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Representing Molecules

Representing Molecules

The structure of a molecule can be shown in many different ways. Sometimes it is easiest to show what a molecule looks like by using different types of diagrams, but at other times, we may decide to simply represent a molecule using its chemical formula or its written name.

Using Formulae to Show the Structure of a Molecule

A chemical formula is an abbreviated (shortened) way of describing a compound. In Chapter 2, we saw how chemical compounds can be represented using element symbols from the periodic table. A chemical formula can also tell us the number of atoms of each element that are in a compound and their ratio in that compound. If the compound is a covalent molecular compound then we can use the molecular formula.

Definition: Molecular Formula

The molecular formula is a concise way of expressing information about the atoms that make up a particular covalent molecular compound. The molecular formula gives the exact number of each type of atom in the molecule.

For example in the figure below the molecular formula of 2-methyl propane is \(\text{C}_{4}\text{H}_{10}\). This tells us that there are 4 carbon atoms and 10 hydrogen atoms in this molecule, i.e the ratio of carbon to hydrogen is \(4:10\)\(2:5\) empirical formula of the molecule.

Note:

The empirical and the molecular formulae can be the same. For example in carbon dioxide the molecular formula is \(\text{CO}_{2}\). This is also the empirical formula since it is the simplest ratio.

Definition: Empirical formula

The empirical formula is a way of expressing the relative number of each type of atom in a chemical compound. The empirical formula does not show the exact number of atoms, but rather the simplest ratio of the atoms in the compound.

The empirical formula is useful when we want to write the formula for network structures. Since network structures may consist of millions of atoms, it is impossible to say exactly how many atoms are in each unit. It makes sense then to represent these units using their empirical formula. So, in the case of a metal such as copper, we would simply write \(\text{Cu}\), or if we were to represent a unit of sodium chloride, we would simply write \(\text{NaCl}\).

Chemical formulae (i.e. the molecular or the empirical formula) therefore tell us something about the types of atoms that are in a compound and the ratio in which these atoms occur in the compound, but they don’t give us any idea of what the compound actually looks like, in other words its shape. To show the shape of compounds we have to use diagrams. The simplest type of diagram that can be used to describe a compound is its structural formula. The structural formula for 2-methyl propane is shown in the figure below.

Diagram showing (a) the chemical, (b) the empirical and (c) the structural formula of 2-methyl propane


Using Diagrams to Show the Structure of a Compound

Diagrams of compounds are very useful because they help us to picture how the atoms are arranged in the compound and they help us to see the shape of the compound. There are three types of diagrams that are commonly used:

  • Wireframe or stick models

    In this model, the bonds between atoms are shown as “sticks”. These “sticks” are coloured to show which atoms are bonding.

  • Ball and stick models

    This is a 3-dimensional molecular model that uses “balls” to represent atoms and “sticks” to represent the bonds between them. The centres of the atoms (the balls) are connected by straight lines which represent the bonds between them.

  • Space-filling models

    This is also a 3-dimensional molecular model. The atoms are represented by spheres.

The table below shows examples of the different types of models for all the types of compounds.

Table: Different Representations for Compounds

 

Covalent molecular

Covalent network

Ionic network

Metallic network

Name of compound

glucose

graphite

silver chloride

zinc

Formula

\(\text{C}_{6}\text{H}_{12}\text{O}_{6}\) or \(\text{CH}_{2}\text{O}\)

\(\text{C}\)

\(\text{AgCl}\)

\(\text{Zn}\)

Stick model

Ball-and-stick model

Space-filling model

Activity: Representing Compounds

A list of substances is given below. Make use of atomic model kits, play dough and toothpicks, or coloured polystyrene balls and skewer sticks to represent each of the substances in three dimensional structures.

  • glucose (\(\text{C}_{6}\text{H}_{12}\text{O}_{6}\))

  • silica (\(\text{SiO}_{2}\))

  • sodium chloride (\(\text{NaCl}\))

  • sulfur (\(\text{S}_{8}\))

  • diamond (\(\text{C}\))

  • graphite (\(\text{C}\))

  • buckyballs(\(\text{C}_{60}\))

  • sucrose (\(\text{C}_{12}\text{H}_{22}\text{O}_{11}\))

  • copper (\(\text{Cu}\))

CanvasMol (www.alteredqualia.com/canvasmol) is a website that allows you to view several compounds. You do not need to know these compounds, this is simply to allow you to see one way of representing compounds.

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