A fullerene counter

You will write a few functions for the cages of carbon. They count bonds and hexagons, test Euler's rule and work out the size of a cage from its number of hexagons.

The idea. A cage with $n$ atoms has $3n/2$ bonds and $n/2 - 10$ hexagons, and always 12 pentagons. For a closed cage, $V - E + F = 2$.

Your tasks.

1. Complete bonds(n) so that it returns the number of bonds.
2. Complete hexagons(n) so that it returns the number of hexagons.
3. Complete euler(n) so that it returns $V - E + F$ for the cage.
4. Complete atoms(h) so that it returns the number of carbon atoms in a cage with h hexagons.