Short answer: 27-13=14 neutrons. Long answer: This is because the atomic number tells you the number of protons and the atomic mass is telling you the overall mass of the atom. The atomic number is important because it distinguishes specific atoms and their locations on the periodic table. Protons have a +1 charge and electrons have a negative charge. On the periodic table, all atoms are neutral this means 0 net charge per atom. That also means protons must equal electrons in a 1:1 ratio because electrons provide a -1 charge. Roughly speaking, protons and neutrons way around 1 amu where electrons weigh a lot less. Why is all this important? It’s important because this should give you an idea as to why you can get the number of neutrons by subtracting the atomic mass by the atomic number.