IB Chemistry SL 2.2.4 VSEPR Model Question Bank

Practise IB Chemistry SL 2.2.4 by counting electron domains, predicting molecular geometry and explaining how lone pairs alter common bond angles.

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 $\mathrm{NH}_{3}$ .

All question bank results loaded

2.2.4—VSEPR model question 1