Atomic number and mass number

• Atomic number

  • the number of protons in the nucleus of an atom
  • also called the proton number
  • all atoms of a given element have the same number of protons
  • atoms of different elements have different numbers of protons
• • an atom contains equal number of protons and electrons
  • protons and electrons have equal and opposite charges
  • so atoms have no overall electrical charge
• Mass number

  • the number of protons and neutrons found in the nucleus of an atom
  • atoms of different elements usually have different mass numbers, but they can be the same
• Calculating numbers of subatomic particles

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