Question

Which sublevel is filled just before $5f$?

• A. $4f$
• B. $7s$
• C. $5d$
• D. $6d$

Answer 1

The correct option is B $7s$

$(n+l)$ value of $5f=5+3=8$. Hence before it the $(n+l)$ value $7$ is filled. Also for $(n+l)$ value $7$, mass value of n will be filled just before $(n+l)$ value $8$, i.e., $n=7,l=0$ This $7s$ orbital is filled just before $5f$