Ionisation energies

• What is the first ionisation energy?

  The energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.
• Ionisation energy rules

  The gas state symbol must be used (g) because ionisation energies are measured for gaseous atoms.

  Always refer to 1 mole of atoms rather than a single atom.

  The lower the ionisation energy, the easier it is to form a positive ion.
• What are the factors affecting ionisation energy?

  Nuclear charge

  Distance from nucleus

  Shielding
• How does nuclear charge affect ionisation energy?

  The more protons there are in the nucleus, the more positively charged the nucleus is and the stronger the attraction for the electrons.
• How does distance from the nucleus affect ionisation energy?

  Attraction falls off very rapidly with distance. An electron close to the nucleus will be more strongly attracted that one further away.
• How does shielding affect ionisation energy?

  As the number of electrons between the outer electrons and the nucleus increases, the outer electrons feel less attraction to the nucleus. This lessening of the pull of the nucleus thanks to the inner electron shells is called shielding
• What does a high ionisation energy mean?

  There’s a high attraction between the electron and the nucleus, so more energy is needed to remove the electron
• What is the second ionisation energy?

  The energy needed to remove an electron from each ion in 1 mole of gaseous 1+ ions, forming 1 mole of gaseous 2+ ions
• Successive ionisation energies

  You can remove all the electrons from an atom, leaving only the nucleus. Each time you remove an electron, there’s a successive ionisation energy.
• Ionisation trends down group 2

  First ionisation energy decreases down group 2.

  This provides evidence that electron shells exist.
• Ionisation trends across period 3

  As you move across period 3, the general trend is for the ionisation energies to increase.
• Ionisation energies and shell structure

  Within each shell, successive ionisation energies increase. This is because electrons are being removed from an increasingly positive ion, there’s less repulsion between electrons and more between the electrons and the nucleus.

  Big jumps in ionisation energy happen when a new shell is broken into.
• How to find group number from the ionisation energies

  Count how many electrons before the first big jump