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

Elements with same number of electrons in the outermost energy shells are grouped into the same groups.
name of the group is based on the number of valence electrons in outermost shells.
A. A forms a +1 ion which means that A was in group 1 with one valence electron and to become stable it has to be give out that electron hence +1 charged. B is in group 7 with 7 valence electrons and to become stable it takes in 1 electron hence -1 charged.
this answer is incorrect

B. A has atomic number 15. then electronic configuration is 1s² 2s² 2p⁶ 3s² 3p³. It has 5 electrons in the third shell, which means A belongs to group 5
B forms -3, for it to taken in 3 electrons and become stable it must be already having 5 electrons in the outer shell, therefore its in group 5.
both A and B are in group 5 therefore this answer is correct

C. A tends to form +2 ion which means it has 2 electrons in the shell, therefore group 2. B gains 2 electrons to become stable so it should have 6 electrons in the outer shell. B is in group 6.
this answer is incorrect.

D.if A gains one electron it should be having 7 electrons therefore A is in group 7 and B has 8 valence electrons therefore in group 8. 
this answer is incorrect

correct answer is B.

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