All oxygen atoms have 8 protons and all sodium ... Note that you can work out the number of neutrons by subtracting the atomic number from the mass number. In this example, it is 35 – 17 ...

For example, the mass number of argon atoms and calcium atoms can both be 40. The atomic number of a chlorine atom is 17 and its mass number is 35. Calculate the number of protons, neutrons and ...