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CaCl2
Two chloride ions balance one calcium ion.
A Level · English Medium · Cambridge · Chemistry
Bonding, formulae and equations
Explain bonding, construct formulae and balance equations without changing formulae.
Shared cross-level core and stretch unit. Start with the core questions, then use the stretch example for deeper work. The same unit is available in related class and board pathways; it is not a full grade-specific or examination-specification course. A teacher should select the appropriate depth and further topics.
Learn the idea
Ionic bonding is electrostatic attraction between oppositely charged ions; covalent bonding involves shared electron pairs. A formula reflects composition. When balancing an equation, change coefficients, not subscripts: altering a subscript changes the substance.
Worked example
Mg2+ and Cl− combine as MgCl2 to balance charge. Hydrogen burning is represented by 2H2 + O2 → 2H2O, with four H and two O atoms on each side.
- Balance charges when writing an ionic formula.
- Count atoms before and after a reaction.
- Use whole-number coefficients and recheck.
Stretch example
For methane, four bonding pairs around carbon repel into an approximately tetrahedral arrangement with angles about 109.5°. Water has two bonding and two lone pairs; stronger lone-pair repulsions reduce its H–O–H angle.
Your written practice
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2
Four hydrogen atoms are needed.
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covalent
Oxygen and hydrogen share electron pairs.
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Its ions can move and carry charge when molten.
Distinguish mobile ions from electrons in a metal.
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H2O2 is hydrogen peroxide, a different substance.
Explain that subscripts determine composition.
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4
Each of four C–H bonds contains a shared pair.
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Two regions of electron density around carbon lie opposite one another, minimising repulsion.
Treat each double bond as one domain.
Equation check
Write and balance three simple reaction equations on paper.
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- Use correct formulae.
- Show atom counts.
- Explain one bonding or structure feature.
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