H2 Chemistry 2 - Chemical Bonding: Shapes and Bond Angles of SF₄ and XeF₂

Step 1 - State the problem and recall VSEPR

Predict the shapes and bond angles of sulfur tetrafluoride and xenon difluoride using VSEPR theory.

Step 1 - State the problem and recall VSEPR

VSEPR stands for Valence Shell Electron Pair Repulsion.

Step 1 - State the problem and recall VSEPR

The idea is that electron pairs around a central atom arrange themselves to minimise repulsion.

Step 1 - State the problem and recall VSEPR

We need to count bonding pairs and lone pairs on the central atom.

Step 2 - Draw the Lewis structure of SF₄

Sulfur is in Group six and has six valence electrons.

Step 2 - Draw the Lewis structure of SF₄

It forms four bonds to fluorine, using four electrons, leaving one lone pair.

Step 2 - Draw the Lewis structure of SF₄

So the steric number is five: four bonding pairs plus one lone pair.

Step 2 - Draw the Lewis structure of SF₄

Five electron pairs give a trigonal bipyramidal electron geometry.

Step 3 - Determine the shape of SF₄

The lone pair occupies an equatorial position to minimise lone pair to bond pair repulsion.

Step 3 - Determine the shape of SF₄

This is because equatorial positions have only two ninety-degree neighbours, while axial positions have three.

Step 3 - Determine the shape of SF₄

Removing one equatorial position from a trigonal bipyramid gives a see-saw or disphenoidal shape.

Step 3 - Determine the shape of SF₄

The axial bond angles are compressed from one hundred and eighty degrees to about one hundred and seventy-three degrees, and the equatorial angles compress from one hundred and twenty degrees to about one hundred and two degrees.

Step 4 - Draw the Lewis structure and shape of XeF₂

Xenon is in Group eight and has eight valence electrons.

Step 4 - Draw the Lewis structure and shape of XeF₂

It forms two bonds to fluorine, using two electrons, leaving three lone pairs.

Step 4 - Draw the Lewis structure and shape of XeF₂

The steric number is five: two bonding pairs plus three lone pairs.

Step 4 - Draw the Lewis structure and shape of XeF₂

All three lone pairs occupy equatorial positions, leaving the two fluorine atoms in axial positions.

Step 4 - Draw the Lewis structure and shape of XeF₂

The molecular shape is linear with a bond angle of one hundred and eighty degrees.

Step 5 - Compare and summarise

Both molecules have a steric number of five, so both start from trigonal bipyramidal electron geometry.

Step 5 - Compare and summarise

But the number of lone pairs differs: one for SF four and three for xenon difluoride.

Step 5 - Compare and summarise

SF four is see-saw shaped; xenon difluoride is linear.

Step 5 - Compare and summarise

Common mistake: placing lone pairs in axial positions. Always put lone pairs equatorial first to minimise ninety-degree repulsions.