IB Chemistry SL 2.2.4 Vsepr Model Questions

Use exam questions to count electron domains, name molecular shapes and explain bond angles using lone-pair repulsion.

Exam points

Count bonding and lone-pair electron domains around the central atom from the Lewis structure.
Name the electron-domain geometry and the molecular geometry separately when lone pairs are present.
Predict how lone-pair repulsion changes a bond angle from the ideal geometry and state the resulting shape/angle.

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The element boron has two naturally occurring isotopes, ${ }^{10} \mathrm{~B}$ and ${ }^{11} \mathrm{~B}$ .

Compare the shapes of the two molecules and explain the difference using valence shell electron pair repulsion theory (VSEPR).

$N H_{3}$ : (trigonal/triangular) pyramidal;
$B F_{3}$ : trigonal/triangular planar;
$\mathrm{NH}_{3}$ has 4 negative centres of charge/three bonding pairs and one lone pair and $\mathrm{BF}_{3}$ has 3 negative centres of charge/three bonding pairs/ OWTTE;
(bond angles) $107^{\circ}$ in $\mathrm{NH}_{3}$ and $120^{\circ}$ in $\mathrm{BF}_{3}$ ;
Accept $107.5^{\circ}$ for $\mathrm{NH}_{3}$ .

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IB Chemistry SL 2.2.4 Vsepr Model Questions question 1