UNIT 2 CHEMISTRY · PEARSON AAQ · A1.3.1
First and Successive Ionisation Energies
FREE CHEMISTRY REVISION · TOPIC 7
Define the process. Balance the charge. Keep every species gaseous.
Distinguish the first electron removal from each successive removal, and write precise equations for the energy change at every stage.
Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · A1.3.1
What you will learn
- Define first and successive ionisation energies in terms of one mole of gaseous species.
- Write equations with correct starting charges, product charges, electrons and state symbols.
- Distinguish an individual successive ionisation energy from the total energy for several removals.
- Explain why successive removals require energy and why their energies increase.
A1.3.1 specifies definitions and equations for first and successive ionisation energies. Using energy jumps to deduce structure is developed in A1.3.2; trends across periods and down groups belong to A1.3.3.
1. What does ionisation mean?
Ionisation is the formation of an ion. In this topic, ionisation energy refers specifically to removing an electron from an isolated gaseous atom or positive ion. The removed electron leaves the species; it is not merely promoted into another bound orbital.
The nucleus attracts negative electrons electrostatically. Energy must be supplied to separate an electron from the remaining positive ion. Ionisation is therefore endothermic: the ionisation energy is positive.
Removing an electron leaves the nucleus unchanged. The atom keeps its element identity and proton number, while its charge increases by one unit: neutral → 1+ → 2+ → 3+.
We use gaseous species to define electron removal independently of the energy needed to break a solid lattice, vaporise a liquid or separate particles in a substance. The element’s normal room-temperature state does not determine the state symbol in an ionisation-energy equation.
2. Define first ionisation energy
The first ionisation energy is the energy required to remove one electron from each atom in one mole of gaseous atoms, forming one mole of gaseous 1+ ions.
Every part matters: one electron per atom, one mole of atoms, and gaseous species. “Energy needed to remove an electron” alone leaves the quantity and starting state unclear.
X(g) → X⁺(g) + e⁻
Energy change = IE₁ > 0
The equation is written for representative particles; the tabulated molar energy applies to one mole of this process. Common units are kJ mol⁻¹. A value of 496 kJ mol⁻¹ means approximately 496 kJ is needed to remove one electron from every atom in one mole of the specified gaseous atoms.
Do not add e⁻(g): write the gaseous state for the atom and ion, and simply e⁻ for the electron. The essential state symbols are X(g) and X⁺(g).
3. Define successive ionisation energies
Successive ionisation energies describe removing electrons one at a time from the same element’s increasingly positive gaseous species. They are distinct stages, each beginning where the preceding stage ended.
Second ionisation energy: the energy required to remove one electron from each ion in one mole of gaseous 1+ ions, forming one mole of gaseous 2+ ions.
X⁺(g) → X²⁺(g) + e⁻
Third ionisation energy: the energy required to remove one electron from each ion in one mole of gaseous 2+ ions, forming one mole of gaseous 3+ ions.
X²⁺(g) → X³⁺(g) + e⁻
More generally, the nth ionisation energy is the energy required to remove one electron from each ion in one mole of gaseous (n − 1)+ ions, producing one mole of gaseous n+ ions. For n = 1 the starting species is a neutral gaseous atom.
X(n−1)+(g) → Xn+(g) + e⁻
The stage number tells you the charge of the product. For the fourth ionisation, the reactant is X³⁺ and the product is X⁴⁺. Every individual ionisation equation produces one electron, however large n is.
| Stage | Starting species | Product ion | Electrons removed in that stage |
|---|---|---|---|
| First | X(g) | X⁺(g) | 1 per atom |
| Second | X⁺(g) | X²⁺(g) | 1 per ion |
| Third | X²⁺(g) | X³⁺(g) | 1 per ion |
| Fourth | X³⁺(g) | X⁴⁺(g) | 1 per ion |
4. Successive stages are not cumulative totals
The second ionisation energy is the energy of the second removal alone. It is not the combined energy needed to remove two electrons from a neutral atom.
Individual second stage:
X⁺(g) → X²⁺(g) + e⁻ [IE₂]
Overall two-electron removal:
X(g) → X²⁺(g) + 2e⁻ [IE₁ + IE₂]
The same distinction applies to three electrons: X(g) → X³⁺(g) + 3e⁻ requires IE₁ + IE₂ + IE₃ per mole of atoms. It is not the definition of IE₃.
5. Why do successive values increase?
For a given element, each electron removal leaves a more positive ion. The nuclear charge stays the same, because the proton number does not change. With fewer electrons, electron–electron repulsion and screening are reduced, the remaining electrons are more strongly bound, and further removal requires more energy.
