An isotope of the element fluorine has 9 protons and 10 neutrons. What is the name of this isotope?
fluorine

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.

Ido not understand. 

The answer is 4 i don't know if its right let me see.