Why does the 4s sublevel fills before the 3d sublevel begins to fill as electrons are added?

Question

Why does the 4s sublevel fills before the 3d sublevel begins to fill as electrons are added?

Answer 1

levels and sublevels are aranged when empty(this is a model,on paper,

like a map) following increasing energy.but when they do fill up,in reallity,the simple order changes.there are some rules to do that,but they are harder to apply.in the periodic table there are 6-7 "key" elements from which rule gets "upset".or else:reality is harder than numerical order.

Answer 2

Electrons go into atomic structures in order of energy of the level.  For the first two levels (1 and 2) the energy differences are great enough that there is no overlap of the sublevels (s, p, d, f).  As the level goes to 3, the splitting of the level into sublevels is close to the level 4, so the overlap occurs and the 4s sublevel has less energy than the 3d sublevel.  There are some tricks to generating the correct order for the energy of the sublevels, or just use the periodic table.