How is the mass number calculated for an element?

The atomic mass number for an element is calculated by adding all of the element's protons and neutrons together. While the atomic mass number for any given element will always be a whole number, atomic mass numbers which appear on periodic tables are averages. Since elements can have isotopes with a different number of neutrons, the atomic masses of an element and its isotopes get averaged together. 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

The mass number of an element is equivalent to the number of Protons and Neutrons in the element. For example Oxygen has eight protons and neutrons, therefore its mass number is 16. You could also calculate this by simply rounding the atomic weight of the element to the nearest whole number. read more

The atomic mass of carbon would be 12.01 grams per mole of carbon atoms. 2) Sum of Protons and Neutrons for a Single Atom. 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. read more