Question

Which transition series includes elements Co and Mo respectively?

• A. $4\text{d}$ and $5\text{d}$
• B. $5\text{d}$ and $6\text{d}$
• C. $3\text{d}$ and $4\text{d}$
• D. $3\text{d}$ and $6\text{d}$

Hint:

1st Series = $3d$ (Sc to Zn); 2nd Series = $4d$ (Y to Cd).

Answer 1

The Correct Option is C

Step 1: Concept The transition series are named after the $d$-orbital being filled: $3d$ (Period 4), $4d$ (Period 5), and $5d$ (Period 6).

Step 2: Meaning Identifying the position of Cobalt (Co) and Molybdenum (Mo) in the periodic table.

Step 3: Analysis Cobalt (Atomic No. 27) belongs to the first transition series ($3d$). Molybdenum (Atomic No. 42) belongs to the second transition series ($4d$).

Step 4: Conclusion Therefore, Co is a $3d$ element and Mo is a $4d$ element. Final Answer: (C)