Step 1: Understanding the Concept:
The transition elements are categorized into $3\text{d}$, $4\text{d}$, $5\text{d}$, and $6\text{d}$ series based on which d-subshell is filling.
Step 2: Key Formula or Approach:
$3\text{d}$ series: Atomic numbers 21-30.
$4\text{d}$ series: Atomic numbers 39-48.
Step 3: Detailed Explanation:
- Cobalt ($\text{Co}$): Atomic number 27. It belongs to the first transition series, where the $3\text{d}$ subshell is filled.
- Molybdenum ($\text{Mo}$): Atomic number 42. It belongs to the second transition series, where the $4\text{d}$ subshell is filled.
Step 4: Final Answer:
$\text{Co}$ and $\text{Mo}$ belong to $3\text{d}$ and $4\text{d}$ series respectively.