You may have heard someone mention the size of atoms, in the media or at school perhaps, and you’ll certainly have heard people talk about how small atoms are. So you may be surprised to hear that
we don’t know how big atoms are - not exactly, only approximately. But why not?
There are two main problems with measuring the size of atoms - other than the fact that they’re definitely too small to measure by eye, even through a microscope:
- Atoms don’t have defined edges
- Atoms can change their size and shape
Atoms don’t have defined edges
We normally talk about electrons in “atomic shells”, which gives the impression of hard, discrete surfaces, like the kernel of a nut. Sometimes, instead, we say electrons travel in “orbitals”. But this conjures up an image of planetary orbitals - specific lines that electrons are restricted to, like a running track. This isn’t a good model. A better description of electrons around a
nucleus is “electron clouds”. These clouds describe fuzzy areas of
electron density with difficult-to-determine edges.
Electron density is the same as negative charge if you assume an electron is a goo smeared out like a cloud, rather than a particle which inhabits a distinct space. This is exactly what an atom is like. We have a fancy name for it:
electron density probability distributions.
