Electron arrangement in Nickel I'm struggling to understand why the element Nickel can be written like so: [Ar]3d84s2, but cannot be written as [Ar]3d10.

I know that the d-orbital can 'house' 10 electrons, and that according to the Aufbau principle, electrons must be 'housed' in the vacant orbital which has the lowest energy, so the first way I presented seems wrong to me. 

I'm sure that there's some confusion on my end here, or rather, I'm missing something that might seem obvious, but I really don't get it.
The available energy levels are arranged in the following order, going from the lowest level to the highest :
$1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p,$  etc. So the $4s$ level is filled before the $3d$.