Question 6
What decides which group an element belongs to?
The right answer is the one marked above.
The number of electrons in its last energy level — its valence electrons. One valence electron puts an element in group I, two in group II, and so on to seven in group VII.
Eight in the last level, or any stable configuration, puts the element in group VIII / 0 — the noble gases. Since it is the last level that decides how an element reacts, the group is also a prediction of its chemistry.