Endothermic Ionisation Energy (Edexcel International A Level Chemistry): Revision Note

Ionisation Energy - Definitions

Ionisation energy

• The ionisation energy (IE) of an element is the amount of energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous ions

• Ionisation energies are measured under standard conditions which are 298 K and 101 kPa

• The units of IE are kilojoules per mole (kJ mol^-1)

• The first ionisation energy (IE₁) is the energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions

  • E.g. the first ionisation energy of gaseous calcium:

Ca (g) → Ca⁺ (g) + e^-          IE₁ = +590 kJ mol^-1

Second and third ionisation energies of an element

• More than one electron can be removed form an atom and each time you remove an electron there is a successive ionisation energy

• These are called second, third ionisation energy and so on

• The second ionisation energy (IE₂) is defined as

  • is the energy required to remove one mole of electrons from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions

• And can be represented as can be represented as

X⁺ (g) → X²⁺ (g) + e^-

OR (for calcium)

Ca⁺ (g) → Ca²⁺ (g) + e^-  IE₂ = 1145 kJ mol^-1

• The third ionisation energy (IE₃) of an element can be represented as

X²⁺ (g) → X³⁺ (g) + e^-

OR (for calcium)

Ca²⁺ (g) → Ca³⁺ (g) + e^-  IE₃ = 4912 kJ mol^-1

Ionisation Energy - Considerations

Orbitals

• Subshells contain one or more atomic orbitals

• Orbitals exist at specific energy levels and electrons can only be found at these specific levels, not in between them

  • Each atomic orbital can be occupied by a maximum of two electrons with opposite spins

• This means that the number of orbitals in each subshell is as follows:

  • s : one orbital (1 x 2 = total of 2 electrons)

  • p : three orbitals ( 3 x 2 = total of 6 electrons)

  • d : five orbitals (5 x 2 = total of 10 electrons)

  • f : seven orbitals (7 x 2 = total of 14 electrons)

• Ionisation energies show periodicity - a trend across a period of the Periodic Table

• As could be expected from their electron configuration, the group 1 metals have relatively low ionisation energy, whereas the noble gases have very high ionisation energies

• The size of the first ionisation energy is affected by four factors:

• Size of the nuclear charge

  • The more protons there are in the nucleus, the more positively charged the nucleus is, and the more strongly electrons are attracted to it

• Distance of outer electrons from the nucleus

  • Attraction falls off very rapidly with distance. An electron close to the nucleus will be much more strongly attracted than one further away

•  Shielding effect of inner electrons

  • As the number of shells between the valence electron and positive nucleus increases, the shielding increases. It then becomes easier to remove the valence electron as the shielding increases. This can occur within an orbit due to the presence of different subshells

•  Spin-pair repulsion

  • Two electrons in the same orbital experience a bit of repulsion from each other. This offsets the attraction of the nucleus, so that paired electrons are removed more easily