1.1.7Physical Chemistry
Ionisation Energy – Trends and Evidence — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 7 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 6Down a Group
Group 2 shows how first ionisation energy changes down a group.
Trend
- Trend First ionisation energy decreases down a group, for example from beryllium to barium.
More shells
- More shells Each new element has an extra electron shell, so the outer electron is further from the nucleus.
More shielding
- More shielding Additional inner shells increase shielding between the nucleus and the outer electron.
Net effect
- Net effect The increase in nuclear charge is outweighed by greater distance and shielding, so less energy is needed to remove the electron.
Section 2 of 6Across Period 3
Across Period 3
General trend
- General trend First ionisation energy increases across a period from sodium to argon.
Reason
- Reason Nuclear charge rises while electrons enter the same shell, so shielding is similar and the attraction increases.
Mg to Al drop
- Mg to Al drop Aluminium’s outer electron is in a 3p orbital, which has higher energy and is further out than the 3s electrons of magnesium.
P to S drop
- P to S drop Sulfur’s outer 3p orbital contains a pair of electrons, and the repulsion between them makes one easier to remove than in phosphorus.
| Change | Elements | Cause |
|---|---|---|
| Small decrease | Mg → Al | Outer electron is in a higher-energy 3p sub-shell |
| Small decrease | P → S | Electron pairing in a 3p orbital causes repulsion |
Section 3 of 6Evidence for Shells and Groups
1
A graph of successive ionisation energies on a log scale shows distinct steps.
2
Each large jump marks the removal of an electron from a new, inner shell closer to the nucleus.
3
Count how many electrons are removed before the first big jump to find the number of outer electrons.
4
The number of outer electrons gives the group number of the element.
5
These patterns support the idea that electrons are arranged in shells with different energies.
Section 4 of 6Period 2 Parallel
Be to B
- Be to B The same effect as Mg to Al appears because boron’s outer electron is in the higher-energy 2p sub-shell.
N to O
- N to O The same reason as P to S: oxygen has paired electrons in a 2p orbital that repel each other.
Noble gases
- Noble gases Noble gases have the highest first ionisation energy in each period because of the high nuclear charge and stable full shells.
Group 1
- Group 1 Group 1 elements have the lowest values in their periods because the outer electron is alone in a new shell.
Section 5 of 6Worked Example
Question
- An element has successive ionisation energies (kJ mol⁻¹) of 578, 1817, 2745, 11577 and 14842.
- Deduce the group of the element.
- Look for the biggest jump between successive values.
- The jump from 2745 to 11577 is far greater than the earlier increases.
- Three electrons were removed before the jump, so three electrons are in the outer shell.
Answer The element is in Group 3 (aluminium has these values).
Section 6 of 6Exam Checklist & Tips
1.1.7 – Key Points for the Exam
- Shielding The reduction of nuclear attraction on an outer electron due to repulsion by inner electrons.
- Sub-shell A set of orbitals of the same type, such as 3p, within a shell.
- Period A horizontal row of the periodic table in which electrons fill the same outer shell.
- Group A vertical column of the periodic table whose elements have the same number of outer electrons.
Common Mistakes
- Explaining the decrease down a group by saying the nuclear charge falls; the nuclear charge actually rises.
- Giving the Mg to Al explanation as shielding by 3s electrons only, instead of the higher energy of the 3p sub-shell.
- Explaining the P to S drop by saying S has a half-filled sub-shell; it is the pairing repulsion that matters.
- Reading the group from the position of the highest value, not the biggest jump in successive values.
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