1.1.4Physical Chemistry
Ionisation Energy
5 min read 5 parts Lesson 5 of 63
Section 1 of 5Definitions
Ionisation energy measures how strongly an atom holds on to its outer electrons.
Definitions
First ionisation energy
- The energy needed to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
Equation
- X(g) → X⁺(g) + e⁻, with state symbols included and the energy quoted in kJ mol⁻¹.
Second ionisation energy
- The energy needed to remove one electron from each ion in one mole of gaseous 1+ ions, giving 2+ ions.
Always larger
- Successive ionisation energies increase because the electron is removed from a more positive ion with a stronger attraction to the nucleus.
Section 2 of 5Trends in First Ionisation Energy
Three factors control ionisation energynuclear charge, distance from the nucleus and shielding.
Down a group
- Decreases because outer electrons are in shells further from the nucleus and are better shielded by inner electrons.
Across a period
- Generally increases because nuclear charge rises while shielding stays about the same, so the outer electrons are held more strongly.
Mg to Al
- Drops because the Al outer electron is in a 3p orbital, which has higher energy and is shielded by the 3s electrons.
P to S
- Drops because in S two electrons share one 3p orbital, and the repulsion between them makes one easier to remove.
Be to B
- Drops for the same reason as Mg to Al, since the outer electron of boron is in a 2p orbital.
Section 3 of 5Successive Ionisation Energies
Big jumps show where a new shell begins.
1
Plot or list successive ionisation energies for one element, remembering the values rise gradually inside one shell.
2
Look for the large jump between two values, which shows that an electron has been taken from an inner shell closer to the nucleus.
3
The number of electrons removed before the jump equals the number of outer-shell electrons and gives the group.
4
For magnesium the values are about 738, 1451, 7733 and 10540 kJ mol⁻¹, and the jump after the second shows two outer electrons.
5
Use a logarithm of the ionisation energy on a graph if the numbers span a wide range, so that each shell shows clearly.
Section 4 of 5Worked Example
Question
- An element has successive ionisation energies (kJ mol⁻¹) of 578, 1817, 2745, 11578 and 14831.
- State its group and justify your answer.
- Look for the biggest increase between successive values.
- The jump from 2745 to 11578 is the largest, so the fourth electron comes from an inner shell.
- Three electrons were removed before the jump, so there are three outer electrons.
- An element with three outer electrons is in Group 3.
Answer The element is in Group 3, because the large jump follows the third ionisation energy.
Section 5 of 5Exam Checklist & Tips
1.1.4 – Key Points for the Exam
- First ionisation energy The energy to remove one electron from each atom in a mole of gaseous atoms.
- Shielding The reduction in attraction of outer electrons to the nucleus caused by inner electrons.
- Nuclear charge The total positive charge of the protons in the nucleus.
- Successive ionisation energies The energies for removing electrons one after another from the same atom.
Common Mistakes
- Leaving out state symbols in the ionisation energy equation, which are required.
- Explaining the Mg to Al drop by saying Al has a smaller nuclear charge, which is false.
- Saying the P to S drop occurs because of shielding, when it is caused by electron repulsion in a shared orbital.
- Stating that ionisation energy decreases when going across the period because atomic radius decreases.