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 = X has Atomic Number 15 and Y has a -3 ion.

First let us discuss all options,
Option 1 is incorrect because X has +1 ion means it belongs to group 1 while Y has -1 ion means it belong to group 7 (Halogens).
In option 2 X is getting oxidized by loosing 2 electron and forming +2 ion hence metallic in nature, while Y is gaining 2 electrons and forming _2 ion.
Option 3 is also incorrect because X has potential to gain one electron, means it has 7 electrons in valence shell, while Y is already having eight electrons.
Last option is correct because X has atomic number 15 (electronic configuration 1s2, 2s2, 2p6, 3s2, 3p3) means it can gain 3 electrons to complete its octet, and Y has gained those three electrons and has -3 charge.

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom. 

answer: mole of the gas

answer: mole of the gasexplanation:

answer: mole of the gasexplanation: this is because 1 mole of a gas is 22.4dm³, which is use in ideal gas