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Molecular shapes for SQA National 5 Chemistry

Molecular shapes

This page covers the following topics:

1. Molecular shapes and bond angles

To determine the shape of molecule, start from counting the number of atoms and lone pairs of electrons attached to the central atom. Then locate the lone pairs of electrons to positions that are further away from the other electron pairs and atoms. Lone pairs of electrons are usually ignored while naming the shape and the following shapes usually result from a particular number of bonds and lone pairs.

Assuming that all atoms bonded to the central atom are the same:
2 bonds โ†’ linear
2 bonds + some lone pairs โ†’ angular/bent
3 bonds โ†’ trigonal planar (120ยฐ)
3 bonds + 1 lone pair โ†’ trigonal pyramidal
4 bonds โ†’ tetrahedral (109.5ยฐ)
4 bonds + 1 lone pair โ†’ see-saw
4 bonds + 2 lone pairs โ†’ square planar (90ยฐ)
5 bonds โ†’ trigonal bipyramidal (90ยฐ and 120ยฐ)
6 bonds โ†’ octahedral (90ยฐ)

Molecular shapes and bond angles

1

Methane has 4 Cโ€”H bonds and no lone pairs of electrons. Sketch the molecule of methane.

Since all of the hydrogen atoms bonded to the central carbon atom are the same, tetrahedral geometry is formed (4 bonds โ†’ tetrahedral geometry).

image

Methane has 4 Cโ€”H bonds and no lone pairs of electrons. Sketch the molecule of methane.

2

Provide the main 3 steps of how to determine molecular geometry.

To determine the shape of molecule, start from counting the number of atoms and lone pairs of electrons attached to the central atom. Then locate the lone pairs of electrons to positions that are further away from the other electron pairs and atoms. Lone pairs of electrons are usually ignored while naming the shape and the following shapes usually result from a particular number of bonds and lone pairs.

1. Count the number of bonds and lone pairs of electrons.
2. Locate lone pairs to the most remote positions.
3. Ignore the lone pairs to determine geometry.

Provide the main 3 steps of how to determine molecular geometry.

3

Molecule XLโ‚… has 5 bonding pairs and 0 lone pairs of electrons. What geometry does it have?

Since all of the atoms bonded to the central atom are the same, trigonal bipyramidal geometry is formed (5 bonds โ†’ trigonal bipyramidal).

trigonal bipyramidal

Molecule XLโ‚… has 5 bonding pairs and 0 lone pairs of electrons. What geometry does it have?

4

The bonds of BFโ‚ƒ are held as far apart as possible, in a trigonal planar geometry. What bond angle are they held at?

Since all of the atoms bonded to the central boron atom are the same, trigonal planar geometry results in 120ยฐ angles between the bonds.

120ยฐ

The bonds of BFโ‚ƒ are held as far apart as possible, in a trigonal planar geometry. What bond angle are they held at?

5

At what angle are bonds held apart from each other in a tetrahedral geometry?

If all of the atoms bonded to the central atom are the same, tetrahedral geometry results in 109.5ยฐ angles between the bonds.

109.5ยฐ

At what angle are bonds held apart from each other in a tetrahedral geometry?

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