Atomic Number and Mass Number

• Atomic number


  The number of protons in the nucleus of an atom
• Mass number


  The total number of protons and neutrons in the nucleus of an atom
• Atoms have no overall charge because the number of electrons is the same as the number of protons
• Information given in the periodic table

  • Element symbol
  • Atomic number
  • Mass number
• Calculating the number of neutrons


  Subtract the atomic number from the mass number
• Atoms of different elements

  • Boron: 4 protons, 5 neutrons, 4 electrons
  • Sodium: 11 protons, 12 neutrons, 11 electrons
  • Fluorine: 9 protons, 10 neutrons, 9 electrons
  • Phosphorus: 15 protons, 16 neutrons, 15 electrons
• Isotopes


  Atoms of the same element with different numbers of neutrons
• Isotopes of carbon


  • 6 protons, 6 neutrons
  • 6 protons, 7 neutrons
  • 6 protons, 8 neutrons
• Isotopes of chlorine


  • 17 protons, 18 neutrons
  • 17 protons, 20 neutrons
• Ions


  Atoms which have an overall charge due to losing or gaining electrons
• Sodium ion


  • 11 protons, 12 neutrons, 10 electrons (1+ charge)
• Oxide ion


  • 8 protons, 8 neutrons, 10 electrons (2- charge)
• Aluminium ion


  • 13 protons, 14 neutrons, 10 electrons (3+ charge)