Topic 3 of 3
Groups and deduction
Apply H1 Group 1/17 trends to new evidence.
A-Level 8873, revised syllabus (2026-2027)
Separate losing electrons from gaining them
Group 1 metals reduce other species; halogens oxidise them.
A reducing agent supplies electrons and is itself oxidised. Group 1 atoms have one outer electron: M → M+ + e-. From lithium through sodium, potassium, rubidium and caesium, the outer electron is farther from the nucleus and more shielded. It is generally lost more easily, so the familiar chemical reactivity increases down the group.
| Element | Outer configuration | Shared behaviour |
|---|---|---|
| Li | 2s1 | Forms a 1+ ion by losing the outer electron |
| Na | 3s1 | Same outer pattern |
| K | 4s1 | Same outer pattern |
| Rb | 5s1 | Same outer pattern |
| Cs | 6s1 | Same outer pattern |
A common reaction is 2M(s) + 2H2O(l) → 2MOH(aq) + H2(g). The metal is oxidised and water is reduced. For this H1 comparison, explain the trend through ease of electron loss; do not import an H2 electrode-potential ranking, which includes additional energetic factors.
An oxidising agent accepts electrons and is itself reduced. X2 + 2e- → 2X-. From chlorine to bromine to iodine, larger size and greater shielding weaken attraction for an incoming electron. Oxidising ability decreases in the order Cl2 > Br2 > I2.
| Mixture | Prediction | Reason |
|---|---|---|
| Cl2 + Br- | Cl2(aq) + 2Br-(aq) → 2Cl-(aq) + Br2(aq) | Chlorine is the stronger oxidant. |
| Br2 + I- | Br2(aq) + 2I-(aq) → 2Br-(aq) + I2(aq) | Bromine is stronger than iodine. |
| I2 + Cl- | No displacement reaction | Iodine is too weak an oxidant to oxidise chloride under these conditions. |
Thermal stability of the hydrogen halides is a different comparison. HCl is more thermally stable than HBr, which is more stable than HI. Down the group, the halogen atom becomes larger, the H-X bond longer and overlap poorer, so bond energy decreases. Less energy is needed to decompose HI: 2HI(g) ⇌ H2(g) + I2(g). This concerns covalent H-X bond breaking, not the intermolecular forces used to explain halogen volatility.
Check your understandingWhy does iodine have a higher boiling point but HI lower thermal stability than the chlorine comparison?Think it through, then reveal the answer
Use several clues to identify an unknown
Predict with a periodic pattern, then check the observations.
Worked example
Identify a Period 3 element
Element X conducts electricity as a solid. Its highest oxide contains X in oxidation state +3 and reacts with both acid and NaOH. Identify X.
- Conductivity suggests a metal among Na, Mg and Al.
- The +3 highest oxide selects aluminium from those candidates.
- The oxide amphoterism agrees with Al2O3, checking the first inference.
X is aluminium. State the independent clues rather than guessing from just one property.
Worked example
Predict an unfamiliar group member
An unfamiliar element has outer configuration ns2 np5 and lies below chlorine. Predict two properties relative to chlorine.
- Seven outer electrons place it in Group 17.
- A higher occupied shell gives greater size and shielding, so it attracts an incoming electron less strongly.
- A larger molecular electron cloud gives stronger instantaneous dipole-induced dipole attractions.
It is a weaker oxidising agent and its diatomic molecules are less volatile than chlorine. These are separate electronic and intermolecular explanations.
When inferring bonding from an oxide or chloride, combine melting behaviour, conductivity in different states and chemical reactions with water. High melting point does not uniquely establish ionic bonding: SiO2 is a covalent network. An acidic chloride solution also does not uniquely establish molecular hydrolysis: hydrated metal ions can release protons.
Worked example
Use an oxide and a chloride to test the same structural model
A Period 3 element forms an oxide that has a very high melting point, does not conduct when molten and does not react with water. Its chloride is a volatile liquid that reacts with water to produce an acidic solution and a white insoluble oxide. Suggest the element and both structures.
- The oxide has strong bonding throughout its structure, because melting needs much energy. Its non-conducting melt argues against an ordinary ionic lattice: mobile ions would carry charge.
- A giant covalent oxide fits these observations. SiO2 is the specified Period 3 example; its extended Si-O network also explains its failure to dissolve in water.
- The chloride is volatile, so the attractions separating its particles are relatively weak. A simple molecular chloride fits; giant ionic chlorides such as NaCl have high melting points.
- SiCl4 hydrolysis gives insoluble SiO2 and HCl, agreeing with both observed products. AlCl3 can also give acidic water, but that single observation cannot explain the entire set.
The evidence supports silicon: giant covalent SiO2 and simple molecular SiCl4. Bonding is inferred from several independent observations, then checked against the reaction products.