# How many neutrons in the ""^56Fe isotope?

""^56Fe has 30 neutrons.
How did I know this? Well, an iron atom has 26 protons, positively charged, massive nuclear particles. The number defines $Z$, the atomic number; $Z = 1 , H$; $Z = 2 , H e$; $Z = 3 , L i$;...........$Z = 26 , F e$.............
The nucleus also contains neutrons, massive, neutrally charged particles, and to a first approximation, the sum of protons and neutrons gives tha mass of the isotope. So if the question mentions iron, I know that $Z = 26$; it is thus simple to specify the isotopic mass if the number of neutrons is given.