15. An electrically neutral atom consists of 15 neutrons, 13 electrons, and a number of protons. What is its mass number?

The charged subatomic particles in an atom are protons and electrons.

For an atom to be electrically neutral, the number of protons must be equal to the number of electrons to cancel out the charge exerted by the electrons.

Therefore, if the atom has 13 electrons, then the atom also has 13 protons. In other words, the atom has an atomic number of 13.

The mass number of an atom is the sum of the number if the atomic number or number of protons and the number of neutrons. Therefore, it’s mass number is 13+15=28.