- 1). Write down the number of protons in the nucleus of the element you are studying. The number of protons is the same as the atomic number of the element on the periodic table. Carbon, for example, has atomic number 6 and hence six protons in its nucleus.
- 2). Write down the number of neutrons. This depends on the isotope you chose to study. Carbon-13, for example, has seven neutrons.
- 3). Add the number of neutrons to the number of protons to find the nominal mass or mass number. The mass number of carbon-13, for example, is 13. Keep in mind that owing to the binding energy for nucleons, the actual mass of carbon-13 will be very slightly different from the nominal mass. For most calculations the nominal mass should be sufficient.
- 4). Look up the exact atomic mass in the table at the National Institute of Standards and Technology Atomic Weights Web page if you need the exact atomic mass. This figure can only be determined experimentally.
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