Which of the following metal(s) in liquid `NH_(3)` with low conc. Is not paramagnetic?

Options

A. `Cs`
B. `Be`
C. `K`
D. `Mg`

Explanation

To determine which metals in liquid ammonia with low concentration are not paramagnetic, we need to analyze the properties of each metal listed in the options: cesium, beryllium, potassium, and magnesium.

### Step-by-Step Solution:

1. **Understand Paramagnetism**:
- Paramagnetic substances have unpaired electrons, which cause them to be attracted to a magnetic field. Conversely, substances with all paired electrons are not paramagnetic.

2. **Analyze the Metals**:
- **Cesium (Cs)**:
- Cesium is an alkali metal with one unpaired electron in its outermost shell (5s¹). Therefore, it is paramagnetic.
- **Beryllium (Be)**:
- Beryllium has a full outer shell (2s²), meaning it has no unpaired electrons. Thus, it is not paramagnetic.
- **Potassium (K)**:
- Potassium, like cesium, is an alkali metal with one unpaired electron (4s¹). Therefore, it is paramagnetic.
- **Magnesium (Mg)**:
- Magnesium also has a full outer shell (3s²), which means it has no unpaired electrons. Thus, it is not paramagnetic.

3. **Dissolution in Liquid Ammonia**:
- Beryllium and magnesium do not dissolve in liquid ammonia due to their high ionization energy and high lattice energy. Therefore, their solutions do not exist in this medium.
- On the other hand, cesium and potassium do dissolve in liquid ammonia and exhibit paramagnetism.

4. **Conclusion**:
- The metals that do not exhibit paramagnetism in liquid ammonia with low concentration are **Beryllium (Be)** and **Magnesium (Mg)**.

### Final Answer:
- **Beryllium (Be) and Magnesium (Mg)** are the metals in liquid ammonia with low concentration that are not paramagnetic.