Periodic table with atomic radius values

How atomic radius is measured

An atom has no sharp edge, so its radius is worked out from the distance between neighboring atoms. The values on this page are van der Waals radii from PubChem: half the distance between the nuclei of two atoms of the same element that touch without being chemically bonded, such as atoms in neighboring molecules.

One picometer is a trillionth of a meter (10⁻¹² m), and 100 pm equals 1 ångström. Other books may quote covalent or metallic radii, which are measured between bonded atoms and come out smaller, so compare values only from the same source. Each cell shows the radius in pm, and elements from fermium onward, with no known value, show a dash.

The trend across periods and down groups

Down a group, atoms get larger because each new period adds an electron shell farther from the nucleus. The alkali metals show this clearly: lithium is 182 pm, sodium 227 pm and potassium 275 pm.

Across a period, atoms generally get smaller. Each step to the right adds a proton to the nucleus, but the new electron joins the same outer shell, where it does little to shield the others. The stronger effective nuclear charge pulls the whole electron cloud inward, so sodium, at 227 pm, is much larger than chlorine, at 175 pm, at the other end of period 3.

Exceptions in this data

Van der Waals radii come from contacts between atoms in real solids, liquids and gases, so the trend across a period is not perfectly smooth. Boron is listed as larger than beryllium, silicon as larger than aluminum, and each noble gas as larger than the halogen beside it. The values across the transition metals are also irregular.

Worked example: sodium and potassium

Potassium (275 pm) is larger than sodium (227 pm) because its outer electron is in the fourth shell instead of the third, farther from the nucleus and better shielded by inner electrons. That electron is easier to remove, which is why potassium reacts more violently with water than sodium does. The same link shows up in its lower first ionization energy: 4.341 eV, compared with 5.139 eV for sodium.

Atoms also change size when they form ions. Positive ions are smaller than their atoms, because they lose electrons, often a whole shell, while negative ions are larger, because the extra electrons repel each other.