Write down the number of 3d electrons in $Co^{2+}$ ion.

7

The correct answer is Option (1) → 7

Cobalt (Co) has atomic number 27.

Electronic configuration of Co (neutral):

$\text{Co} : [Ar]\,3d^7\,4s^2$

Formation of Co²⁺:

• Electrons are removed first from 4s, then from 3d.
• Remove 2 electrons from $4s^2$:

$\text{Co}^{2+} : [Ar]\,3d^7$

Number of 3d electrons in Co²⁺: 7