they are lighter than air. they have full outer electron shells. they have excess electrons to share. they easily react with other elements.
I gave my students this multiple choice question: On the periodic table, the elements in group 18 (8A) (helium, neon, argon, etc.) are grouped together because

Only one answer is correct among these 4 choices:
- they are lighter than air.
- they have full outer electron shells.
- they have excess electrons to share.
- they easily react with other elements.

Could you tell me which one is correct?

they have full outer electron shells.