Question

Which of the following pairs of elements could possibly be in the same group? X has a 1+ ion; Y has a 1- ion. X tends to form a 2+ ion; Y tends to gain two electrons. X will tend to gain one electron; Y has eight valence electrons. X has an atomic number 15; Y forms a 3- ion.

Answer 1

Let's eliminate these one by one.
The first pair would not be the same, as X would most likely be in group IA, and Y would be in group VIIA, because of their tendency to gain and lose electrons.
The second pair would also violate the same rule, but X would most likely be in group IIA, and Y would most likely be in group VIA.
The third pair would not be the same, as X is most likely in group VIIA, and since Y has eight valence electrons, it is most likely a noble gas.
The final pair has X with atomic number 15, making it phosphorous. Phosphorous wants to gain 3 electrons to have a full octet of 8 outer "valence" electrons, and Y would also like to gain 3 electrons. This means it is possible that the final pair would be in the same group.

Answer 2

Answer is: X has an atomic number 15; Y forms a 3- ion.

1)  X has a 1+ ion; Y has a 1- ion is not correct, because X is probably metal (lost one electron, group 1) and Y is nonmetal (gain one electron, group 17).

2) X tends to form a 2+ ion; Y tends to gain two electrons is not correct, because X lost two electrons (probably metal from group 2).

3) X will tend to gain one electron; Y has eight valence electrons is not correct, because X is probably from group 17 and Y is noble gas (group 18).

4) Y is phosphorus (atomic number 15, group 15) and Y is nonmetal from group 15.