Why does the 4s orbital fill before the 3d orbital?

Answer 1

even though it would seem that 3d orbital is lower in energy it is actually higher in energy. That is why the 4s is filled first.

Answer 2

When both the 4s and 3d orbitals are unfilled, the 4s orbitals are filled first because Z*is higher in the 4s orbital. However, as soon as the 3d orbital begins to fill with electrons, the 3d electronscan “shield” the 4s electrons from nuclear charge, which reduces Z*and makes it easier toremove electrons from the 4s orbital than the 3d. In essence, this phenomenon arises because the3d electrons shield the 4s electrons, but the reverse is not true to a significant extent

Answer 3

Easy way to remember orbitals is to check out the periodic table. It's your best cheat guide, and you rarely take a chem test without it. 4s fill first because it takes less energy. Nature is lazy like no other and doesn't want to expend energy so it takes the easy route.

Answer 4

I assume that you have studied the quantum numbers - principal quantum no., azimuthal quantum no., magnetic quantum no. & spin quantum no.

principal quantum no. is denoted by 'n' & its the no. of shell. eg: for 4s its 4 & for 3d its 3.
azimuthal quantum no. is denoted by 'l'. for 's' orbital, its value = 0.
for p orbital, its value = 1,
for d orbital, its value = 2.

now your answer-
orbitals fill up in the order of increasing value of ( n+l);
for 4s, (n+l) = 4+0 = 4.
for 3d, (n+l) = 3+2 = 5.

since the value of 4s is less, so it will fill up first.
this is simply called as the (n+l) rule.

Answer 5

because its energy level is lower than 3d.