Electron arrangement

Electron arrangement

1. Shells

   • Electrons are arranged in energy levels (electron shells)
     • Shells have sub-shells

2. Sub-shells

   • Each shell/sub-shell is made up of electron orbitals which can each hold 2 electrons

3. Orbitals

   • Each sub-level consists of electron orbitals (region of space in which the electron spends most of its time)
     • Orbitals are regions of space that electrons are MOST LIKELY to be in
   • Each orbital can hold 2 electrons with opposite spins (one electron spins clockwise and one anti-clockwise
   • When we work out the order in which orbitals are filled, you MUST ALWAYS consider the Hund's Rule and the Aufbau Principle!
     • Hund's Rule: Electrons prefer to occupy orbitals on their own, and only pair when no empty orbitals of the same energy are available.
     • Aufbau Principle: Electrons enter the LOWEST energy orbital available first.

4. Ions

   • The highest energy electrons are lost when an ion is formed.
     • Exception: 4s electrons are lost before 3d!

5. Ionisation energy

   • Evidence for how the electrons are arranged in atoms comes from ionisation energies
   • 1st ionisation energy = Energy required to remove one mole of electrons from an atom in its gaseous state.
     • As a result, as you go down a group atoms get bigger, there is more shielding and therefore weaker attraction from nucleus to electron in outer shell!
       • General trend in the 1st IE: as you move across a period, there is an increased nuclear charge (i.e. more protons), atoms get smaller and results in much stronger attraction from nucleus to electrons in outer shell.