If an atom has an atomic number of 15 and an atomic mass of 31, how many protons, neutrons and electrons does the atom contain?
This is the isotope phosphorus-31. It has 15 protons, 15 electrons, and 16 neutrons.
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.