Question

The _____ orbital is filled immediately after the 3p orbital.

• A. 4s
• B. 3d
• C. 4p
• D. 4d

Answer 1

The correct option is A 4s

Aufbau principle states that an electron occupies orbitals in order from lowest energy to highest. Electrons are filled in orbitals in order of n+l . For subshells with the same n+l value, electrons are filled first in the subshell with lower value of n.
$$\begin{array}{cc}Orbital& n+lvalue\\ 3p& 4\\ 4s& 4\\ 3d& 6\\ 4p& 6\\ 4d& 7\end{array}$$
Therefore, 4s will be filled after 3p as the value of n for 4s orbital is greater than 3p.