Name the (i) lightest and the (ii) heaviest element (in terms of density)among the transition elements.

Answer

To answer the question regarding the lightest and heaviest transition elements in terms of density, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Transition Elements**:
Transition elements are defined as d-block elements that have partially filled d-orbitals. Elements like zinc, cadmium, and mercury are not considered transition metals because they have completely filled d-orbitals.

2. **Identifying the Lightest Transition Element**:
To find the lightest transition element, we look for the element with the lowest density among the transition metals. Scandium (Sc) is the first transition element in the periodic table and has a low density.
- **Density of Scandium**: Approximately 3.43 g/cm³.

3. **Identifying the Heaviest Transition Element**:
For the heaviest transition element, we need to identify the element with the highest density. Osmium (Os) and Iridium (Ir) are known to be the heaviest transition metals.
- **Density of Osmium**: Approximately 22.59 g/cm³.
- **Density of Iridium**: Approximately 22.61 g/cm³.
Iridium has a slightly higher density than osmium.

4. **Conclusion**:
- The lightest transition element is **Scandium (Sc)** with a density of 3.43 g/cm³.
- The heaviest transition element is **Iridium (Ir)** with a density of 22.61 g/cm³.

### Final Answer:
- (i) Lightest transition element: **Scandium (Sc)**
- (ii) Heaviest transition element: **Iridium (Ir)**