Question

an atom of cr [z=24] loses 2 electrons . how many unpaired electrons shall be there in cr2+?
a]4
b]3
c]2
d]1

Answer 1

Answer is 4 unpaired electrons

Answer 2

The answer is a) 4.

Explanation:  

Chromium has the atomic number 24.

The electronic configuration for Cr in the ground state is :

Cr = 1s², 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵.

Cr 2+ has the electronic configuration:  

1s², 2s² 2p⁶ 3s² 3p⁶ 3d⁴.

We can write the electronic configuration of Cr2+ as [Ar]3d⁶ 4s²(4s has higher energy than 3d).  

Cr2+ means we are removing 2 electrons from the highest energy level - 4s, and we so the electronic configuration for Cr2+ will be [Ar]3d⁶.

We have to put 6 electrons in the d orbital. But d has only 5 orbitals. According to Hund’s rule, we will put all of the 5 in different orbitals. Since all the 5 orbitals have electrons, the last electron should also be paired. So, we have 1 orbital with 2 electrons and 4 shells having unpaired electrons.