ionisation energy

• First ionisation energy

  The energy required to remove 1 mole of electrons from 1 mole of gaseous atoms
• first ionisation energy equation

  X(g) -> X+(g) + e-
• 3rd ionisation energy

  The energy required to remove 1 mole of electrons from 1 mole of gaseous 2+ ions
• 3rd ionisation energy equation

  X2+(g) -> X3+(g) + e-
• X axis of ionisation energy graph

  Number of electrons removed
• Y axis of ionisation energy graph

  log10 (ionisation energy)
• Ionisation is what type of process?

  Endothermic as it requires energy so it will have a positive value
• What is the value of ionisation energy based on?

  The electrostatic attraction between the nucleus and the outermost electron(s)
• The electrostatic attraction between positively charged nucleus and the outermost negatively charged electrons is affected by

  -distance from nucleus (atomic radius)

  -number of protons (nuclear charge)

  -shielding from inner energy levels
• Shielding

  The more energy levels between the positive nucleus and the negative electron that is being removed, the less energy is required because there is a weaker attraction
• Atomic radius

  -the bigger the atom, the further away the outer electrons are from the nucleus so there are weaker forces of attraction between the nucleus and outer electrons so less energy is required to remove the electron so electron is easier to remove
• Nuclear charge

  The more protons in the nucleus, the bigger the forces of attraction between the nucleus and the outermost negatively charged electrons so more energy is required to remove the electron
• Successive ionisation energy

  The removal of more than one electron from the same atom
• Why are successive ionisation energies greater than the previous ones?

  -the outer electron is being removed from a more positive species and it is also slightly closer to the positive nucleus

  -this means that there is stronger electrostatic attraction on the remaining electrons

  -this means that more energy is required to overcome the attraction
• Why does 1st Ionisation energy significantly increase in Jump from one energy level to another

  -This is because there is a big decrease in shielding and the outer electron is much closer and more strongly attracted to the positively charged nucleus

  -this means that there is stronger electrostatic attraction

  -this means that more energy is required to overcome the attraction
• Why does the 1st ionisation energy decrease down a group?

  -Despite the increase in nuclear charge (number of protons increases), there is more shielding (number of energy levels increases) and the distance from the outermost electron to the nucleus increases (atomic radius increases). Therefore there are weaker forces of attraction between the nucleus and outermost electron.

  -it also proves Niels Bohr model of the atom is correct and provides strong evidence for shells
• Why does ionisation energy generally increase across a period?

  Because shielding stays the same, nuclear charge increases (number of protons increases) and distance of outermost electron to nucleus decreases (atomic radius decreases). Therefore there are stronger forces of attraction between the nucleus and outermost electron.
• Exceptions for ionisation energy trends

  Group 3 and 6 are lower than expected
• Evidence for atoms having sublevlels

  1st ionisation energy decreases at aluminium
• Evidence for electron repulsion in an orbital

  1st ionisation energy decreases at sulfur
• Why does the group 3 element have a lower first ionisation energy than expected/( the group 2 element)

  Group 3 has an outermost electron in p sublevel (group 2 has 2 electrons in s sublevel and ends in s2). Despite having a greater nuclear charge, it has a lower 1st ionisation energy because p sublevel is higher in energy (further from the nucleus) than s sublevel. Therefore there are weaker forces of attraction between the nucleus and outermost electron.
• Why does group 6 have a lower first ionisation energy than expected/( the group 5 element)

  Because there is a pair of electrons in the p orbital; these undergo mutual repulsion. The electron repulsion means less energy is needed to remove an electron from an orbital with 2 electrons in than one with 1 electron in.
• 2nd ionisation energy pattern

  Same pattern, Shifts one to the left