Select the correct statement (s).

Options

A. (a) Co-ordination no. of an atom at a lattice point in sample cubic arrangement is 6
B. (b) Co- ordination no. of an atom at octahedral site 8.
C. (c) Co-ordination no. of an atom at lattice point in hcp arrangment is 6
D. (d) Co-ordination no. of an atom at octahedral site is 6

Answer

To solve the question, we need to evaluate the given statements about coordination numbers in different types of crystal arrangements. Let's analyze each statement step by step.

### Step 1: Analyze the first statement
**Statement A:** Coordination number of an atom at a lattice point in a simple cubic arrangement is 6.

- In a simple cubic arrangement, atoms are located only at the corners of the unit cell. Each corner atom is surrounded by 6 neighboring atoms (one on each face of the cube).
- Therefore, the coordination number for a simple cubic structure is indeed 6.

**Conclusion:** Statement A is correct.

### Step 2: Analyze the second statement
**Statement B:** Coordination number of an atom at an octahedral site is 8.

- An octahedral site is formed when an atom is surrounded by 6 other atoms, arranged in an octahedral geometry (one atom above, one below, and four in the plane).
- Thus, the coordination number for an octahedral site is 6, not 8.

**Conclusion:** Statement B is incorrect.

### Step 3: Analyze the third statement
**Statement C:** Coordination number of an atom at a lattice point in HCP arrangement is 6.

- In a hexagonal close packing (HCP) arrangement, the coordination number is determined by the number of nearest neighboring atoms surrounding a central atom.
- In HCP, each atom is surrounded by 12 other atoms (6 in the same layer and 3 in each of the layers above and below).
- Therefore, the coordination number for HCP is 12, not 6.

**Conclusion:** Statement C is incorrect.

### Step 4: Analyze the fourth statement
**Statement D:** Coordination number of an atom at an octahedral site is 6.

- As discussed earlier, an octahedral site has 6 nearest neighboring atoms.
- Therefore, the coordination number for an octahedral site is indeed 6.

**Conclusion:** Statement D is correct.

### Final Conclusion
The correct statements are:
- Statement A: Coordination number of an atom at a lattice point in a simple cubic arrangement is 6. (Correct)
- Statement D: Coordination number of an atom at an octahedral site is 6. (Correct)

### Summary of Correct Statements:
- A and D are correct.

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