[ 2.2.2.3 ] shapes of molecules & ions

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[ https://chemrevise.org/wp-content/uploads/2022/01/2.2.2-bonding-and-structure.pdf ] + [ https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FChemistry%2FA-level%2FNotes%2FOCR-A%2F2-Foundations-in-Chemistry%2FDetailed%2F2.2.%20Electrons%2C%20Bonding%20and%20Structure.pdf ]

Last updated 10:27 AM on 10/8/26
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11 Terms

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Electron Group Repulsion Theory

States that groups of electrons around a central atom all have negative charge an repel each other, so they move as far apart as possible to minimise the repulsion between them.

2
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Groups of Electrons

Regions where electrons are concentrated around a central atom. This includes bonding pairs of electrons (single, double, triple, & dative) and lone pairs of electrons.

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<p><strong>2</strong> Electron Groups in Outer Shell of Central Atom:</p>

2 Electron Groups in Outer Shell of Central Atom:

  • shape → linear

  • bond angles between 2 adjacent electron groups → 180°


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<p><strong>3 </strong>Electron Groups in Outer Shell of Central Atom:</p>

3 Electron Groups in Outer Shell of Central Atom:

  • shape → trigonal planar

  • bond angles between 2 adjacent electron groups → 120°


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<p><strong>4</strong> Electron Groups in Outer Shell of Central Atom:</p>

4 Electron Groups in Outer Shell of Central Atom:

  • shape → tetrahedral

  • bond angles between 2 adjacent electron groups → 109.5°


6
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<p><strong>6</strong> Electron Groups in Outer Shell of Central Atom:</p>

6 Electron Groups in Outer Shell of Central Atom:

  • shape → octahedral

  • bond angles between 2 adjacent electron groups → 90°


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Lone Pairs’ Affect on Shape:

  • lone pair is not shared with another atom so it lies closer to the central atom and repels more strongly than bonded groups of electrons

  • the final shape taken up by the atoms is not affected just the shape taken up by the electron groups

  • each lone pair reduces the bond angle by around 2.5°


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<p>Features of Trigonal Planar (Non-linear):</p>

Features of Trigonal Planar (Non-linear):

  • 2 groups of bonding electrons + 1 lone pair

  • bond angle → 117.5° (120°-2.5°)

  • eg. SO2 , GeCl2


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<p>Features of Tetrahedral (Pyramidal):</p>

Features of Tetrahedral (Pyramidal):

  • 3 groups of bonding electrons + 1 lone pair

  • bond angle → 107° (109.5°-2.5°)

  • eg. NH3 , PCl3


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<p>Features of Tetrahedral (Non-linear):</p>

Features of Tetrahedral (Non-linear):

  • 2 groups of bonding electrons + 2 lone pairs

  • bond angle → 104.5° (109.5°-5°)

  • eg. H2O , SCl2


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How to Predict Shape of Molecules:

  1. Draw a dot-and-cross diagram

  2. Count up all the electron groups and identify any lone pairs