Topic 2 · page 7 of 15

Ionisation energy

Ionisation energy measures how tightly an atom holds an electron. It is always endothermic, and its size depends on the nuclear charge, the shielding, and the sub-shell the electron comes from.

12 min read2.82.10

Read first: Molecules and 2+ ions

Defining ionisation energy 2.8

The mass spectrometer tells you what an atom weighs. It says nothing about where the electrons are. To learn that, chemists measure how much energy it takes to pull an electron away from an atom. The more tightly an electron is held, the more energy it takes.

In symbols, for any element X:

X(g)→XX+(g)+eX− \ce{X(g) -> X+(g) + e-}

The value is measured per mole, so the unit is kJ mol⁻¹. For magnesium it is +738 kJ mol⁻¹:

Mg(g)→MgX+(g)+eX− \ce{Mg(g) -> Mg+(g) + e-}

Take another electron off the same atom and you have the second, then the third.

The third ionisation energy follows the same pattern: one electron from each ion in a mole of gaseous 2+ ions, giving gaseous 3+ ions.

Equation for magnesium Energy / kJ mol⁻¹
1st Mg(g)→MgX+(g)+eX−\ce{Mg(g) -> Mg+(g) + e-} +738
2nd MgX+(g)→MgX2+(g)+eX−\ce{Mg+(g) -> Mg^2+(g) + e-} +1451
3rd MgX2+(g)→MgX3+(g)+eX−\ce{Mg^2+(g) -> Mg^3+(g) + e-} +7733

Notice the pattern in every equation: one electron is removed and gaseous species are on both sides.

Why gaseous? In a solid or liquid, neighbouring particles interact with each other. A gaseous atom is on its own, so the only thing you measure is the attraction between its nucleus and the electron.

All ionisation energies are endothermic

An electron is attracted to the positive nucleus. To pull it away, you must supply energy to overcome that attraction. So every ionisation energy is endothermic, with a positive sign.

Successive ionisation energies get larger. After one electron has gone, the particle left behind has the same nuclear charge but fewer electrons. The remaining electrons are pulled in more tightly, and the next electron has to leave a particle that is already positive.

What controls the size of an ionisation energy 2.10

Ionisation energy measures the attraction between the nucleus and the electron being removed. Three things change that attraction.

Factor What changes Effect on ionisation energy
Nuclear charge more protons in the nucleus greater attraction, so higher
Shielding and distance more inner shells between the nucleus and the electron weaker attraction, so lower
Sub-shell the electron comes from a higher-energy sub-shell, or from an orbital that already holds another electron easier to remove, so lower

Shielding is the repulsion between the outer electron and the inner electrons. The inner electrons push the outer electron away, so it feels less of the pull of the nucleus. An outer electron in a new shell is also further from the nucleus, and attraction falls quickly with distance.

Sub-shells and electron pairs are explained on Orbitals and shells. For now, remember that the electron's own energy level and any repulsion from a partner in the same orbital also matter.

Worked exampleWhy is the first ionisation energy of beryllium higher than lithium?

Li: 520 kJ mol⁻¹. Be: 900 kJ mol⁻¹.

  1. Both electrons come from the 2s sub-shell, with the 1s electrons inside. Shielding is about the same.
  2. Beryllium has 4 protons and lithium has 3, so the nuclear charge is greater.
  3. The outer electron in beryllium is pulled harder, so more energy is needed.

Worked exampleWhy is the first ionisation energy of potassium lower than sodium?

Na: 496 kJ mol⁻¹. K: 419 kJ mol⁻¹.

  1. Potassium has more protons (19, against 11), which increases the attraction.
  2. But potassium's outer electron is in the 4th shell, not the 3rd. It is further from the nucleus and is shielded by more inner shells.
  3. The extra distance and shielding outweigh the extra nuclear charge, so the attraction is weaker and less energy is needed.

Check yourself

  1. Define the first ionisation energy of sodium and write an equation for it, with state symbols.
  2. Write an equation for the third ionisation energy of aluminium.
  3. Explain why the second ionisation energy of magnesium (1451 kJ mol⁻¹) is larger than the first (738 kJ mol⁻¹).
  4. Explain why every ionisation energy has a positive sign.
  5. Explain why the first ionisation energy of magnesium (738 kJ mol⁻¹) is higher than that of sodium (496 kJ mol⁻¹).
  6. A student writes the equation for the second ionisation energy of calcium as Ca(s) → Ca²⁺ + 2e⁻. Give two errors and write the correct equation.

Answers cover these until you have tried the questions

  1. The energy required to remove one electron from each atom in one mole of gaseous sodium atoms, to form one mole of gaseous Na⁺ ions. Na(g)→NaX+(g)+eX−\ce{Na(g) -> Na+(g) + e-}
  2. AlX2+(g)→AlX3+(g)+eX−\ce{Al^2+(g) -> Al^3+(g) + e-}
  3. The second electron is removed from a 1+ ion. The nuclear charge is the same but there is one fewer electron, so the remaining electrons are pulled in more tightly and the attraction on the electron being removed is greater.
  4. Energy must be supplied to overcome the attraction between the positive nucleus and the negative electron, so the process is endothermic (ΔH\Delta H is positive).
  5. Both outer electrons are in the 3s sub-shell, with the same inner electrons, so shielding is about the same. Magnesium has 12 protons and sodium has 11, so the nuclear charge is greater and the attraction for the outer electron is stronger.
  6. Errors: the starting species should be Ca⁺(g), not Ca(s); and only one electron should be removed. Correct equation: CaX+(g)→CaX2+(g)+eX−\ce{Ca+(g) -> Ca^2+(g) + e-}