Question

An electrically neutral atom has an atomic mass of 30.973 and an atomic number of 15. How many neutrons does this atom have?_______________

Answer 1

Answer: 16 neutrons

Explanation: The atomic number for any element is the number of protons in its atomic nuclei. In a neutral atom, the number of electrons are the same as the number of protons. The atomic mass (mass number) is the sum of the protons and neutrons in the nucleus. Mass number can vary due to the fact that the number of neutrons can vary. Isotopes are named for their mass numbers.

So, this element has 15 protons (atomic number), 15 electrons, and 16 neutrons (mass number minus atomic number).

This isotope is phosphorus-31, which has atomic number 15.