An isotope of the element gallium has a mass of 71 and an atomic number of 31. the number of electrons, protons, and neutrons respectively in a neutral atoms of this isotope is

31, 31, 40. An atom of _31^71Ga has 31 electrons, 31 protons, and

40 neutrons.

The atomic number is 31, so the atom has 31 protons (+).

If the atom is neutral, it must contain 31 electrons (-).

Mass number = protons + neutrons or  

A = p + n

71 = 31 + n

n = 71- 31 = 40 neutrons