Down the group density of elements increases, this is a trend in the periodic table. In which of these pairs is this trend not true?

Options

A. Br, I
B. Na, K
C. Ru, Os
D. Nb, Ta

Answer

To solve the question regarding the trend of density increasing down a group in the periodic table, we will analyze the given pairs of elements. The key is to identify any exceptions to the general trend.

### Step-by-Step Solution:

1. **Understand the Trend**: The general trend in the periodic table is that the density of elements increases as you move down a group. This is because, as you go down, the atomic mass increases while the volume also increases, but the increase in mass is usually greater than the increase in volume.

2. **Identify the Pairs**: The pairs given in the question are:
- BrI (Bromine and Iodine)
- NaK (Sodium and Potassium)
- RuOs (Ruthenium and Osmium)
- Nbt (This seems to be a typo or unclear, but we will focus on the other pairs)

3. **Analyze Each Pair**:
- **BrI (Bromine and Iodine)**: Both are halogens, and as we go down the group from Bromine to Iodine, the density does increase (Bromine has a density of about 3.12 g/cm³, and Iodine has a density of about 4.93 g/cm³).

- **NaK (Sodium and Potassium)**: Sodium (Na) has a density of about 0.97 g/cm³, while Potassium (K) has a density of about 0.86 g/cm³. Here, the density actually decreases as we move from Sodium to Potassium, which is an exception to the trend.

- **RuOs (Ruthenium and Osmium)**: Both are transition metals, and the density increases as we move from Ruthenium (density ~ 12.37 g/cm³) to Osmium (density ~ 22.59 g/cm³).

- **Nbt**: Since this is unclear, we will not analyze it further.

4. **Conclusion**: The pair where the trend of increasing density down the group does not hold true is **Sodium (Na) and Potassium (K)**.

### Final Answer:
The pair in which the trend of increasing density down the group is not true is **Sodium (Na) and Potassium (K)**.