Silicon (Si) atomic mass is 28.085. To find the mass number of most common isotope we need to round atomic mas to wholes. 28.085≈28 is atomic mass Atomic number(number of protons) is an ordinal number of an element in the Periodic Table. For silicons it is 14. Mass number=number of protons(atomic number)+number of neutrons. Number of neutrons=Mass number - Number of Protons(atomic number) Number of neutrons= 28 - 14 = 14
