Ionisation Energy

Ionisation Energy

1. Definition

   • When atom loses electron it becomes positive ion. Been ionised. Energy is needed to remove electrons.
   • The 1st ionisation energy of element is amount of energy needed to remove 1 electron from each atom in mole of atoms of element in gaseous state.
   • General symbol for ionisation energy is ΔHi + for 1st ionisation energy it is ΔHi1.
     • Process may be shown by example of calcium as: Ca(g) → Ca⁺ (g) + e⁻
     • ∆Hi1 = +590kJmol⁻¹

2. Elements

   • Energy needed to remove 2nd electron from each ion in mole of gaseous ions is second ionisation energy. For calcium:
     • Ca⁺(g) → Ca²⁺(g) + e⁻; ∆Hi₂ = +1150kJmol⁻¹
     • 2nd ionisation energy much larger than 1st. Can continue removing electrons until only nucleus of atom is left.
   • See following for any 1 element.
     • Successive ionisation energies increase. As each electron is removed from an atom, remaining ion becomes more positively charged. Moving next electron away from increased positive charge is more difficult + next ionisation energy is even larger.
     • 1/more particularly large rises within set of ionisation energies of each element (except hydrogen + helium).
     • Ionisation energies of elements measured mainly by:
       • Using calculations based on light emitted by element when it's heated strongly, or when an electric current is passed through it in gaseous state.
       • Using electron bombardment of element in gaseous state.
       • Data may be interpreted in terms of atomic no.s of elements + their simple electronic configurations.

3. Factors influencing ionisation energies

   • Size of positive nuclear charge
     • Charge affects all electrons in an atom. Increase in nuclear charge with atomic no. will tend to cause increase in ionisation energies.
     • More protons. greater nuclear charge. greater attraction felt by outer electrons.
   • Distance of electron from nucleus
     • All forces of attraction decrease rapidly as distance between attracted bodies increases. Thus, attraction between nucleus of an atom + electron is less for electrons nearer to nucleus.
     • Further a shell is from nucleus, lower are the ionisation energies for electrons in that shell.
     • More shells an atom has + therefore further the outer electrons are from nucleus, less attraction the experience.
   • Shielding effect by electrons in filled inner shells
     • All electrons negatively charged + repel each other. Electrons in filled inner shells repel electrons in outer shell + reduce effect of positive nuclear charge.
     • Shielding effect upon electron, lower the energy required to remove it + thus lower ionisation energy.
     • Inner shells of electrons repel outer ones. More inner shells, larger effect + lower attraction felt by outer electrons.