Answer: By definition, if the atomic number = 7 , then there are 7 protons, 7 electrons, and 8 neutrons!
Step-by-step explanation:
The atomic number is by definition the number of protons, positively charged particles, contained within the atomic nucleus. So if there are 7 protons, there MUST be 7 electrons, 7 negatively charged particles. Why? Because matter is electrically neutral, and positive and negative particles must be equal.
Electrons have negligible mass; the mass number depends on the number of protons contained within the nucleus, PLUS the number of neutrons. Given that we have 15 N , there must be 8 neutrons contained within the nucleus. does this help?