Answer to Question #218644 in General Chemistry for Joe

Question

1. Why does the 4s orbital get filled with electrons before any electrons can be placed into a 3d orbital? 

Answer 1

According to Aufbau principle , electrons first occupy the lowest energy orbital available to them and enter into higher energy orbitals only after the lower energy orbitals are filled . Therefore , 3d orbital is higher in energy than 4s . And hence electrons fill up in 4s before filling up in 3d .