How many electrons are there in second principal energy
level(n=2) of a phosphorus atom?

8 electrons

Explanation:

Phosphorous is the element of group 15 and third period. The atomic number of chlorine is 15 and the symbol of the element is P.

The electronic configuration of the element phosphorus is:-

Thus, from the electronic configuration shown above,

In n = 2, the total number of electrons are 8 which are present in

the electron configuration that you gave me has 16 electrons ergo it has 16 protons ergo it is the element sulfur.