Two dimensional close packed structure can be generated by stacking the rows of close packed spheres. This can be done in two different ways.
(I) The second row may be placed in contact with the first one, such that the spheres of the second row are exactly above those of first row. The spheres of the two rows are aligned vertically as well as horizontally.
If we call the first row as 'A' type of row, the second row being exactly same as the first one is also of 'A' type. Similarly, we may place more rows to obtain AAAA.... type arrangement
(II) In this type, the second row may be placed above the first one in a staggered manner such that its spheres. fit in depressions of first row. if the arrangement of spheres in the first row is called 'A' type the one in the second row is different and may be called 'B' type. When the third row is placed adjacent to the second in staggered manner, its spheres are aligned with those of first layer. Hence this layer is also 'A' type. The spheres of similarly placed fourth row will be aligned with those of the second row ('B' type). Hence this arrangement is of ABAB.... type
The type of voids generated in type (I) & (II) respectively are

Options

A. tetrahedral, octahedral
B. octahedral, tetrahedral
C. triangular, square
D. square, triangular

Answer

To solve the problem of identifying the types of voids generated in two-dimensional close-packed structures, we will analyze the two arrangements described in the question step by step.

### Step 1: Understand the Two Arrangements
1. **Arrangement I (AAAA... Type)**:
- In this arrangement, the spheres are stacked directly on top of each other. Each row is identical, and the spheres are aligned both vertically and horizontally.
- This results in a square close packing.

2. **Arrangement II (ABAB... Type)**:
- In this arrangement, the second row is staggered such that the spheres fit into the depressions of the first row. The first row is of type A, the second row is of type B, and this pattern continues.
- This results in a hexagonal close packing.

### Step 2: Identify the Voids in Arrangement I (AAAA... Type)
- In the square close packing (AAAA... arrangement), the voids created are surrounded by four spheres.
- When you visualize the arrangement, if you consider the centers of these four spheres, they form a square shape.
- Therefore, the voids in this arrangement are **square voids**.

### Step 3: Identify the Voids in Arrangement II (ABAB... Type)
- In the hexagonal close packing (ABAB... arrangement), the voids are surrounded by three spheres.
- When you visualize this arrangement, the centers of these three spheres form a triangular shape.
- Therefore, the voids in this arrangement are **triangular voids**.

### Conclusion
- The types of voids generated in the two arrangements are:
- Arrangement I (AAAA... Type): **Square voids**
- Arrangement II (ABAB... Type): **Triangular voids**

### Final Answer
The correct answer to the question is **Option D: Square and Triangular**.

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