In short: The first ionisation energy equation removes one electron from a gaseous atom to make a gaseous 1+ ion. For magnesium it is Mg(g) → Mg⁺(g) + e⁻, and the value is +738 kJ mol⁻¹. Ionisation is always endothermic.
Ionisation energy equations come up often, and the marks go quietly. The chemistry is simple. The detail is not: every species, charge and state symbol has to be exactly right.
Spot the mistake
In the clip, a student answers the question "Write the equation and give the enthalpy change for the first ionisation energy of magnesium" in three lines: the equation, the word "endothermic", and +738 kJ mol⁻¹. One line would lose a mark.
The equation was written with magnesium as a solid:
Mg(s) → Mg⁺(g) + e⁻
The mistake is the state symbol on the left. Ionisation energies are measured on gaseous atoms, so the correct equation is:
Mg(g) → Mg⁺(g) + e⁻
The word "endothermic" and the value +738 kJ mol⁻¹ were both fine.
Rich's tip: Read every symbol, not just the numbers. This is where the marks go.
The definition
The first ionisation energy is the energy required to remove one electron from each atom in one mole of gaseous atoms, to form one mole of gaseous 1+ ions. Learn it word for word. The key words are one mole, gaseous and 1+ ions.
The second ionisation energy
The second ionisation energy removes an electron from each ion in one mole of gaseous 1+ ions, to form one mole of gaseous 2+ ions:
Mg⁺(g) → Mg²⁺(g) + e⁻
Notice that the left-hand side is the ion from the first equation. Each equation takes away exactly one electron, so the charge goes up by one each time.
Why ionisation is always endothermic
The outer electron is attracted to the positive nucleus. Energy has to be put in to overcome that attraction and pull the electron away, so the enthalpy change is positive. That is why the values are written with a plus sign. It is never exothermic, however easy the electron is to remove.
Common slips
- Wrong state symbols: writing (s) for a metal, or leaving the symbols out. Both sides need them: (g) for the atom, (g) for the ion.
- Wrong charge: the first ionisation gives 1+, the second gives 2+. Check that the charges balance on both sides.
- The electron on the wrong side: e⁻ is a product, so it goes on the right. Writing it on the left describes an electron being added.
- Writing more than one electron: the first ionisation energy removes one electron only. Mg → Mg²⁺ + 2e⁻ is not the first ionisation.
- Units: kJ mol⁻¹, with the sign included.
Linking it to trends
Once you can write the equations, explain the trends in the values. Across a period the nuclear charge increases while the shielding stays about the same, so the outer electron is held more tightly. For the full pattern, including the dips, see First ionisation energy across Period 3.
If you want to practise writing equations like this, Dexter, the A Level Chemistry tutor I built, can ask you for ionisation equations in your course and point out the symbols you missed.