Answer: 2

Detailed Reasoning:

We are looking for elements from the given list that exhibit only one non-zero oxidation state. The elements given are:

O (Oxygen)

Cl (Chlorine)

F (Fluorine)

N (Nitrogen)

P (Phosphorus)

Sn (Tin)

Tl (Thallium)

Na (Sodium)

Ti (Titanium)

Let's analyze each:

Oxygen (O):

Oxygen typically shows multiple non-zero oxidation states, such as -2 (most compounds), -1 (peroxides), and even positive oxidation states in compounds like OF₂ (+2). Hence, it has more than one non-zero oxidation state.

Chlorine (Cl):

Chlorine exhibits a wide range of oxidation states: -1 (HCl), +1 (HOCl), +3 (HClO₂), +5 (HClO₃), and +7 (HClO₄). Clearly multiple non-zero states.

Fluorine (F):

Fluorine is the most electronegative element and almost always exhibits an oxidation state of -1 in its compounds. It does not show other stable non-zero oxidation states. Thus, fluorine has only one non-zero oxidation state: -1.

Nitrogen (N):

Nitrogen shows many oxidation states: from -3 (ammonia, NH₃) to +5 (in nitrates, NO₃⁻), passing through several intermediate states (+1, +2, +3, +4). Many non-zero oxidation states.

Phosphorus (P):

Phosphorus can exist in -3 (PH₃), +3 (PCl₃), and +5 (PCl₅) states. Multiple non-zero oxidation states.

Tin (Sn):

Tin commonly shows +2 and +4 oxidation states. Hence more than one non-zero oxidation state.

Thallium (Tl):

Thallium typically shows +1 and +3 oxidation states. Again, multiple non-zero oxidation states.

Sodium (Na):

Sodium, in stable compounds, is almost always present as Na⁺ (oxidation state +1). It does not commonly display any other non-zero oxidation state. So sodium has only one non-zero oxidation state.

Titanium (Ti):

Titanium can exhibit +2, +3, and +4 oxidation states, among others. Multiple non-zero oxidation states.

Conclusion:

Only fluorine (F) and sodium (Na) have exactly one non-zero oxidation state.

Number of such elements = 2