Ionisation Energy & Electronic Configuration (CIE AS Chemistry)

• Energy is required to remove an outer shell electron as this involves breaking the attractive forces between the electron and the positively charged nucleus
• There are several factors which affect the magnitude of the ionisation energy:
• Nuclear charge
  • Positive nuclear charge increases with increasing number of protons
  • The greater the positive charge, the greater the attractive forces between the outer electron(s) and the nucleus
  • More energy is required to overcome these forces so ionisation energy increases with increasing nuclear charge

• Shielding
  • Electrons repel each other and electrons occupying the inner shells repel electrons located in shells further outside the nucleus and prevent them from feeling the full effect of the nuclear charge
  • The greater the shielding effect is, the weaker the attractive forces between the positive nucleus and the negatively charged electrons
  • Less energy is required to overcome the weakened attractive forces so ionisation energy decreases with increasing shielding effects

Diagram demonstrating nuclear attraction and shielding

Shielding makes it easier to remove the outermost electrons

• Atomic/ionic radius
  • The larger the radius, the greater the distance between the nucleus and the outer shell electron(s)
  • Increasing distance weakens the strength of the attractive forces
  • Larger atoms/ions also result in greater shielding due to the presence of more inner electrons
  • Less energy is required to remove the outer shell electron(s) so ionisation energy decreases with increasing atomic/ionic radius

• Spin-pair repulsion
  • Spin pair repulsion occurs when the electron being removed is spin paired with another electron in the same orbital
  • The proximity of the like charges of electrons in the orbital results in repulsion
  • Less energy is required to remove one of the electrons so ionisation energy decreases when there is spin-pair repulsion

Summary of factors affecting ionisation energies of atoms

Nuclear charge and shielding link to the atomic / ionic radius