When removal reaches an inner principal shell, the increase can be especially large: those electrons are closer to the nucleus and less shielded by inner electrons than the outer-shell electrons removed earlier. You will use these large jumps as evidence for shell occupancy in A1.3.2.
Do not write “the nucleus gains protons” or “the nuclear charge increases after each removal”. The net charge of the ion increases; the charge of its nucleus does not. Also distinguish successive values for one element from first ionisation energies of different elements.
6. Worked examples
Example 1 · First ionisation of sodium
- Start from neutral gaseous sodium atoms, even though sodium is normally solid.
- Remove one electron and write Na⁺ as the ion.
- Na(g) → Na⁺(g) + e⁻
- Charge check: 0 on the left; (+1) + (−1) = 0 on the right. Atom check: one Na on each side.
Definition: energy required to remove one electron from each atom in one mole of gaseous sodium atoms, forming one mole of gaseous sodium 1+ ions.
Example 2 · Second ionisation of magnesium
- Second stage means the starting ion has charge 1+.
- Remove one electron, making the product charge 2+.
- Mg⁺(g) → Mg²⁺(g) + e⁻
- Charge check: +1 on the left; +2 − 1 = +1 on the right.
Mg(g) → Mg²⁺(g) + 2e⁻ is a balanced overall process, but it represents IE₁ + IE₂ rather than IE₂ alone.
Example 3 · Third ionisation of aluminium
- The third electron is removed from Al²⁺, not Al or Al³⁺.
- The product is Al³⁺ and one electron is released.
- Al²⁺(g) → Al³⁺(g) + e⁻
- Charge check: +2 = +3 − 1. Define the quantity for one mole of gaseous Al²⁺ ions.
Al³⁺(g) → Al⁴⁺(g) + e⁻ would instead be aluminium’s fourth ionisation.
Example 4 · Repair an incorrect fourth-ionisation equation
Proposed: Si(g) → Si⁴⁺(g) + 4e⁻.
- The proposed equation starts from a neutral atom and removes four electrons overall.
- A fourth ionisation must start from the species after three removals, Si³⁺.
- Write Si³⁺(g) → Si⁴⁺(g) + e⁻.
- Check charge: +3 = +4 − 1. Only one electron leaves in this stage.
The original overall equation is charge-balanced, but its energy is IE₁ + IE₂ + IE₃ + IE₄. Charge balance alone is not enough to identify the correct stage.
Example 5 · Calculate a total from successive values
A question supplies magnesium values IE₁ = 738 and IE₂ = 1451 kJ mol⁻¹, rounded for this calculation. Find the energy for Mg(g) → Mg²⁺(g) + 2e⁻.
- The process contains first and second removals.
- Add the two molar energies: 738 + 1451.
- Total = 2189 kJ mol⁻¹.
- For 0.250 mol of gaseous Mg atoms: E = 0.250 × 2189 = 547 kJ (3 significant figures).
1451 kJ mol⁻¹ is only the Mg⁺ → Mg²⁺ stage. The amount calculation is an application of the molar definition; remember that energy for a sample is in kJ, while molar ionisation energy is in kJ mol⁻¹.
7. Common misconceptions and exam tips
- “Use the element’s normal state.” Both starting atom/ion and resulting ion are gaseous in these equations.
- “The second ionisation removes two electrons at once.” It removes one electron from a 1+ ion.
- “Ionisation energy is negative because electrons leave.” Electron removal requires an energy input: positive, endothermic.
- “A 3+ product needs three electrons on the right.” Only for an overall neutral-to-3+ process; IE₃ itself produces one electron.
- “Successive removal changes the element.” Proton number is unchanged.
- “Charge-balanced means it must be the requested stage.” Check the starting charge and number of electrons too.
Equation check: correct element → starting charge n − 1 → product charge n → (g) for both species → one e⁻ on the right → equal total charge.
For definitions, include the molar amount, gaseous state, starting charge and one-electron removal. For equations, write clear superscript charges: Mg²⁺ means one magnesium ion with charge 2+, whereas 2Mg⁺ means two ions of charge 1+.
Your revision summary
- IE₁: X(g) → X⁺(g) + e⁻.
- IE₂: X⁺(g) → X²⁺(g) + e⁻.
- Each successive stage removes one electron from an increasingly positive gaseous species.
- Molar ionisation energies are positive and usually given in kJ mol⁻¹.
- A multi-electron overall process needs the sum of its individual ionisation energies.
Retrieval challenge: define IE₂ without looking, then write the first, third and fifth ionisation equations for a generic element X.
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