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: Use the period number:
Co is in period 4, so its d electrons are in the 3d series. Mo is in period 5, so its d electrons are in the 4d series.

Step 2: Pick:
3d and 4d is option C.

Final Answer:
Co sits in period 4 (3d) and Mo in period 5 (4d). \[ \boxed{\text{(C) }\text{3d and 4d}} \]