for al, its atomic number is 13 and its mass number is 27. how many neutrons does it have?

 the atomic number is the number of protons in an atom, the mass number is the number of protons and neutrons, the number o protons is the difference between the mass number and the atomic number,  N =A-Z. and for the above atom, the number of neutrons is 27-13=14 neutrons.