Which of the following molecules does not have zero entropy even at zero kelvin? `CO, CO_(2), NaCl, NO`

Options

A. `CO,CO_(2)`
B. `CO, NO`
C. `CO_(2), NaCl`
D. NaCl

Answer

To determine which of the following molecules does not have zero entropy even at zero Kelvin, we need to analyze the properties of each molecule: CO (carbon monoxide), CO₂ (carbon dioxide), NaCl (sodium chloride), and NO (nitric oxide).

### Step-by-Step Solution:

1. **Understanding Entropy and the Third Law of Thermodynamics**:
- The third law of thermodynamics states that the entropy of a perfect crystalline substance approaches zero as the temperature approaches absolute zero (0 K).
- However, this law applies only to substances that are perfectly ordered at absolute zero.

2. **Analyzing Each Molecule**:
- **CO (Carbon Monoxide)**: CO is a diatomic molecule that does not form a perfectly ordered crystalline structure at absolute zero. Therefore, it retains some entropy even at 0 K.
- **CO₂ (Carbon Dioxide)**: CO₂ is a linear molecule and can form a symmetrical structure. It can achieve a perfectly ordered state, leading to zero entropy at absolute zero.
- **NaCl (Sodium Chloride)**: NaCl is an ionic compound that can form a perfect crystal lattice structure. Thus, it has zero entropy at absolute zero.
- **NO (Nitric Oxide)**: Similar to CO, NO is a diatomic molecule that does not exhibit perfect order at absolute zero, meaning it also retains some entropy.

3. **Conclusion**:
- Based on the analysis, CO and NO do not have zero entropy at absolute zero because they do not form perfectly ordered crystalline solids. In contrast, CO₂ and NaCl can achieve zero entropy at 0 K.
- Therefore, the molecules that do not have zero entropy even at zero Kelvin are CO and NO.

### Final Answer:
The molecules that do not have zero entropy even at zero Kelvin are **CO and NO**.