Electron structure

• The further a shell (energy level) is from the nucleus, the higher its energy, and the larger its principal quantum number.
• Not all electrons in an energy level have exactly the same energy, so they are divided into sub-shells.
• Sub-shells have different numbers of orbitals- each orbital can hold up to 2 electrons:

  • s subshell- 1 orbital- 2 electrons
  • p subshell- 3 orbitals- 6 electrons
  • d subshell- 5 orbitals- 10 electrons
• An atom fills up the sub shells:

  • in order of increasing energy (3d is higher in energy than 4s)
  • electron orbitals in a sub-shell singly before pairing
• when ionising- add/remove an electron from the highest energy occupied sub-shell.
• 1st Ionising energy- the energy needed to remove 1 mole of electrons from 1 mole of gaseous atoms.

  M(g) -> M+(g) + e-
• Factors affecting IE:

  • Nuclear charge- more protons lead to a stronger attraction of positive nucleus and outer electrons
  • Distance from nucleus- an electron closer to nucleus will be much more strongly attracted
  • Sheilding- the number of electrons between outer electrons and nucleus increases, outer electrons feel less attraction as outer electrons repelled by electrons in complete inner shells.
• 2nd IE- the energy needed to remove 1 mole of electrons from 1 mole gaseous 1+ ions.

  M+(g) + e- -> M2+(g) + 2e-
• 2nd IE are greater than 1st as the electron is being removed from a positive ion.
• 2nd IE of group 1 elements are higher than others as the 2nd electron removed comes from a shell closer to the nucleus so there is a stronger attraction.
• IE decreases down a group...

  • electrons in shells further from the nucleus (distance increases)
  • more shielding
  • attraction decreases
• IE generally increases across a period...

  • same distance and shielding
  • number of protons increases
  • attraction increases
• Drop between groups 2-3... (e.g. Al and Mg)

  • Al outer electrons in 3p orbital, Mg in 3s
  • 3p has slightly higher energy than 3s
• Drop between groups 5-6... (e.g. P and S)

  • in P, electrons removed from a singly-occupied orbital
  • in S, electrons removed from orbital with 2 electrons
  • repulsion between the pair in an orbital means its easier to remove from shared orbitals