9701 · 3.5
Shapes of molecules flashcards
Revision flashcards for Cambridge 9701 Shapes of molecules (syllabus 3.5). Flip, recall, then mark a real past-paper question.
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What is the fundamental principle of VSEPR theory?
Electron pairs in the valence shell of a central atom repel each other and arrange themselves in space to be as far apart as possible, minimising repulsion.
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What is a 'bonding pair' of electrons?
A region of electron density located between the nuclei of two bonded atoms, forming a covalent bond.
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What is a 'lone pair' of electrons?
A pair of valence electrons that is not involved in bonding and belongs exclusively to one atom.
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Why do lone pairs repel more strongly than bonding pairs?
Lone pairs are only attracted to one nucleus, so their electron cloud is more diffuse and occupies more space. Bonding pairs are localised between two nuclei and are less repulsive.
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State the order of repulsion strength between electron pairs.
Lone Pair - Lone Pair (LP-LP) > Lone Pair - Bonding Pair (LP-BP) > Bonding Pair - Bonding Pair (BP-BP).
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What is the shape and bond angle of a methane molecule, CH₄?
Shape: Tetrahedral. Bond angle: 109.5°. It has 4 bonding pairs and 0 lone pairs.
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What is the shape and bond angle of an ammonia molecule, NH₃?
Shape: Trigonal pyramidal. Bond angle: approx. 107°. It has 3 bonding pairs and 1 lone pair.
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What is the shape and bond angle of a water molecule, H₂O?
Shape: Bent or V-shaped. Bond angle: approx. 104.5°. It has 2 bonding pairs and 2 lone pairs.
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What is the shape and bond angle of a boron trifluoride molecule, BF₃?
Shape: Trigonal planar. Bond angle: 120°. It has 3 bonding pairs and 0 lone pairs.
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How do you treat a double or triple bond when using VSEPR theory?
A double or triple bond is treated as a single region of electron density (or a single 'electron domain'). For example, in CO₂, the two double bonds are treated as two domains, leading to a linear shape.
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What is the difference between 'electron pair geometry' and 'molecular shape'?
Electron pair geometry describes the arrangement of all electron pairs (bonding and lone). Molecular shape describes only the arrangement of the atoms, ignoring the lone pairs in the final name.