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 = 18 neutrons.

For example, the formula for carbon dioxide is CO 2. It contains one carbon atom and two oxygen atoms. Using the mass numbers from the periodic table, the A r of carbon is 12, and the A r of ...

For example, a helium atom has an atomic mass of 4, but an atomic number (or positive charge) of 2. Since electrons have almost no mass, it seemed that something besides the protons in the nucleus ...