# Atomic theory¶

## Historic approaches to atomic theory¶

### Niels Bohr¶

If the mass of one molecule of a substance is $M$ a.m.u (atomic mass units) then one gramme molecule (aka a mole) is defined to be $M$gm (grammes).

Avogadro's number (commonly written $N$) is a universal constant which is defined as the number of molecules in one mole of any substance. There are many experiments from which its value can be deduced. $$N \approx 6 \times 10^{23}$$

1 a.m.u = (\frac{1}{N})gm

In the same way that a gram molecule (mole) is defined, a gram ion can also be defined. The total charge of one gram ion of any substance is defined to be $Ne$ where $e$ is the charge of a single ion of that substance.

## Intermolecular forces¶

There are two types of force between molecule – the attractive and repulsive forces (denoted $F_W$ and $F_R$ respectively). The attractive force is dominant at about 2-3Å and the repulsive force is dominant at about 1-2Å.

Intermolecular forces decrease quickly as distance increases.