What is the electron configuration for Gallium?

Question

Google results show: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p¹

or [Ar] 3d10 4s2 4p1

However, when based on the electron configuration diagram and the periodic table, doesn't 4s come before 3d? Shouldn't it be

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p¹

or [Ar] 4s² 3d¹⁰ 4p¹ ?

What is the reason for writing 3d first?

Answer 1

You will see it BOTH ways, and while you are correct that 4s fills before 3d, it is often common to write the 3d in front of the 4s when writing the configuration just to keep the order of principle quantum number, i.e. 3 before 4, etc. Both are actually correct and it depends on who is grading the question.

Answer 2

Hello,

The reason for this is due to The aufbau principle, where electron fill atomic orbitals of the lowest available energy levels before occupying higher levels. Thus, 4s is filled before 3d.