Periodicity — IB Chemistry SL Study Guide
For: IB Chemistry SL candidates sitting IB Chemistry SL.
Covers: IB Topic 3 — periodic table arrangement, trends in atomic radius / ionization energy / electronegativity / melting point across periods and down groups, alkali metals (group 1), halogens (group 17), period 3 oxides.
A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the IB Chemistry SL style for educational use. They are not reproductions of past IBO papers.
1. Why Periodicity Matters
Topic 3 ties Topic 2 (atomic structure) to Topics 4-10 (chemistry of compounds). The periodic table organises elements so that periodic trends in properties emerge from electron-configuration patterns. About 5-7% of the IB Chemistry SL exam covers Topic 3 directly, but periodic reasoning underlies most multiple-choice and short-answer questions.
2. Periodic table layout
Group: vertical column. Elements share the same outer-shell configuration → similar chemistry. 18 groups; key ones:
- Group 1: alkali metals (1 valence e⁻).
- Group 2: alkaline earth metals (2 valence e⁻).
- Group 17: halogens (7 valence e⁻).
- Group 18: noble gases (full shell).
Period: horizontal row. Period number = number of occupied electron shells.
Blocks: s-block (groups 1-2), p-block (13-18), d-block (transition metals).
3. Trend: atomic radius
Down a group: increases. More electron shells, outer e⁻ farther from nucleus.
Across a period (left to right): decreases. More protons in same shell pull electrons closer; shielding stays roughly constant.
Implication: lithium (period 2 group 1) has small radius; caesium (period 6 group 1) is much larger.
4. Trend: ionization energy
Ionization energy = minimum energy to remove the most loosely held electron.
Down a group: decreases. Outer e⁻ farther from nucleus, easier to remove.
Across a period: increases. More protons, smaller radius, e⁻ harder to remove.
Anomalies: B < Be (full 2s subshell stable in Be); O < N (paired 2p e⁻ in O is easier to remove than half-filled 2p in N).
Successive ionization energies always increase. A jump in IE between th and th ionization indicates removal from a new shell — useful for determining group number.
5. Trend: electronegativity
Electronegativity = an atom's tendency to attract bonding electrons (Pauling scale: F=4.0 most, Cs=0.7 least).
Down a group: decreases. Across a period: increases.
Most electronegative: F (top right). Most electropositive: Fr (bottom left). Used to predict bond polarity (covalent if Δχ < 0.5, polar covalent 0.5-1.7, ionic > 1.7).
6. Trend: melting point
Melting point trends are NOT monotonic — they depend on bonding type:
Period 3 example:
- Na, Mg, Al: metallic bonding, increasing m.p. (more delocalised e⁻).
- Si: giant covalent network — very high m.p. (~1414 °C).
- P, S, Cl, Ar: simple molecular (P₄, S₈, Cl₂, Ar) — low m.p., increasing molar mass increases dispersion forces.
Group 1: m.p. decreases down (weaker metallic bonding as atomic radius increases). Group 17: m.p. increases down (heavier molecules, stronger dispersion forces).
7. Group chemistry
Group 1 (alkali metals): react vigorously with water → metal hydroxide + H₂. Reactivity increases down the group (lower IE, easier to lose 1 e⁻).
Group 17 (halogens): react with metals to form ionic halides. Reactivity decreases down the group (lower electronegativity, less drive to gain 1 e⁻).
Displacement reactions: a halogen will displace any halogen below it in the group from solution: .
8. Period 3 oxides
| Element | Oxide | Bonding | Acid/Base |
|---|---|---|---|
| Na | Na₂O | Ionic | Basic |
| Mg | MgO | Ionic | Basic |
| Al | Al₂O₃ | Ionic-covalent | Amphoteric |
| Si | SiO₂ | Giant covalent | Acidic |
| P | P₄O₁₀ | Simple molecular | Acidic |
| S | SO₃ | Simple molecular | Acidic |
| Cl | Cl₂O / Cl₂O₇ | Simple molecular | Acidic |
Trend: basicity → amphoteric → acidic moving across the period. Tied to bonding shift: ionic (left) → covalent (right).
9. Worked Example
Predict and explain: (a) Which has higher first ionization energy: Mg or Al? (b) Which has higher electronegativity: Cl or Br? (c) Which has lower melting point: Na or Al?
Solution.
(a) Trend: across period 3, IE increases from Na to Mg to Al... but wait — Mg actually has higher first IE than Al! In Mg, the e⁻ removed is from a stable filled 3s subshell. In Al, the e⁻ removed is from 3p (singly occupied), which is at slightly higher energy than 3s and easier to remove. So Mg > Al in first IE — exception to the across-period trend.
(b) Cl is in period 3, group 17. Br is in period 4, group 17. Down the group, electronegativity decreases, so Cl > Br.
(c) Both metallic bonded. Down group 1 m.p. decreases (Na to K to Rb...) — but Na and Al are different groups. Across period 3, m.p. increases from Na to Mg to Al (Al has 3 valence e⁻ vs 1 for Na, more delocalised → stronger metallic bond). So Na < Al in m.p.
10. Common Pitfalls
- Period vs group trends: students often mix up direction. Memorise: down a group, atomic radius increases; across a period, decreases.
- IE exceptions: B < Be, O < N (in period 2); Al < Mg, S < P (in period 3). Subshell stability arguments.
- Melting point trend: not monotonic across a period because bonding type changes (metallic → covalent network → simple molecular).
- Halogen reactivity: decreases down the group (opposite to alkali metals).
11. Practice Questions
- Place in order of increasing first ionization energy: Na, Si, S, Cl, Ar.
- Predict the bonding type and acid/base nature of MgO and SO₃ when dissolved in water.
- Which alkali metal would react most vigorously with cold water: Li, Na, K, or Cs? Explain.
12. Quick Reference Cheatsheet
- Down a group: atomic radius ↑, IE ↓, electronegativity ↓, m.p. depends on bonding.
- Across a period: atomic radius ↓, IE ↑, electronegativity ↑.
- Most electronegative: F. Most electropositive: Fr (or Cs).
- Group 1 reactivity: increases down (Li < Na < K < Rb < Cs).
- Group 17 reactivity: decreases down (F > Cl > Br > I).
- Period 3 oxides: Na₂O basic → Al₂O₃ amphoteric → SO₃ acidic.
13. What's Next
Periodicity unifies Topic 2 (Atomic Structure) with Topic 4 (Bonding) and onward — the bond type formed is predictable from electronegativity differences, and reaction trends follow IE/electronegativity gradients. Use Ollie for any specific trend prediction or anomaly explanation.