Periodic Landscapes - Atomic Radii
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Atomic radii

The atomic radius is half the distance apart of two atoms of the element in its normal state. The atomic radius of a metal depends on its co-ordination numbers, or number of nearest neighbours. For a non-metal the atomic radius is half the distance between a pair of bonded atoms of the element. Molecules such as oxygen and nitrogen are multiply bonded so the atomic radii of these elements are calculated from molecules such as hydrogen peroxide (HOOH) and hydrazine (H2NNH2) which contain only single bonds. Gases like neon, which are atomic rather than molecular and are not known to form compounds, are more of a problem. Their radii are calculated from the structures the elements adopt at absolute zero.
The size of an atom depends on the control the positively charged nucleus can exercise over the negatively charged electrons. From an electron’s point of view going down a group is like changing from a double bass to a violin. The nucleus has less control over the ‘high pitched’ electrons consequently the atomic radii increase. Across a series the electron tunes are more complex. In general the atomic radii decrease from left to right across the Periodic Table.
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