The number of neutrons in the nucleus of an atom can be determined by
(1) adding the atomic number to the mass number
(2) subtracting the atomic number from the mass number
(3) adding the mass number to the atomic mass
(4) subtracting the mass number from the atomic number

(2) subtracting the atomic number from the atomic mass number, because the mass number is the total number of protons and neutrons in an atom and the atomic number is the number of protons in an atom.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.

Hcn is the correct answer leave a good rate and .