A researcher wants to experiment with an element that reacts like Nitrogen (N) but had a greater atomic mass. Which element should she select?

If the researcher is looking for an element that reacts similarly to nitrogen (N) but has a greater atomic mass, the best choice would be Arsenic (As).

Here's a brief comparison of the elements listed:

- **Na** (Sodium): This is an alkali metal and does not have similar chemical properties to nitrogen.
- **S** (Sulfur): Although it is in a neighboring group in the periodic table, sulfur has different reactivity compared to nitrogen and has a lower atomic mass than arsenic.
- **As** (Arsenic): This element is in the same group (15) as nitrogen and can exhibit similar chemical behavior, especially in oxidation states like -3, +3, and +5. Arsenic has a greater atomic mass than nitrogen.
- **Si** (Silicon): This element is in group 14 and does not behave similarly to nitrogen.

Therefore, the researcher should select **Arsenic (As)**.