When the mass number of oxygen is 16, how many neutrons does oxygen have in its nucleus?
Mass number equals number of sub atomic particles in the nucleus.
Atomic number gives the number of protons, which equals the number of electrons. The atomic number for oxygen is 8, so the atom has 8 electrons and 8 protons.
Protons and neutrons are found in the nucleus, so the number of neutrons is found by subtracting the proton number from the mass number I.e. 16 - 8 = 8
Oxygen exists in isotopic form and has an isotope with mass number 18. This atom will have 10 neutrons as 18 - 8 = 10