electrons in the outer shell. neutrons in the outer shell. electrons in the nucleus. protons in the nucleus.
I gave my students this multiple choice question: The atomic number of an element refers to the number of

Only one answer is correct among these 4 choices:
- electrons in the outer shell.
- neutrons in the outer shell.
- electrons in the nucleus.
- protons in the nucleus.

Could you tell me which one is correct?

protons in the nucleus.