Here's why:

* Atomic Number: The atomic number of an element represents the number of protons in its nucleus.

* Nitrogen: The element nitrogen (N) always has 7 protons.

* Isotopes: Nitrogen-15 is an isotope of nitrogen. Isotopes of an element have the same number of protons but differ in the number of neutrons. The "15" in nitrogen-15 refers to its mass number (protons + neutrons).

So, even though nitrogen-15 has a different number of neutrons than regular nitrogen (nitrogen-14), it still has the same number of protons, and thus the same atomic number (7).