[kandyfloss]

I'm studying the structure of atom at the moment.I wanted to know the reason why the orbitals in an atom are arranged in the way they are?For ex:- why is 3d orbital above 4s orbital? and why does the energy gap decrease when the no. orbitals increase?

[Borek]

I doubt you will find better answer than "when you calculate energy levels, they behave this way".

[kandyfloss]

We haven't really learnt to calculate energy levels.I kinda came up with an answer :- the electrons occupy the 4s orbital first with opposite spins since only 2 electrons can fit into 4s orbital,and it's much more stable than singally occupying the 3d orbital.And when 4s is filled,the effective nuclear attraction is more on 4s than on 3d so it's below 3d,so it's closer to the nucleus..It was a total guess,i don't think it's anywhere near precise.

[Borek]

It won't be fast before you will know how to calculate energy levels. As of now, you just have to believe what you are told.