Ionisation Energy

• First Ionisation energy:

  The amount of energy(enthalpy) required to remove one mole of electrons from one mole atoms(g) in the gaseas state to produce one mole of unipositive cations (g) and one mole of electrons
• Second Ionisation Energy:

  The amount of energy required to remove one mole of electrons from one mole of unipositive cations of the gaseas state to form one mole of dipositive cations in the gaseas state and one mole of electrons
• Factors affecting Ionisation energy:

  Atomic Radius - Distance between the nucleus and the outer electrons

  Nuclear charge - (Number of protons) the greater the number of protons the stronger attraction between the outer electrons and the nucleus

  Shielding - Outer shell electrons are repelled by electrons in the inner shell so more inner shell electrons = more repulsion = weaker attraction
• Successive ionisation energy of oxygen:

  Gradual increase in ionisation energy each time we remove and electron from the outer shell this is because the remaining electrons move slightly closer to the nucleus causing ionisation energy to increase

  
  

  There is a massive increase in ionisation energy as we remove the 7th electron this is because it is in the first shell therefore it has much smaller atomic radius so a greater attraction to the nucleus Furthermore, experience No shielding
• First Ionisation energy decreases as we go down a group:

  This is because as we go further down a group atomic radius increases, therefore a weaker attraction between nucleus and outer shell electrons

  Going down the group number of inner shells increases, therefore outer shell electrons experience more repulsion from inner electrons(shielding) causing a lower ionisation energy
• First ionisation energy increases as we go across a period:

  As we move across a period the nuclear charge increases meaning the number of protons in the nucleus increases meaning a stronger attraction between nucleus and outer shell electrons

  As we move across a period atomic radius decreases meaning a stronger attraction between nucleus and outer shell electrons

  There are a few exceptions of this trend
• Exceptions to the trend across a period group 2

  Boron has 2s2 2p1 whereas Li has 2s1 and Be has 2s2, Therefore boron has electron in the 2p sublevel which has a higher energy further away from nucleus therefore less energy required to remove an outer electron from boron

  Oxygen has a drop because compared to N, O2 has a pair of electrons with opposite spins (in p orbital), meaning they repel therefore it takes less energy to remove an outer electron from O2