Atomic Structure (AS)

• Isotopes are atoms of the same element with different numbers of neutrons.
• The mass number is equal to the sum of protons and neutrons.
• 1800 - John Dalton described atoms as solid spheres and said that different spheres made up the different elements.
• 1897 - J.J. Thomson discovered the electron. This showed that atoms weren't solid and indivisible. The 'solid sphere' idea of atomic structure was replaced with the 'plum pudding model'.
• 1911 - Ernest Rutherford conducted an experiment firing protons at a gold film and studied the deflection of protons. He changed the model so that atoms had a positive core and that electrons orbited the nucleus leaving empty space between.
• 1913 - Neils Bohr proposed that electrons orbit an atoms nucleus in particular orbits. This changed the atom model to add in different energy levels and 'layers' of electrons.
• 1932 - James Chadwick used atomic weight and charge to predict the existence of another sub atomic particle. He concluded that it had neutral charge and called them neutrons, which were very hard to find.
• Relative isotopic mass - mass of an atom of an isotope compared with 1/12th the mass of an atom of carbon-12.
• Relative atomic mass - the ratio of the average mass of an atom of an element to 1/12th of the mass of an atom of carbon-12
• Relative molecular mass - the ratio of the average mass of a molecule of an element or compound to 1/12th of the mass of an atom of carbon-12
• Electron shells are made up of sub-shells and orbitals.
• Work out electron configurations by filling the lowest energy levels first.
• Ionisation is the removal of one or more electrons.
• The drop in ionisation energy between groups 2 and 3 shows sub-shell structure.

  • Aluminium's outer electron is in a 3p orbital rather than a 3s.
  • The 3p orbital has a slightly higher energy than the 3s orbital, so the electron is to be found further from the nucleus.
• The drop in ionisation energy between groups 5 and 6 is due to electron repulsion.

  • In both phosphorus and sulfur atoms, the shielding is identical and the electron is being removed from an identical orbital (3p)
  • The electron being removed from phosphorus is on a singly-occupied orbital but in sulfur, the electron is being removed from an orbital containing two electrons.
  • The repulsion between the two electrons in Sulfur's orbital means the electrons are easier to remove.