Question

The atomic number of an element in $35$. What is the total number of electrons present in all the p-orbitals of the ground state atom of the element?
A.$6$
B.$11$
C.$17$
D.$23$

Answer 1

The Aufbau principle dictates the manner in which electrons are filled in the atomic orbitals of an atom in its ground state. It states that electrons are filled into atomic orbitals in the increasing order of orbital energy level. According to the Aufbau principle, the available atomic orbitals with the lowest energy levels are occupied before those with higher energy levels. The order in which the energy of orbitals increases can be determined with the help of the (n+l) rule. Here, ‘n’ refers to the principal quantum number and ‘l’ is the azimuthal quantum number.
The order in which the orbitals are filled with electrons is: $1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,6s,4f,5d,6p,7s,5f,6d,7p,8s,$ and so on.
If two orbitals share equal (n+l) values, the orbital with the lower n value is said to have lower energy associated with it and hence it is filled first than the orbital having the same energy but with a higher value of ‘n’.

Complete answer:
We’ve been given that the atomic number of the element is $35$, so according to Aufbau principle, electronic configuration of this element is
$1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^2}3{d^{10}}4{p^5}$
From electronic configuration, we can clearly see that:
Number of p electrons in $2p$=$6$
Number of p electrons in $3p$=$6$
Number of p electrons in $4p$=$5$
Therefore, total number of p electrons= $6 + 6 + 5 = 17$
Hence, an element with atomic number $35$ has a total number of $17$electrons present in all the p-orbitals.

Option (C) is correct.

Note:
The electron configuration of chromium is $\left[ {Ar} \right]3{d^5}4{s^1}\;$and not $\left[ {Ar} \right]3{d^4}4{s^2}\;$ (as suggested by the Aufbau principle). This exception is attributed to several factors such as the increased stability provided by half-filled subshells and the relatively low energy gap between the $3d$ and the $4s$ subshells. Half filled subshells feature lower electron-electron repulsions in the orbitals, thereby increasing the stability. Similarly, completely filled subshells also increase the stability of the atom. Therefore, the electron configurations of some atoms disobey the Aufbau principle (depending on the energy gap between the orbitals). For example, copper is another exception to this principle with an electronic configuration corresponding to $\left[ {Ar} \right]3{d^{10}}4{s^1}$. This can be explained by the stability provided by a completely filled $3d$ subshell.