Question

In which of the following orbitals electron will be filled first?

• A. $5f$
• B. $6d$
• C. $7s$
• D. $7p$

Answer 1

The correct option is C $7s$

According to Aufbau principle, electrons are filled into atomic orbitals in the increasing order of orbital energy level and the available atomic orbitals with the lowest energy levels are occupied before those with higher energy levels.
The orbital in which electron is filled first is decided by “$n+l$” rule.
$$\begin{array}{cc}\text{Orbital}& (n+l)\\ 5f& 8\\ 6d& 8\\ 7s& 7\\ 7p& 8\end{array}$$
So, $7s$ is will be filled first.