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 of the metals in liquid NH₃ with low concentration is not paramagnetic, we need to analyze the options provided: A) Cg, B) Beryllium, C) Potassium, and D) Magnesium.

### Step-by-step Solution:

1. **Understanding Paramagnetism**:
- Paramagnetic substances have unpaired electrons in their atomic or molecular orbitals, which allow them to be attracted to a magnetic field.
- Conversely, substances with all paired electrons are not paramagnetic.

2. **Analyzing Each Metal**:
- **A) Cg (likely a typo for a metal, assuming it refers to a known metal)**: Without specific information, we cannot determine its magnetic properties.
- **B) Beryllium (Be)**:
- Beryllium has an atomic number of 4. Its electron configuration is 1s² 2s².
- All electrons are paired, indicating that Beryllium is not paramagnetic.
- **C) Potassium (K)**:
- Potassium has an atomic number of 19. Its electron configuration is [Ar] 4s¹.
- It has one unpaired electron in the 4s orbital, making it paramagnetic.
- **D) Magnesium (Mg)**:
- Magnesium has an atomic number of 12. Its electron configuration is 1s² 2s² 2p⁶ 3s².
- All electrons are paired, indicating that Magnesium is also not paramagnetic.

3. **Conclusion**:
- From the analysis, both Beryllium and Magnesium have all their electrons paired and therefore are not paramagnetic.
- The correct answer to the question is B) Beryllium and D) Magnesium.

### Final Answer:
Beryllium and Magnesium are the metals in liquid NH₃ with low concentration that are not paramagnetic.