Atomic Number and Atomic Mass

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  • The illustration below shows the chemical symbol for the hypothetical element "X".
  • The number of protons in the nucleus is represented by "Z", the atomic number
    • All the isotopes of an element have the same "Z"
  • The atomic mass of the element (number of protons plus the number neutrons) is represented by "A"
    • "A" is usually place to the left above the element symbol
  • The number of neutrons in the nucleus is equal to A minus Z

chemical symbol

  • Two different forms, or isotopes, of carbon are shown below:
    • Carbon-12: with 6 protons and 6 neutrons and an atomic mass of 12
    • Carbon-14: with 6 protons and 8 neutrons, and an atomic mass of 14
carbon isotopes

Adapted from Understanding Radiation (EPA)