[huskywolf]

Hello
Can someone explain clearly why 4s comes infront of 3d in electron config
I know its got to do with energy levels but how do I calculate this
and why in Cu does 4s come after 3d?

[Fridushka]

Well write down the electronic configuration..most probably there will be something related with stability..

[huskywolf]

the little electrons like to fill the empty subshells on their own?
like people like to take an empty bus seat..instead of sitting beside another person(unless its a good looking girl)

[Ranadeep]

Refer to Aufbau Principle in some standard text book ..

[stewie griffin]

I agree with sitting next to cute girls on the bus. Always chose the seat next to the cute girl 
Anyway, I'd just like to add a bit to the help that others have already provided. 
When we have the free atoms in the gas state, data indicates that the 4s is indeed lower than the 3d. Ok fine. But in solution and when complexed with ligands, the 3d orbitals are actually lower in energy than the 4s. So even though we write the configuration of iron (Fe) as 4s²3d⁶, really when we do actual chemistry with iron we consider the iron to be 3d⁸. This is just FYI in case it bothers you for some reason to think that the 4s has to be below the 3d. It doesn't. However, for a gen chem exam, stick with the model of having the 4s orbitals lower in energy as that's what is taught in gen chem (and you don't want to lose those simple points  )