The mass number is the sum of protons and neutrons. Since we have selenium-79, that means the sum of protons and neutrons add up to 79.
The atomic number of selenium is 34. Since atomic number represents number of protons, that will be 34.
Take the difference of the mass number and the atomic number to get the number of neutrons. So, 79-34 will get you 45 neutrons.
The number of electrons depends on whether the atom has a charge. If there is no charge provided, that means the atom is in its ground state, or most stable state. The number of electrons would be equal to the number of protons. Therefore, the number of electrons of selenium is 34.
