For nuclide ¹⁵₇X, find proton and neutron numbers and identify the element.

Solution & explanation

1. Read atomic number Z = 7 and mass number A = 15.

2. Proton number equals Z.

   $$p=7$$

3. Mass number counts protons plus neutrons.

   $$n=A-Z$$

4. Subtract.

   $$n=15-7=8$$

5. Z = 7 identifies nitrogen, so the isotope is nitrogen-15.

6. The printed phrase “nitrogen-7” confuses atomic number with isotope mass number.

7 protons; 8 neutrons; nitrogen-15