Determining Molecular Shape

Determining molecular shape

To predict the shape of a covalent molecule, follow these steps:

  1. Draw the molecule using a Lewis diagram. Make sure that you draw all the valence electrons around the molecule’s central atom.

  2. Count the number of electron pairs around the central atom.

  3. Determine the basic geometry of the molecule using the table below. For example, a molecule with two electron pairs (and no lone pairs) around the central atom has a linear shape, and one with four electron pairs (and no lone pairs) around the central atom would have a tetrahedral shape.

Tip:

The central atom is the atom around which the other atoms are arranged. So in a molecule of water, the central atom is oxygen. In a molecule of ammonia, the central atom is nitrogen.

The table below gives the common molecular shapes. In this table we use A to represent the central atom, X to represent the terminal atoms (i.e. the atoms around the central atom) and E to represent any lone pairs.

Number of bonding electron pairs

Number of lone pairs

Geometry

General formula

\(\text{1}\) or \(\text{2}\)

\(\text{0}\)

linear

\(\text{AX}\) or \(\text{AX}_{2}\)

\(\text{2}\)

\(\text{2}\)

bent or angular

\(\text{AX}_{2}\text{E}_{2}\)

\(\text{3}\)

\(\text{0}\)

trigonal planar

\(\text{AX}_{3}\)

\(\text{3}\)

\(\text{1}\)

trigonal pyramidal

\(\text{AX}_{3}\text{E}\)

\(\text{4}\)

\(\text{0}\)

tetrahedral

\(\text{AX}_{4}\)

\(\text{5}\)

\(\text{0}\)

trigonal bipyramidal

\(\text{AX}_{5}\)

\(\text{6}\)

\(\text{0}\)

octahedral

\(\text{AX}_{6}\)

Table: The effect of electron pairs in determining the shape of molecules. Note that in the general example \(\text{A}\) is the central atom and \(\text{X}\) represents the terminal atoms.

The common molecular shapes.

Photo created from images by Ben Mills on Wikipedia

The common molecular shapes in 3-D.

In the figures above, the green balls represent the lone pairs (E), the white balls (X) are the terminal atoms and the red balls (A) are the center atoms.

Of these shapes, the ones with no lone pairs are called the ideal shapes. The five ideal shapes are: linear, trigonal planar, tetrahedral, trigonal bypramidal and octahedral.

One important point to note about molecular shape is that all diatomic (compounds with two atoms) compounds are linear. So \(\text{H}_{2}\), \(\text{HCl}\) and \(\text{Cl}_{2}\) are all linear.

Example: Molecular Shape

Question

Determine the shape of a molecule of \(\text{BeCl}_{2}\)

Step 1: Draw the molecule using a Lewis diagram

d99dffbce0c1aa429a763e6da6899537.png

The central atom is beryllium.

Step 2: Count the number of electron pairs around the central atom

There are two electron pairs.

Step 3: Determine the basic geometry of the molecule

There are two electron pairs and no lone pairs around the central atom. \(\text{BeCl}_{2}\) has the general formula: \(\text{AX}_{2}\). Using this information and the table above, we find that the molecular shape is linear.

Step 4: Write the final answer

The molecular shape of \(\text{BeCl}_{2}\) is linear.

Example: Molecular Shape

Question

Determine the shape of a molecule of \(\text{BF}_{3}\)

Step 1: Draw the molecule using a Lewis diagram

c1a3805cbc745f394e3b59051e452472.png

The central atom is boron.

Step 2: Count the number of electron pairs around the central atom

There are three electron pairs.

Step 3: Determine the basic geometry of the molecule

There are three electron pairs and no lone pairs around the central atom. The molecule has the general formula \(\text{AX}_{3}\). Using this information and the table above, we find that the molecular shape is trigonal planar.

Step 4: Write the final answer

The molecular shape of \(\text{BF}_{3}\) is trigonal planar.

Example: Molecular Shape

Question

Determine the shape of a molecule of \(\text{NH}_{3}\)

Step 1: Draw the molecule using a Lewis diagram

de9be96a1509930afd139454ce4f1532.png

The central atom is nitrogen.

Step 2: Count the number of electron pairs around the central atom

There are four electron pairs.

Step 3: Determine the basic geometry of the molecule

There are three bonding electron pairs and one lone pair. The molecule has the general formula \(\text{AX}_{3}\text{E}\). Using this information and the table above, we find that the molecular shape is trigonal pyramidal.

Step 4: Write the final answer

The molecular shape of \(\text{NH}_{3}\) is trigonal pyramidal.

Tip:

We can also work out the shape of a molecule with double or triple bonds. To do this, we count the double or triple bond as one pair of electrons.

Optional Discussion: Building molecular models

With friends, try to build a number of molecules using jellytots (or chewing gums or any other similar substance) to represent the atoms in the molecule, and toothpicks to represent the bonds between the atoms. In other words, the toothpicks will hold the atoms (jellytots) in the molecule together. Try to use different coloured jellytots to represent different elements.

You will need jellytots, toothpicks, labels or pieces of paper.

On each piece of paper, write the words: “lone pair”.

You will build models of the following molecules:

\(\text{HCl}\), \(\text{CH}_{4}\), \(\text{H}_{2}\text{O}\), \(\text{BF}_{3}\), \(\text{PCl}_{5}\), \(\text{SF}_{6}\) and \(\text{NH}_{3}\).

For each molecule, you need to:

  • Determine the molecular geometry of the molecule

  • Build your model so that the atoms are as far apart from each other as possible (remember that the electrons around the central atom will try to avoid the repulsions between them).

  • Decide whether this shape is accurate for that molecule or whether there are any lone pairs that may influence it. If there are lone pairs then add a toothpick to the central jellytot. Stick a label (i.e. the piece of paper with “lone pair” on it) onto this toothpick.

  • Adjust the position of the atoms so that the bonding pairs are further away from the lone pairs.

  • How has the shape of the molecule changed?

  • Draw a simple diagram to show the shape of the molecule. It doesn’t matter if it is not \(\text{100}\%\) accurate. This exercise is only to help you to visualise the 3-dimensional shapes of molecules. (See the figures above to help you).

Do the models help you to have a clearer picture of what the molecules look like? Try to build some more models for other molecules you can think of.

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This is a lesson from the tutorial, Bonding and Atomic Combinations and you are encouraged to log in or register, so that you can track your progress.

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