Ionisation Energies - CGP

• What is ionisation?

  Removing electrons from an atom or molecule
• What is the first ionisation energy?

  Energy needed to remove one electron from an atom
• How is the first ionisation energy defined?

  For 1 mole of gaseous atoms forming 1 mole of ions
• Write the equation for the first ionisation of oxygen.

  O(g) → O+ (g) + e-
• Why must the gas state symbol (g) be used in ionisation energies?

  Because ionisation energies are measured for gaseous atoms
• What does a lower ionisation energy indicate?

  It is easier to form a positive ion
• What factors affect ionisation energy?

  • Nuclear charge: More protons increase attraction
  • Distance from nucleus: Closer electrons are more attracted
  • Shielding: Inner electrons reduce attraction for outer electrons
• How does nuclear charge affect ionisation energy?

  More protons increase attraction for electrons
• What happens to attraction as distance from the nucleus increases?

  Attraction falls off rapidly with distance
• What is shielding in the context of ionisation energy?

  Inner electrons reduce attraction for outer electrons
• Why is the second ionisation energy higher than the first?

  Removing an electron from a positive ion requires more energy
• Write the equation for the second ionisation of oxygen.

  O+(g) → O2+(g) + e-
• What is the general equation for the nth ionisation?

  X^((n-1)+)(g) → X^(n+)(g) + e^(-)
• Write the equation for the fifth ionisation of oxygen.

  O^(4+)(g) → O^(5+)(g) + e^(-)
• What is the trend of first ionisation energy down Group 2?

  • First ionisation energy decreases down Group 2
  • Extra electron shells shield outer electrons
  • Outer electrons are further from the nucleus
• What is the trend of first ionisation energy across Period 3?

  • First ionisation energy generally increases
  • More protons lead to stronger nuclear attraction
  • Little extra shielding effect across the period
• What causes the drop in ionisation energy between Groups 2 and 3?

  Sub-shell structure affects ionisation energy
• Why does the first ionisation energy of aluminium drop compared to magnesium?

  Aluminium's outer electron is in a higher energy 3p orbital
• What causes the drop in ionisation energy between Groups 5 and 6?

  Electron repulsion makes electrons easier to remove
• How can successive ionisation energies provide evidence for shell structure?

  • Successive ionisation energies increase within each shell
  • Big jumps occur when removing electrons from a new shell
  • Graphs show how many electrons are in each shell
• How can you determine the group number from successive ionisation energies?

  Count electrons removed before the first big jump
• What happens to ionisation energy as more electrons are removed?

  • Successive ionisation energies increase
  • Electrons are held more strongly by the nucleus
  • Less repulsion among remaining electrons
• What do big jumps in ionisation energy indicate?

  • A new electron shell is being broken into
  • Indicates a change in electron configuration
  • Provides evidence for the shell structure of atoms