Question

Electronic configuration of a transition element X in +2 oxidation state is $[Ar]3d^5$. What is its atomic number?

• A. 25
• B. 26
• C. 27
• D. 24

Answer 1

Answer: (A)

25

Answer 2

The electronic configuration of X in +2 oxidation state is $[Ar]3d^5$. Since transition elements lose electrons from the outermost ns orbital first, the two electrons lost to form the +2 ion must have come from the 4s orbital. Thus, the electronic configuration of the neutral atom is $[Ar]4s^2 3d^5$. The atomic number is the total number of electrons in the neutral atom. Atomic number = 18 (from Ar) + 2 (from 4s) + 5 (from 3d) = 25.