Elements showing only one non-zero oxidation state
- The elements that show only one non-zero oxidation state are those that have a stable electron configuration and do not readily gain or lose electrons to form multiple oxidation states.

Explanation of elements with one non-zero oxidation state
- Oxygen (O): Oxygen typically has an oxidation state of -2 in most of its compounds, as it tends to gain two electrons to achieve a stable octet configuration. However, in some compounds, such as peroxides (e.g., H2O2), oxygen can have an oxidation state of -1.
- Chlorine (Cl): Chlorine usually has an oxidation state of -1 in its compounds, as it tends to gain one electron to achieve a stable octet configuration. However, in some compounds, such as hypochlorites (e.g., NaClO), chlorine can have an oxidation state of +1.
- Fluorine (F): Fluorine typically has an oxidation state of -1 in its compounds, as it tends to gain one electron to achieve a stable octet configuration. It rarely exhibits any other oxidation state.
- Nitrogen (N): Nitrogen can have multiple oxidation states, including -3, -2, +1, +2, +3, +4, and +5. However, in most of its compounds, nitrogen has an oxidation state of -3 or +3, depending on the compound.
- Phosphorus (P): Phosphorus can have multiple oxidation states, including -3, +1, +3, and +5. However, in most of its compounds, phosphorus has an oxidation state of +5.
- Tin (Sn): Tin can have multiple oxidation states, including -4, +2, and +4. However, in most of its compounds, tin has an oxidation state of +2.
- Thallium (Tl): Thallium can have multiple oxidation states, including +1 and +3. However, in most of its compounds, thallium has an oxidation state of +1.
- Sodium (Na): Sodium typically has an oxidation state of +1 in its compounds, as it tends to lose one electron to achieve a stable octet configuration. It rarely exhibits any other oxidation state.
- Titanium (Ti): Titanium can have multiple oxidation states, including +2, +3, and +4. However, in most of its compounds, titanium has an oxidation state of +4.

Conclusion
Among the given elements, only Oxygen and Sodium show only one non-zero oxidation state. Oxygen has an oxidation state of -2 in most compounds but can also exhibit an oxidation state of -1 in peroxides. Sodium has a fixed oxidation state of +1 in its compounds.