module 3.1

• ionisation refers to the process of removing one or more electrons from an atom or molecule. this requires an input of energy, so ionisation is an endothermic process
• the first ionisation energy is the energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions

  Since ionisation requires energy input, the values for ionisation energies are always positive.
• nuclear charge - atoms with more protons in their nucleus have a stronger positive charge. this creates stronger electrostatic attraction between the nucleus and the outer electrons
• atomic radius: electrostatic attraction drop off steeply with increasing distance. electrons in smaller atoms are held closer to the nucleus so the attraction is greater
• electron shielding - inner electron shells cancel out some of the positive nuclear charge, reducing the attraction on the outer electrons. more electron shells provide more shielding
• Ionisation energy decreases down groups because:

  nuclear charge - increases down the group as more protons are added, increasing attraction for electrons
• Ionisation energy decreases down groups because:

  atomic radius: increases down the group as more electron shells are added, moving electrons away from the nucleus
• Ionisation energy decreases down groups because:

  electron shielding: increases down the group as more inner electron shells reduce nuclear attraction
• Ionisation energy generally increases across periods because:

  nuclear charge: increases as more protons added across periods
• Ionisation energy generally increases across periods because:

  atomic radius: decreases across periods as extra electrons added to the same shell
• Ionisation energy generally increases across periods because:

  electron shielding: stays similar across periods with no extra inner shells
• the atomic radius and shielding effects down groups are greater than the nuclear charge effect, leading to an overall decrease in ionisation energies as you move down a group
• the increasing nuclear charge effect outweighs the similar shielding across periods, so ionisation energies generally increase as you move across a period
• the drop between groups 2 and 3

  in group 3, the electron is removed from a p orbital rather than an s orbital like in group 2

  p orbitals have a slightly higher energy than s orbitals, so the outermost electron is on average further from the nucleus

  the p orbitals also experiences additional shielding from the nucleus provided by the s electrons

  as a result, less energy is required to remove the outermost p electron from the group 3 element compared to removing the outermost s electron from the group 2 element
• the drop between groups 5 and 6

  in group 5, the electron is removed from a singly occupied orbital

  in group 6, the electron is removed from an orbital containing 2 electrons

  the paired electrons in the group 6 element experience greater electron-electron repulsion

  as a result, less energy is needed to remove one of these paired electrons in the group 6 element compared to the unpaired electron in the group 5 element
• electrons can be sequentially removed until only the bare nucleus remains. the energy to remove each successive electron is called the successive ionisation energies

  e.g the second ionisation energy is the energy needed to remove 1 electron from each ion in 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions
• Second ionisation energy (IE2) of lithium

  Li+(g) → Li2+(g) + e− ΔHIE2 = +7,298 kJ mol−1
• as successive electrons are removed from the same shell, the remaining electrons experience greater electrostatic attraction to the increasingly positive nucleus

  this increased nuclear attraction requires more energy to remove the next electron from that shell

  for magnesium, the second ionisation energy is slightly higher than the first as these electrons are both being removed from the 3s subshell
• when reaching a new inner electron shell, there is a big increase in the ionisation energy needed to remove the first electron in that new shell. this happens because the attraction to the nucleus is much greater for inner shell electrons closer to the nucleus
• for magnesium, there is a large jump from the second ionisation energy to the third as the 3s subshell is now full, requiring the next electron to be removed from the inner 2p subshell