Question

Which one among the following principal energy levels has exactly four subshells?
(A) $1$
(B) $2$
(C) $3$
(D) $4$
(E) $5$

Answer 1

The principal energy level was denoted by the symbol $'n'$ which corresponds to shell or orbital. The principal energy level is nothing but a principal quantum number. The number of subshells in the principal energy level is equal to the number of that principal energy level or principal quantum number.

Complete Step By Step Answer:
Quantum numbers are the values that were used to represent or describe the energy levels in atoms or molecules. They are four different quantum numbers. They are a principal quantum number which was represented by $'n'$ , an azimuthal quantum number which was represented by $'l'$ , a magnetic quantum number which was represented by $'m'$ and spin quantum number which was represented by $'s'$ .
The principal quantum number is nothing but the principal energy level. The value of a principal quantum number is the number that was written before the subshell. For example, the subshell $3s$ has the value of the principal quantum number as $'3'$ . This quantum number was introduced by Niels Bohr.
Given that the principal energy level has exactly four subshells. Thus, the value of the principal quantum number or principal energy level will be $4$ .
Option D is the correct one.

Note:
Each starting element in the period of a periodic table has a new principal energy level. Hydrogen is the first element in the first period and has only one principal energy level, whereas the element in the second period is lithium has the two principal energy levels.