Assertion (A) : End-centred unit cells contain one constituent particle at each face besides the ones at its corners.
Reason (R) : Unit cells are of two types, primitive and centred unit cells.

Options

A. Both Assertion (A) and Reason (R) are correct statements, and Reason (R) is the correct explanation of the Assertion (A).
B. Both Assertion (A) and Reason (R) are correct statements, but Reason (R) is not the correct explanation of the Assertion (A).
C. Assertion (A) is correct, but Reason (R) is incorrect statement.
D. Assertion (A) is incorrect, but Reason (R) is correct statement.

Answer

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided.

### Step 1: Understanding the Assertion (A)
The assertion states that "End-centred unit cells contain one constituent particle at each face besides the ones at its corners."

- In an end-centred cubic unit cell, there are indeed atoms located at the corners of the cube. However, there are only two faces that have atoms, specifically the two opposite faces. Each of these face-centered atoms contributes half of its volume to the unit cell. Therefore, the assertion is incorrect because it implies that there is an atom at every face, which is not true.

### Step 2: Understanding the Reason (R)
The reason states that "Unit cells are of two types, primitive and centred unit cells."

- This statement is true. Unit cells can be classified into two main types: primitive unit cells, which contain lattice points only at the corners, and centred unit cells, which have additional lattice points at the center or faces of the unit cell.

### Step 3: Conclusion
Now we can conclude:
- The assertion (A) is false because it incorrectly describes the positioning of atoms in end-centred unit cells.
- The reason (R) is true as it correctly classifies unit cells into primitive and centred types.

Thus, the correct option is that the assertion is incorrect, but the reason is correct.

### Final Answer:
- Assertion (A) is false.
- Reason (R) is true.
- The correct option is: Assertion is incorrect but reason is correct.

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