How do you find the mass number of an atom?

The hydrogen atom may contain a neutron, but it usually doesn't, so the mass number of hydrogen is 1. Oxygen, on the other hand, has an equal number of proteins and neutrons, which raises its mass number to 16. Subtracting an element's mass number from its atomic mass tells you the number of protons in its nucleus. read more

To calculate the atomic mass of a single atom of an element, add up the mass of protons and neutrons. Example: Find the atomic mass of an isotope of carbon that has 7 neutrons. You can see from the periodic table that carbon has an atomic number of 6, which is its number of protons. read more

Using the periodic table of elements, find the atomic number of the element with mass numbers that need to be calculated. The atomic number of an element is equivalent to its number of protons. For example, the atomic number of sodium is 11. read more