Atoms and molecules

Introduction: the atom as the building block of matter

We have now seen that different materials have different properties. Some materials are metals and some are non-metals; someare electrical or thermal conductors, while others are not. Depending on the properties of these materials, they can be used in lots of useful applications.But what is it exactly that makes up these materials? In other words, if we were to break down a material into the parts that make it up, what would we find? Andhow is it that a material's microscopic structure (the small parts that make up the material) is able to give it all these different properties?

The answer lies in the smallest building block of matter: the atom . It is the type of atoms, and the way in which they are arranged in a material, that affects the properties of that substance.

It is not often that substances are found in atomic form. Normally, atoms are bonded (joined) to other atoms to form compounds or molecules . It is only in the noble gases (e.g. helium, neon and argon) that atoms are found individually and are not bonded to other atoms. We will look at thereasons for this in a later chapter.

Molecules

Molecule

A molecule is a group of two or more atoms that are attracted to each other by relatively strong forces or bonds.

Almost everything around us is made up of molecules. Water is made up of molecules, each of which has two hydrogen atoms joined to one oxygen atom. Oxygen is a molecule that is made up of two oxygen atoms that are joined to one another. Even the food that we eat is made up ofmolecules that contain atoms of elements such as carbon, hydrogen and oxygen that are joined to one another in different ways. All of these are known as small molecules because there are only a few atoms in each molecule. Giant molecules are those where there may be millions of atoms per molecule. Examples of giantmolecules are diamonds , which are made up of millions of carbon atoms bonded to each other and metals , which are made up of millions of metal atoms bonded to each other.

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 byusing different types of diagrams , but at other times, we may decide to simply represent a molecule using its chemical formula or its written name.

1. Using formulae to show the structure of a molecule. A chemical formula is an abbreviated (shortened) way of describing a molecule, or some other chemical substance. Inthe chapter on classification of matter, we saw how chemical compounds can be represented using element symbols from the Periodic Table. A chemical formulacan also tell us the number of atoms of each element that are in a molecule and their ratio in that molecule. For example, the chemical formula for a molecule of carbon dioxide isCO ₂ The formula above is called the molecular formula of that compound. The formula tells us that in one molecule of carbon dioxide, there is one atom of carbon and two atoms of oxygen. Theratio of carbon atoms to oxygen atoms is 1:2.

   Molecular formula

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

   A molecule of glucose has the molecular formula: C ₆ H ₁₂ O ₆ . In each glucose molecule, there are six carbon atoms, twelve hydrogen atoms andsix oxygen atoms. The ratio of carbon:hydrogen:oxygen is 6:12:6. We can simplify this ratio to write 1:2:1, or if we were to use the element symbols, the formulawould be written as CH ₂ O. This is called the empirical formula of the molecule.

   Empirical formula

   This is a way of expressing the relative number of each type of atom in a chemical compound. In most cases, the empiricalformula 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 a giant molecule . Since giant molecules may consist of millions of atoms, it is impossible to say exactly howmany atoms are in each molecule. It makes sense then to represent these molecules using their empirical formula. So, in the case of a metal such ascopper, we would simply write Cu, or if we were to represent a molecule of sodium chloride, we would simply write NaCl.Chemical formulae therefore tell us something about the types of atoms that are in a molecule and the ratio in which these atoms occur in the molecule, but they don't give us any idea of what the molecule actually lookslike, in other words its shape . To show the shape of molecules we can represent molecules using diagrams.Another type of formula that can be used to describe a molecule is its structural formula . A structural formula uses a graphical representation to show a molecule's structure( [link] ).

   

2. Using diagrams to show the structure of a molecule Diagrams of molecules are very useful because they help us to picture how the atoms are arranged in the molecule and they help us to see the shape of themolecule. There are two types of diagrams that are commonly used:
   • 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 (theballs) are connected by straight lines which represent the bonds between them. A simplified example is shown in [link] .

     

   • Space-filling model This is also a 3-dimensional molecular model. The atoms are represented by spheres. [link] and [link] are some examples of simple molecules that are represented in different ways.

     

     

   [link] shows the bonds between the carbon atoms in diamond, which is a giant molecule . Each carbon atom is joined to four others, and this pattern repeats itself until a complex lattice structure is formed. Each black ball in the diagram represents a carbon atom, and each line represents the bondbetween two carbon atoms. Note that the carbon atoms on the edges are actually bonded to four carbon atoms, but some of these carbon atoms have been omitted.

   

This website allows you to view several molecules. You do not need to know these molecules, this is simply to allow you to see one way ofrepresenting molecules.

Atoms and molecules

1. In each of the following, say whether the chemical substance is made up of single atoms, simple molecules or giant molecules.
   1. ammonia gas (NH ₃ )
   2. zinc metal (Zn)
   3. graphite (C)
   4. nitric acid (HNO ₃ )
   5. neon gas (Ne)
2. Refer to the diagram below and then answer the questions that follow:

   

   1. Identify the molecule.
   2. Write the molecular formula for the molecule.
   3. Is the molecule a simple or giant molecule?
3. Represent each of the following molecules using its chemical formula , structural formula and ball and stick model .
   1. Hydrogen
   2. Ammonia
   3. sulphur dioxide