charge felt by the outer electrons rises steadily, and it pulls the whole outer shell in closer.

Covalent radii for period 3 show the steady shrink (values from the widely used 2008 compilation by Cordero and colleagues):

Element	Na	Mg	Al	Si	P	S	Cl
Covalent radius (pm)	166	141	121	111	107	105	102

Our element pages and the atomic radius trend map use the van der Waals radius instead — the size of an atom that isn’t bonded to anything. Those values are harder to measure and bumpier across a period (some period 3 elements come out larger than their left-hand neighbours), but the overall direction is the same, and down a group they’re very clear.

Down a group: more shells

Going down a group, each element adds a whole new electron shell. The outer electrons are further from the nucleus, and the extra inner shells shield them. Even though the nuclear charge is much larger, the added distance wins.

Group 1 from our data:

Element	Li	Na	K	Rb	Cs	Fr
Radius (pm)	182	227	275	303	343	348

Ranking atoms by size (exam technique)

1. Different periods? More shells means bigger. Potassium (period 4) is larger than chlorine (period 3).
2. Same period? Further left is bigger. Sodium is larger than chlorine.
3. Diagonal comparisons (like Al vs. C) can be settled by combining both rules: aluminium is lower and further left, so it’s bigger.
4. When the two rules point opposite ways (for example lithium, higher and further left, versus sulfur, lower and further right), the trends alone can’t decide — look up real radius data.

Ionic radius

When atoms form ions, their size changes a lot:

• Cations are smaller than their parent atoms. Na → Na⁺ loses its entire outer shell, and the remaining electrons feel a stronger pull. Na⁺ is roughly half the radius of an Na atom.
• Anions are larger than their parent atoms. Cl → Cl⁻ gains an electron in the same shell, increasing repulsion without adding protons.

Isoelectronic series

Ions with the same number of electrons are called isoelectronic. For them, size depends only on nuclear charge — more protons, smaller ion:

O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³⁺

All five have 10 electrons (like neon), but aluminium’s 13 protons pull them in far tighter than oxygen’s 8.

The lanthanide contraction

Normally, atoms in period 6 are bigger than those above them in period 5. But across the lanthanides (elements 57–71), electrons fill 4f orbitals, which shield very poorly. By the time you reach hafnium, the nucleus has 14 extra protons that the 4f electrons barely offset. The result: hafnium is almost exactly the size of zirconium above it, tantalum matches niobium, and so on. That’s why these pairs are so chemically similar, and why period 6 metals are so dense. See rare earth elements.

Why atomic size matters

Atomic radius underlies other trends:

• Bigger atoms hold outer electrons more loosely → lower ionization energy and more metallic behaviour.
• Smaller atoms attract bonding electrons more strongly → higher electronegativity.
• Bond lengths, and therefore bond strengths, depend on atomic size.

Quick answers

Which element has the largest atomic radius? Cesium among measured elements; francium is predicted to be similar. See largest and smallest atoms.

Why doesn’t adding electrons make atoms bigger across a period? They go into the same shell, while the added protons increase the pull on that whole shell.

Is atomic radius the same as ionic radius? No. Cations are smaller and anions larger than their neutral atoms.