Working out the shapes of molecules using VSEPR theory

This page follows on from the shapes of molecules webpage, where you met the five basic shapes found in molecules that contain only bonding pairs of electrons and no lone pairs. Here, you will be introduced to the basic steps used to work out the shapes of molecules without lone pairs of electrons. The diagram below should remind you of these five fundamental molecular shapes. We will now look at the valence shell electron pair repulsion (VSEPR) theory — the model used to predict the shapes of simple molecules and ions. Despite its impressive name, VSEPR theory is very straightforward to use and makes determining molecular shapes quick and easy.

The 5 basic shapes of molecules with only bonding pairs of electrons

Using VSEPR rules to work out the shapes of molecules

The best way to learn VSEPR is to work through a few examples and determine the shapes of some simple molecules; so let's get started!

Carry out the following steps in order to find the shape of the molecule in question:

1. Identify the central atom and the number of valence electrons it has. This is easily done: use the periodic table to find what group the central atom is in and this will give the number of valence electrons it contains; this is shown in the table below:

   group in periodic table	1	2	3	4	5	6	7	8
   number of valence electrons	1	2	3	4	5	6	7	8

2. Count the number of atoms which are covalently bonded to the central atom in the molecule; now each atom bonded to the central atom will contribute 1 electron to the covalent bond, with the other electron coming from the central atom.
3. Add up the total number of electrons and divide by 2 to get the number of electron pairs.
4. Once we know the number of electron pairs present in a molecule we can then easily work out its basic shape.

Example 1: What shape is a molecule of beryllium dichloride (BeCl₂)?

Simply work through the rules listed above:

1. Be is the central atom in this molecule and it is in group 2 in the periodic table; it will therefore have 2 valence electrons in its outer shell.
2. Two chlorine atoms are bonded to the central atom; each chlorine atom will contribute 1 electron in forming a covalent bond to the beryllium atom. So we have 2 electrons in total from the chlorine atoms.
3. This gives a total number of electrons in the valence shells as 4 electrons; dividing by 2 gives 2 electron pairs. Now the table below may look familiar if you have previously read the page on—"The shapes of molecules"—it shows that a molecule with 2 electron pairs will be linear in shape.
   

   Number of electron pairs	Shape of molecule	Name of molecular shape
   2		linear
   3		trigonal planar
   4		tetrahedral

   
   
4. A molecule with two bonding pairs of electrons will be linear. This allows the electrons in the two covalent bonds to get as far apart as possible to minimise the repulsion between them; so BeCl₂ is a linear molecule with bond angles of 180°.

Example 2: What shape is a molecule of methane (CH₄)?

1. Carbon is the central atom in a methane molecule and being in group 4 of the periodic table it has 4 valence electrons.
2. Four hydrogen atoms are bonded to the central carbon atom with each hydrogen atom contributing 1 electron. So we have 4 electrons in total from the hydrogen atoms.
3. The total number of electrons in the valence shell is therefore 8 electrons; dividing by 2 gives 4 electron pairs; so the shape of a CH₄ molecule will be based on a tetrahedral structure with bond angles of 109.5°.

Example 3: What shape is a molecule of phosphorus pentachloride (PCl₅)?

1. Phosphorus is the central atom and it is in group 5 of the periodic table so it will have 5 valence electrons.
2. Five chlorine atoms are bonded to the central phosphorus atom and each chlorine atom will contribute 1 electron when it forms a covalent bond to the phosphorus atom. So we have 5 electrons in total from the chlorine atoms.
3. The total number of electrons in the valence shells is therefore 10 electrons; dividing by 2 gives 5 electron pairs; so the shape of a PCl₅ molecule will be based on a trigonal bipyramidal structure with bond angles of 120° and 90°.

Example 4: What shape is a molecule of sulfur hexafluoride (SF₆)?

1. Sulfur is the central atom in this molecule and since it is in group 6 of the periodic table it will have 6 valence electrons in its outer shell.
2. Six fluorine atoms are bonded to the central sulfur atom and each fluorine atom will contribute 1 electron to each of the covalent bonds formed with the sulfur atom. So we have 6 electrons in total from the fluorine atoms.
3. The total number of electrons in the valence shells is therefore 12 electrons; dividing by 2 gives 6 electron pairs; so the shape of an SF₆ molecule will be based on an octahedral structure with bond angles of 90°.

Self-check

Work out the shapes of the molecules and bond angles in each of the molecules in the flashcards below; then click the flashcards to check your answers.

Guess the Shape

Tap a card to reveal the VSEPR shape and typical bond angle(s). No lone pairs on the central atom in this set.

Reveal all Reset Shuffle

BeH₂

Guess the shape

Linear

Bond angle: 180°

BCl₃

Guess the shape

Trigonal planar

Bond angle: 120°

SiCl₄

Guess the shape

Tetrahedral

Bond angle: 109.5°

PF₅

Guess the shape

Trigonal bipyramidal

Bond angles: 120° (equatorial), 90°/180° (axial)

SeF₆

Guess the shape

Octahedral

Bond angle: 90°