Of these nonmetals, which one is likely to be the least reactive? oxygen (o) sulfur (s) chlorine (cl) fluorine (f)

The correct answer is Sulfur (S).

Explanation:

Chemical reactivity is defined as the tendency of an element to loose of gain electrons.

Non-metals are the elements which gains electrons and hence, their chemical reactivity will be the tendency to gain electrons.

Their chemical reactivity increases as we move from left to right in a period because the valence shell come closer to the nucleus as we move from left to right. Thus, the addition of new electron in the valence shell becomes easier due to greater attraction between nucleus and valence electron.

Their chemical reactivity decreases as we move from top to bottom in a group. As, the electrons get added up in the new shell, the electron in the outermost orbital gets far away from the nucleus. And hence, the electron will be difficult to gain. Thus, the chemical reactivity of non-metals decreases.

For the given options:

Oxygen (O) belongs to Group 16 and Period 2 of the periodic table.

Sulfur (S) belongs to Group 16 and Period 3 of the periodic table.

Chlorine (Cl) belongs to Group 17 and Period 3 of the periodic table.

Fluorine (F) belongs to Group 17 and Period 2 of the periodic table.

Order of reactivity of non-metals follows:

Thus, the correct answer is Sulfur (S).