Question

Which transition series includes elements Co and Mo respectively?

• A. 4d and 5d
• B. 5d and 6d
• C. 3d and 4d
• D. 3d and 6d

Hint:

Co (Z = 27) is in the 3d series and Mo (Z = 42) is in the 4d series.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The transition series is named after the d subshell that is being filled: 3d (Sc to Zn), 4d (Y to Cd), 5d (La, Hf to Hg) and 6d (Ac, Rf onwards).

Step 2: Locate Co:
Cobalt has atomic number 27 and the configuration \([\text{Ar}]3d^74s^2\), so it belongs to the 3d series.

Step 3: Locate Mo:
Molybdenum has atomic number 42 and the configuration \([\text{Kr}]4d^55s^1\), so it belongs to the 4d series.

Step 4: Conclusion:
Co is in 3d and Mo is in 4d, which is option C. The other options place Co in 4d or 5d or Mo in 5d or 6d, which is wrong.

Final Answer:
Co is a 3d element and Mo is a 4d element. \[ \boxed{\text{(C) }\text{3d and 4d}} \]