Redox and Group 1/2 trends
Introduced| English | Meaning |
|---|---|
| oxidising agent/ˈɒksɪdaɪzɪŋ ˈeɪdʒənt/ | A substance that accepts electrons from another substance and is itself reduced. |
| oxidation number/ˌɒksɪˈdeɪʃn ˈnʌmbə/ | The formal charge an atom would have if bonding electrons were assigned according to agreed electronegativity rules. |
| peroxides | Compounds containing a peroxide O–O unit with oxygen in oxidation state −1, such as hydrogen peroxide. |
| shielding/ˈʃiːldɪŋ/ | The reduction in nuclear attraction experienced by an electron because of repulsion from other electrons. |
Why is the oxidising agent 氧化剂 reduced?
- Magnesium reacting with chlorine loses electrons, while chlorine gains them. The words agent and process describe different sides of the same transfer.
- Track electron loss and gain before naming the agents.
Use oxidation-number 氧化数 bookkeeping
- Oxidation-number values add to the species charge. Elements are zero; oxygen is usually −2 but −1 in peroxides 过氧化物.
- Hydrogen is usually +1 but −1 in metal hydrides. In MnO₄⁻, x + 4(−2) = −1, so manganese is +7.
Find the oxidation number of Mn in MnO₄⁻. Use O = −2.
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x + 4(−2) = −1, so x = +7.
Combine half-equations
- Mg → Mg²⁺ + 2e⁻ and Cl₂ + 2e⁻ → 2Cl⁻ contain equal electron amounts, so the electrons cancel.
- Mg is the reducing agent and is oxidised. Cl₂ is the oxidising agent and is reduced. Check atoms and total charge.
For Al reducing Cu²⁺, multiply Al → Al³⁺ + 3e⁻ by two and Cu²⁺ + 2e⁻ → Cu by three. The overall equation is 2Al + 3Cu²⁺ → 2Al³⁺ + 3Cu.
In Mg + Cl₂ → Mg²⁺ + 2Cl⁻, which is the oxidising agent?
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Cl₂ accepts electrons and is reduced, so it oxidises magnesium.
Complete the process name: loss of electrons is _____.
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Oxidation is electron loss and an increase in oxidation number.
Explain two different group trends
- Down Groups 1/2, greater radius and shielding 屏蔽 weaken attraction to outer electrons. Electron loss becomes easier and reactivity generally rises.
- Small cations polarise carbonate/nitrate ions strongly. Their compounds are less thermally stable; stability generally rises down the group.
Increasing shielding down Group 1 makes the outer electron harder to remove.
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Greater shielding and radius reduce attraction to the outer electron, making loss easier.
Separate observations from explanations
- Group 2 hydroxide solubility rises down the group, while sulfate solubility falls. These are solubility trends, not electron-loss equations.
- Flame colours come from excited electrons falling to lower levels. Sodium gives yellow, potassium lilac, calcium brick-red and barium apple-green.
“The oxidising agent loses electrons” is false. It accepts electrons and is itself reduced.
Use oxidation-number sums, electron balance and a specific particle model; do not use “more reactive” as its own explanation.
Which two Group 2 trends are correct?
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Hydroxides and sulfates have opposite solubility trends. Small cations polarise anions more; brick-red is calcium’s characteristic flame colour.