3.1.3Additional lessons
Periodic Trends — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 39 of 98
Part 1 of 6Atomic and Ionic Radius
Trends are explained by nuclear charge, shielding and distance of the valence electrons.
Across a period
- Across a period Nuclear charge increases while shielding stays similar, so the valence electrons are pulled closer and radius decreases.
Down a group
- Down a group Extra electron shells increase the distance and shielding, so atomic radius increases.
Cations
- Cations A metal loses electrons and often an entire shell, so a cation is smaller than its atom, e.g. Na 186 pm and Na⁺ 102 pm.
Anions
- Anions Extra electrons repel one another, so an anion is larger than its atom, e.g. Cl 99 pm and Cl⁻ 181 pm.
Part 2 of 6Ionisation Energy
1
First ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms: X(g) → X⁺(g) + e⁻.
2
Across a period it generally increases because nuclear charge rises and radius falls, so electrons are held more strongly.
3
There is a small drop from Mg to Al because the 3p electron is higher in energy and slightly shielded by the 3s electrons.
4
There is another drop from P to S because in sulfur two electrons share a 3p orbital and repel each other.
5
Down a group it decreases because the valence electron is further from the nucleus and more shielded.
| Element | Na | Mg | Al | P | S | Cl | Ar |
|---|---|---|---|---|---|---|---|
| First IE / kJ mol⁻¹ | 496 | 738 | 578 | 1012 | 1000 | 1251 | 1521 |
Part 3 of 6Electronegativity and Melting Point
Electronegativity and Melting Point
Electronegativity across
- Electronegativity across It increases across a period because the stronger nuclear charge attracts bonding electrons more strongly.
Electronegativity down
- Electronegativity down It decreases down a group because bonding electrons are further from the nucleus and more shielded.
Melting point, metals
- Melting point, metals Across Na, Mg, Al melting point rises as metallic bonding strengthens with more electrons per atom.
Melting point, non-metals
- Melting point, non-metals Giant covalent silicon is very high, while P, S and Cl form molecules with weak forces and low melting points.
Part 4 of 6Writing Good Explanations
Nuclear charge
- Nuclear charge State that the number of protons increases across a period, so attraction on valence electrons increases.
Shielding
- Shielding Inner electrons shield valence electrons, and this changes little across a period but increases down a group.
Distance
- Distance Valence electrons in higher shells are further from the nucleus, so they are held less strongly.
Compare clearly
- Compare clearly Use the words more, less, greater and smaller to compare, and refer to both elements.
Part 5 of 6Worked Example
Question Explain why the first ionisation energy of sodium (496 kJ mol⁻¹) is lower than that of magnesium (738 kJ mol⁻¹).
- Both elements lose a 3s electron and have the same number of inner shells, so shielding is similar.
- Magnesium has 12 protons and sodium 11, so the nuclear charge is greater.
- The 3s electron in magnesium is attracted more strongly and the atom is slightly smaller, so more energy is needed to remove it.
Answer Magnesium has a higher nuclear charge with similar shielding, so its outer electron is held more strongly.
Part 6 of 6Exam Checklist & Tips
3.1.3 – Key Points for the Exam
- Ionisation energy The energy needed to remove one mole of electrons from one mole of gaseous atoms.
- Shielding The reduction in attraction from the nucleus due to inner electrons.
- Atomic radius Half the distance between nuclei of two bonded atoms of an element.
- Electronegativity The ability of an atom to attract bonding electrons.
Common Mistakes
- Saying the nuclear charge decreases down a group.
- Explaining trends only by distance and forgetting shielding.
- Forgetting the drops at Al and S in period 3 ionisation energies.
- Stating that an anion is smaller than its atom.
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