Atoms and the Periodic Table
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The structure of the atom
  • An atom is mainly empty space
  • It is thought that atoms are made of even smaller particles called subatomic particles.


  • The centre of an atom
  • Contains protons and neutrons so is positively charged
  • Occupies a small volume of the atoms, but is very dense


  • Electrons orbit the nucleus in energy levels or shells
  • Carry a negative charge and are held in orbit by electrostatic attraction
    Atomic or Proton number
  • Number of protons in the nucleus
  • The number of protons in an atom determines what element it is
  • No. of protons (+) = No. of electrons (-) so an atom has no overall charge on it
    Mass number
  • The total number of protons and neutrons found in the nucleus. Electron arrangement in an atom
  • Electrons move very fast around the nucleus in energy levels or shells
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  • The innermost energy level can only hold 2 electrons
  • The next energy level can hold up to 8 electrons
  • The third energy level can hold up to 18 electrons
  • The electrons always fill the lowest available energy levels, so the innermost available shells are always filled before the outer ones.
  • The way the electrons are arranged is called the electron structure or electron configuration.
  • Atoms with a full outer shell of electrons are very unreactive.
  • Atoms of the same element which have different numbers of neutrons.
  • Most isotopes are stable, but a few have unstable nuclei.
  • If a nucleus has too many or too few neutrons it may split up or disintegrate.
  • If the unstable nucleus breaks up, the isotope is said to be radioactive.

  • When the nucleus breaks up it is said to decay, so the process is often referred to as radioactive decay.


    • Elements in the same group have similar properties because they have a similar electron arrangement
    • Group I or Alkali metals - Elements whose atoms have 1 outer-shell electron
    • Group II or Alkaline Earth Metals - Elements whose atoms have 2 outer-shell electrons
    • Group III - Elements whose atoms have 3 outer-shell electrons
    • Group IV - Elements whose atoms have 4 outer-shell electrons
    • Group V - Elements whose atoms have 5 outer-shell electrons
    • Group VI - Elements whose atoms have 6 outer-shell electrons
    • Group VII or Halogens - Elements whose atoms have 7 outer-shell electrons
    • Group 0, sometimes called group 8 or Noble Gases - Elements whose atoms have full outer shells so they are very unreactive.

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