IB Chemistry SL 2.2.4 VSEPR Model

Practise moving from a Lewis structure to electron-domain geometry, molecular geometry and approximate bond angles. Count bonding and non-bonding domains, apply stronger…

Exam points

count bonding and lone-pair domains around the central atom from a Lewis structure
name electron-domain and molecular geometries separately when lone pairs are present
predict how lone-pair repulsion changes a bond angle from the ideal geometry

[Maximum number: 4]

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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2.2.4—VSEPR model question 1