Ionisation Energy & Electronic Configuration (Cambridge International AS Chemistry)

Ionisation Energies: Electronic Configuration

Using Successive Ionisation Energy Data

• Each ionisation energy refers to the energy required to remove one mole of electrons from one mole of a specific species in the gaseous state

  • First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms
  • Successive ionisation energies are the energy required to remove one mole of electrons from one mole of gaseous positive ions

• As electrons are removed, the ion becomes more positively charged

  • This increases the electrostatic attraction between the nucleus and remaining electrons
  • Therefore, ionisation energies increase progressively

• A large jump in successive ionisation energies indicates that:

  • An electron is being removed from a new, inner shell closer to the nucleus
  • These electrons experience a stronger attraction to the nucleus and require more energy to remove

How to deduce electronic configuration

• Look for the largest increase between two successive ionisation energies
• The number of electrons removed before this jump = number of outer-shell (valence) electrons
• For Groups 1 and 2 this is the group number; for Groups 13 to 18 it is the group number minus 10 (for example, 7 outer-shell electrons means Group 17)

Worked Example Deduce the electronic configuration of element Z, a Period 3 element, using successive ionisation energy data.

IE	5th	6th	7th	8th
IE (kJ mol-1)	6542	9362	11,018	33,606

Answer

• You are told that the element is in Period 3

• Period 3 elements have electrons up to the 3rd shell:

  • 3s and 3p subshells are included

• There is a large jump between the 7th and 8th IE, so element Z has 7 electrons in its outer shell and must be in Group 17

• So element Z is in Period 3 and Group 17:

  • This corresponds to chlorine, Cl

• The electronic configuration of Cl is:

  • 1s²2s²2p⁶3s²3p⁵