Answer:
An atom must be similar in size.
The substituting atom must be from the same group.
Step-by-step explanation:
A solid solution is a solid mixture containing a minor component uniformly distributed within the crystal lattice of another component called the major component. Metal alloys are typical examples of a solid solution.
Note that only atoms of similar size can substitute each other in a solid solution. Usually atoms of elements in the same group have atomic sizes that vary within narrow ranges. Hence atoms of elements in the same group have similar atomic sizes and can substitute each other in a solid solution.