Introduction:
The crystal structure of a solid is described by the unit cell, which is the smallest repeating unit of the crystal lattice. The number of atoms that are present in a unit cell depends on the crystal structure. There are different types of crystal structures, such as face-centered cubic (fcc) and body-centered cubic (bcc).

BCC structure:
In the bcc structure, there are two atoms per unit cell located at the corners and one atom at the center of the cube. When all atoms along the body diagonal are removed, there will be no atoms at the center of the cube. Therefore, the number of atoms per unit cell will be reduced from 2+1=3 to 2.

FCC structure:
In the fcc structure, there are four atoms per unit cell located at the corners and one atom at the center of each face of the cube. When all atoms along the body diagonal are removed, there will be no atoms at the corners of the cube. Therefore, the number of atoms per unit cell will be reduced from 4 to 3.

Defects in Solids:
Defects in solids are the imperfections in the crystal lattice that affect the properties of the material. These defects can be classified into two types: point defects and line defects. Point defects include vacancies, interstitials, and substitutional defects. Line defects include dislocations and stacking faults.

Conclusion:
The number of atoms per unit cell in bcc and fcc structures can be affected by the removal of atoms along the body diagonal. Defects in solids can affect the properties of the material and can be classified into point defects and line defects.