Question

The number of electrons in 3d orbital for element with Atomic number $26$ is-
A)4
B)6
C) 8
D)10

Answer 1

The element with atomic number $26$ is Iron(Fe).Write its electronic configuration using Aufbau rule, according to which the electrons are filled from lowest to highest energy orbital, and you’ll get the answer.

Complete step by step answer:
Given, the atomic number of elements is $26$.
We have to find the number of electrons in 3d orbital.
First we have to identify the element.Since the atomic number is $26$ which means the element has a total of $26$ electrons. Iron is the element that has atomic number $26$ .
Now we need to write its electronic configuration and we will use the Aufbau principle for this. According to aufbau principle, electrons first occupy those orbitals whose energy is lowest so the order in which the orbitals are filled is 1s,2s,2p,3s,2p,4s,3d,4p,5s and so on.
On using Aufbau principle-Now in Iron,2 electrons are in 1s subshell,2 are in 2s,6 are in 2p,2 are in 3s and 6 electrons in 3p subshell.
Now, the electrons will first enter 4s subshell instead of 3d because 4s has lower energy than 3d.So 2 electrons are in 4s and 6 electrons are in 3d.
$\Rightarrow$ E.C.=$1s^2,2s^2,2p^6,3s^23p^6,4s^2,3d^6$ .

Hence there are 6 electrons in 3d, so the correct answer is ‘B’.

Note:
Here, The electronic configuration can also be written in the following way-
we can also write [Ar] in place of $1s^2,2s^2,2p^6,3s^2,3p^6$ in the electronic configuration as Argon is a noble gas and its atomic number is 18 which is equal to the electrons filled up to 3p.So instead of writing the long version, noble gas shortcut can also be used.