Wayne Tregidga, of Buller High School, asks :-

How big is an atom?

Owen Curnow, a chemist at the University of Canterbury, responded.

An atom is typically about 0.1nm in diameter, ie about 10 million of them side by side would cover a millimetre in length.

This seemingly innocuous question is filled with many complications, not the least of which is: how do you define the size of an atom? This is a bit like asking, "how wide is the Clutha river?", which, of course, depends on where and how you measure it.

For an atom, it depends on how you measure it since there is no definite boundary for where an atom begins and ends (in principle, every atom extends for infinity!). It turns out that there are several definitions for the size of an atom, depending mostly on the type of atom. For metals such as sodium, we can use half the distance between two atoms in the metallic state (this is called the "metallic radius").

Another way is to find a compound with an element-element bond and use half of the distance between them - this is called the covalent radius. Unfortunately, this depends very much on which compound you choose. For non-metallic elements, such as chlorine, the covalent radius works quite well. Chlorine forms the simple molecule in which a pair of chlorine atoms are bonded together. Another definition is: half of the closest distance that two non-bonded atoms can get; this is called the van der Waal's radius. For example, if we freeze chlorine gas down into the solid state, we can measure the closest distance between a chlorine atom of one pair to a chlorine atom of another pair. An additional definition is the radius that encloses 90 per cent of the electron cloud. This has to be calculated, but works well for noble gases like helium that don't form compounds, and can also be done for all atoms equally.