Atomic and ionic radii

An atom has no sharp edge, so its 'radius' is measured from the distance between atoms:

• Covalent radius: half the bond length between two identical atoms joined by a single bond. The Cl–Cl bond is 198 pm, so chlorine's covalent radius is 99 pm.
• Metallic radius: half the distance between the centres of two neighbouring atoms in a metal crystal (copper: 128 pm).

Two trends:

• Across a period, atoms get smaller (lithium 152 pm down to fluorine 64 pm). The electrons are added to the same shell, while the nuclear charge keeps increasing, so the outer electrons are pulled in more tightly.
• Down a group, atoms get bigger (lithium 152 pm up to caesium 262 pm), because a new shell is added each time.

Ions differ in size from their atoms:

• A cation (positive ion) is smaller than its atom: it has fewer electrons, but the same nucleus pulls on them more strongly. Na⁺ is much smaller than Na.
• An anion (negative ion) is larger than its atom: the extra electron adds repulsion between the electrons, and each feels less pull.

Isoelectronic species have the same number of electrons. Among them, size depends only on the number of protons: the more protons, the smaller the species, because the same electrons are pulled harder. For the 10-electron species: Al³⁺ < Mg²⁺ < Na⁺ < F⁻ < O²⁻ < N³⁻.

The noble gases do not form bonds easily, so their radii are van der Waals radii (half the distance between non-bonded atoms), which are naturally larger. They should not be compared with covalent radii.