This is an example of CONDOMINANCY.
Condominancy is said to occur when two different alleles are combined together to form a different allele, in which the alleles are expressed equally and none of them is dominant. In the example given above, the black rat population mated with the white rat population to produce offspring that are grey in colour. Thus, the white and the black colour of the parents are not dominant, but are equally expressed to give gray colour.
1) they have similar chemical properties.
for example fluorine from the 17th column is a non metal so does all the elements of that group.
2)they have same valence electrons in their outermost shell
example
electronic configuration of
hydrogen 1, lithium 2,1, sodium 2,8,1 and so on.
3) have same valency.
Remain stable is the answer