Which of the following crystals do not crystallise in HCP structure.

Options

A. Na
B. Be
C. Ca
D. Ba

Explanation

To determine which of the following crystals do not crystallize in the Hexagonal Close Packing (HCP) structure, we need to analyze the given options and compare them with known metals that form HCP lattices.

### Step-by-Step Solution:

1. **Understand HCP Structure**:
- The Hexagonal Close Packing (HCP) structure is a type of crystal lattice arrangement where atoms are packed closely together in a hexagonal shape.
- Common metals that crystallize in HCP include Zirconium, Cadmium, Magnesium, Titanium, Zinc, and Beryllium.

2. **List the Given Options**:
- The question provides a set of metals. We need to identify which of these do not form HCP structures.

3. **Identify Known HCP Metals**:
- From the information provided, we know that Beryllium, Magnesium, Titanium, and Zinc crystallize in HCP structure.
- Therefore, we need to check the remaining options to see if they are known to crystallize in HCP.

4. **Analyze Each Option**:
- **Sodium**: Sodium does not crystallize in HCP; it typically forms a body-centered cubic (BCC) structure.
- **Beryllium**: Beryllium does crystallize in HCP.
- **Calcium**: Calcium also does not crystallize in HCP; it typically forms a face-centered cubic (FCC) structure.
- **Zirconium**: Zirconium does crystallize in HCP.

5. **Determine the Correct Answer**:
- From the analysis, Sodium and Calcium do not crystallize in HCP structure. However, the question asks for the option that does not crystallize in HCP. If we assume the options are numbered, we can conclude that the correct option is the one that includes Sodium or Calcium.

### Final Answer:
The crystals that do not crystallize in HCP structure are Sodium and Calcium.