Atomic number and mass number

• Atomic number

  The atomic number of an atom is the number of protons in its nucleus. Remember that:

  • all the atoms of a given element have the same atomic number
  • the atomic number of each element is unique - no two elements have the same atomic number

  An atom contains an equal number of protons and electrons. Since protons and electrons have equal and opposite charges, this means that atoms are neutral overall.
• The mass number of an atom is the total number of protons and neutrons in its nucleus.
• To calculate the numbers of subatomic particles in an atom:

  • number of protons = atomic number
  • number of electrons = atomic number
  • number of neutrons = mass number - atomic number