Atomic radius
The key idea
Atomic radius is not a single directly measurable hard boundary; several operational definitions are used, such as covalent, metallic and van der Waals radii.
Why it matters
The definition must be stated before comparing values. Periodic trends are useful, but different radius definitions should not be mixed.
A picture to remember
The definition must be stated before comparing values. Periodic trends are useful, but different radius definitions should not be mixed.