The ccp lattice is formed by the element Y. The number of octahedral voids generated would be equal to the no of atoms of Y present in it. Since all the octahedral voids are occupied by the atoms of X, the number would also be equal to that of the element Y. Therefore the formula of the compound is XY.

The compound formed by two elements X and Y can be determined by analyzing the arrangement of atoms in the crystal lattice. In this case, the atoms of element Y (anion) make up a cubic close-packed (ccp) structure, while the atoms of element X (cation) occupy all the octahedral voids within the ccp arrangement.

1. Identifying the Crystal Structure:
To determine the formula of the compound, we first need to understand the crystal structure. A ccp structure is also known as a face-centered cubic (FCC) structure. In this arrangement, each atom at the corners of a cube is shared by eight adjacent unit cells, while each atom on the face of the cube is shared by two adjacent unit cells. This arrangement forms a close-packed structure with the highest packing efficiency.

2. Identifying the Atom Arrangement:
Next, we need to understand the arrangement of atoms within the ccp structure. In the ccp arrangement, there are four layers of atoms. The first layer consists of atoms in the corners of the cube, the second layer consists of atoms on the faces of the cube, the third layer consists of atoms in the corners again, and the fourth layer consists of atoms on the faces. This sequence repeats throughout the crystal lattice.

3. Occupying Octahedral Voids:
The octahedral voids in a ccp structure are located at the center of each face of the unit cell. These voids are created by the close-packed arrangement of atoms. In this case, the atoms of element X (cation) occupy all the octahedral voids within the ccp structure.

4. Determining the Formula:
Since the atoms of element X occupy all the octahedral voids, the number of atoms of element X is equal to the number of octahedral voids. The number of octahedral voids in a ccp structure is equal to the number of atoms in the face-centered planes, which is equal to the number of atoms in the corners of the cube.

Therefore, the formula of the compound can be determined by comparing the number of atoms of element X and element Y. The ratio of X to Y can be calculated by dividing the number of octahedral voids (X) by the number of atoms in the ccp structure (Y).

For example, if there are 4 octahedral voids and 8 atoms in the ccp structure, the formula of the compound would be X4Y8.

In conclusion, the formula of the compound formed by elements X and Y can be determined by analyzing the crystal structure and atom arrangement. By occupying all the octahedral voids within the ccp structure, the atoms of element X form a compound with the atoms of element Y. The specific formula can be determined by comparing the number of atoms of X and Y.