Ionisation Energies

• Ionization energy generally decreases down a group on the periodic table.
• First ionisation energy 


  the energy required to remove one electron from each atom in one mole of gaseous atoms to make one mole of gaseous 1+ atoms.
• Atomic radius


  The distance between the outer shell electrons and the nucleus, meaning less nuclear attraction, so outer shell electrons are easier to lose. Additionally, the distance between the nucleus and the bonding electrons of another atom, making it harder to share electrons as the attraction is weaker.
• Shielding

  The inner shell electrons repel the outer shell electrons, reducing the attraction between outer electrons and the nucleus.
• Nuclear charge

  The more protons there are in the nucleus, the greater the attraction between negative electrons and the positive nucleus.
• Second ionisation energy is always greater than first ionisation energy because there are more protons than electrons in a positive ion, so there is greater attraction between the nucleus and the outer shell electrons.
• First ionisation energy of oxygen is lower than the first ionisation energy of nitrogen.
• The first ionisation energy of boron is lower than the first ionisation energy of beryllium.
• Boron has a lower first ionisation energy than beryllium because the 2p subshell has higher energy than the 2p subshell of beryllium. Therefore, the 2p electron in boron is easier to remove than one of the 2s electrons in beryllium.
• Oxygen has a lower first ionisation energy than nitrogen as, in oxygen, there has to be one electron pair in the 2p subshell. The paired electrons in the 2p orbital repel other electrons, making it easier to remove.
• Who made the Periodic Table?

  Dmitri Mendeleev
• The modern periodic table arranges elements by proton number.
• A high ionisation energy means that there is a strong attraction between the electron and the nucleus, so more energy is needed to overcome the attraction and remove the electron.
• Decrease of ionisation energy down a group

  Greater shielding; greater atomic radius.
• Ionisation energy increases across a period

  Nuclear charge is increasing; no change in shielding or radius.