The mass number,
In other words, the mass number tells you how many protons and neutrons are located inside the nucleus of a given atom.
#color(blue)(ul(color(black)("mass number = no. of protons + no. of neutrons")))#
As you know, the number of protons present inside the nucleus is given by the atomic number,
You can thus rewrite the above equation as
#color(blue)(ul(color(black)(A = Z + "no. of neutrons")))#
Take, for example. silver,
As you can see, silver has an atomic mass of
In this case, you'd have
#107.87 ~~ 108 ->#rounded to the nearest whole nubmer
This means that the most common isotope of silver is silver-108. This isotope has a mass number equal to
You know that silver has an atomic number equal to
#A = Z + "no. of neutrons" implies "no. of neutrons" = A - Z#
you can say that the silver-108 isotope has
#"no. of neutrons" = 108 - 47 = 61#
neutrons located inside its nucleus.