Ionisation Energies

Ionisation Energies

1. Introduction

   • Measures how easily atom loses electrons to form positive ions
   • First Ionisation energy is the energy required to remove one electron from each atom in one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions.

2. Factors affecting Ionisation Energy

   • Electrons are held together in a shell by attraction from the nucleus
   • First electron lost will be the highest energy level
     • Experience least attraction from the nucleus
   • 1. Atomic Radius
     • Greater the distance = less nuclear attraction
       • Forces of attraction decrease sharply as distance increases
         • Atomic Radius has a large effect
   • 2. Nuclear Charge
     • More protons = greater the attraction between nucleus and outer electrons
   • 3. Electron Shielding
     • Electrons are negatively-charged
       • Inner-shell electrons repel each other
         • Repulsion causes a shielding effect
           • Reduces attraction between nucleus and outer electrons

3. Successive Ionisation Energies

   • Element has as many ionisation energies, as there are electrons
   • Example: Helium
     • Has two electrons and two ionisation energies
       • He --> He+ + e- (1st IE)
       • He+ --> He2+ + e- (2nd IE)
       • Second is greater than the first
     • In a helium atom, two protons are attracting two electrons in the 1s sub-shell
       • After 1st electron is lost, the single electron is pulled closer to the helium nucleus
         • Nuclear attraction on remaining electron increases
           • More IE needed to remove 2nd electron
   • Successive IE are defined in the same way as the first ionisation energy
   • Second IE is the energy required to remove one electron from each atom in one mole of gaseous 1+ ions of an element to form one mole of gaseous 2+ ions

4. Successive IEs & Shells

   • Provide important evidence for different electron energy levels in an atom
   • Large increase between 7th and 8th IEs
     • Suggest that 8th electron must be removed from a different shell
       • Closer to the nucleus with less shielding
         • First shell (n=1, closer to the nucleus) contains 2 electrons
         • Second shell (n=2, outer shell) contains 7 electrons
   • Making Predictions
     • Number of electrons in outer shell
     • Group of the element in the Periodic Table
     • Identity of the element

5. Trends in 1st IEs

   • Provide important evidence for the existence of shells and sub-shells
     • 1st IEs for first 20 elements
       • General increase in 1st IEs across each period
       • Sharp decrease in 1st IE between the end and beginnings between two periods

6. Trends in 1st IE down a group

   • Atomic radius increase
   • Nuclear attraction on outer electrons decreases
   • More inner shells so shielding increases
   • 1st IEs decreases

7. Trends in 1st IE across a period

   • Nuclear charge increases
   • Same shell; similar shielding
   • Nuclear attraction increases
   • Atomic radius increases
   • 1st IE increases

8. Sub-shell trends in 1st IE

   • Shows general increases across both Period 2 & 3
     • Does drop in certain places
       • Same places = periodic causes
         • Links to sub-shells and orbital-filling
   • Across Period 2
     • Rise from Li to Be
     • Fall to B
     • Rise to C & N
     • Fall to O
     • Rise to F & Ne

9. Comparisons

   • Be to B
     • Fall in IE
       • Start and filling of 2p sub-shell
       • Easier to remove 2p (B)
         • Compared to 2s (Be)
   • N & O
     • Highest energy level in 2p
     • Pairs repel one another
       • Easier to remove electron from oxygen atom than Oxygen
     • 1st IE of Oxygen < 1st IE of Nitrogen