Give the reason why the electron configuration of copper is 1s2 2s2 2p6 3s2 3p6 3d10 4s1 and not 1s2 2s2 2p6 3s2 3p6 3d9 4s2.

The electron configuration $1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^1$ is preferred because a fully filled $3d^{10}$ subshell is more stable than a partially filled $3d^9$ subshell.

One electron from the $4s$ orbital is promoted to the $3d$ orbital to achieve this more stable, fully filled $3d^{10}$ configuration.

Iron forms two common chlorides: iron(II) chloride ($FeCl_2$) and iron(III) chloride ($FeCl_3$).

In $FeCl_2$, the oxidation state of iron is +2. In $FeCl_3$, the oxidation state of iron is +3.
The two oxidation states are $\boxed{\text{+2 and +3}}$.

Iron is used as a catalyst in the Haber process.
The chemical process is the $\boxed{\text{Haber process}}$.