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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; some are 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? And how 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. This is similar to building materials. We can use bricks, steel, cement, wood, straw (thatch), mud and many other things to build structures from. In the same way that the choice of building material affects the properties of the structure, so the atoms that make up matter affect the properties of matter.

It is not often that substances are found in atomic form (just as you seldom find a building or structure made from one building material). 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 looked at some of the reasons for this in earlier chapters.

Compounds

Definition 1: Compound
A compound is a group of two or more different atoms that are attracted to each other by relatively strong forces or bonds.

Almost everything around us is made up of molecules. The only substances that are not made of molecules, but instead are individual atoms are the noble gases. 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 of molecules 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 giant molecules 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.

As we learnt in (Reference) atoms can share electrons to form covalent bonds or exchange electrons to form ionic bonds. Covalently bonded substances are known as molecular compounds. Ionically bonded substances are known as ionic compounds. We also learnt about metallic bonding. In a metal the atoms lose their outermost electrons to form positively charged ions that are arranged in a lattice, while the outermost electrons are free to move amongst the spaces of the lattice.

We can classify covalent molecules into covalent molecular structures and covalent network structures. Covalent molecular structures are simply individual covalent molecules and include water, oxygen, sulphur (S8S8) and buckminsterfullerene (C60C60). All covalent molecular structures are simple molecules. Covalent network structures are giant lattices of covalently bonded molecules, similar to the ionic lattice. Examples include diamond, graphite and silica (SiO2SiO2). All covalent network structures are giant molecules.

Examples of ionic substances are sodium chloride (NaClNaCl) and potassium permanganate (KMnO4KMnO4). Examples of metals are copper, zinc, titanium, gold, etc.

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.

  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. In the chapter on classification of matter, 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 molecule and their ratio in that molecule. For example, the chemical formula for a molecule of carbon dioxide is CO2CO2 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. The ratio of carbon atoms to oxygen atoms is 1:2.
    Definition 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 of each type of atom in the molecule.
    A molecule of glucose has the molecular formula: C6H12O6C6H12O6. In each glucose molecule, there are six carbon atoms, twelve hydrogen atoms and six 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 formula would be written as CH2OCH2O. This is called the empirical formula of the molecule.
    Definition 3: Empirical formula
    This is a way of expressing the relative number of each type of atom in a chemical compound. In most cases, 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 a giant molecule. Since giant molecules may consist of millions of atoms, it is impossible to say exactly how many 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 as copper, 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 looks like, 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 (Figure 1).
    Figure 1: Diagram showing (a) the molecular, (b) the empirical and (c) the structural formula of isobutane
    Figure 1 (CG10C2_001.png)
  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 the molecule. 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 (the balls) are connected by straight lines which represent the bonds between them. A simplified example is shown in Figure 2.
      Figure 2: A ball and stick model of a water molecule
      Figure 2 (CG10C2_002.png)
    • Space-filling model This is also a 3-dimensional molecular model. The atoms are represented by spheres. Figure 3 and Figure 4 are some examples of simple molecules that are represented in different ways.
      Figure 3: A space-filling model and structural formula of a water molecule. Each molecule is made up of two hydrogen atoms that are attached to one oxygen atom. This is a simple molecule.
      Figure 3 (CG10C2_003.png)
      Figure 4: A space-filling model and structural formula of a molecule of ammonia. Each molecule is made up of one nitrogen atom and three hydrogen atoms. This is a simple molecule.
      Figure 4 (CG10C2_004.png)
    Figure 5 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 bond between 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.
    Figure 5: Diagrams showing the microscopic structure of diamond. The diagram on the left shows part of a diamond lattice, made up of numerous carbon atoms. The diagram on the right shows how each carbon atom in the lattice is joined to four others. This forms the basis of the lattice structure. Diamond is a giant molecule.
    Figure 5 (CG10C2_005.png)

Note: Interesting Fact :

Diamonds are most often thought of in terms of their use in the jewellery industry. However, about 80% of mined diamonds are unsuitable for use as gemstones and are therefore used in industry because of their strength and hardness. These properties of diamonds are due to the strong covalent bonds (covalent bonding will be explained later) between the carbon atoms in diamond. The most common uses for diamonds in industry are in cutting, drilling, grinding, and polishing.

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 of representing molecules.

Figure 6: Ball-and-stick view of ethanol from http://alteredqualia.com/canvasmol/
Figure 6 (canvasmol.png)

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 (NH3NH3)
    2. zinc metal (ZnZn)
    3. graphite (CC)
    4. nitric acid (HNO3HNO3)
    5. neon gas (HeHe)
    Click here for the solution
  2. Refer to the diagram below and then answer the questions that follow:
    Figure 7
    Figure 7 (CG10C2_006.png)
    1. Identify the molecule.
    2. Write the molecular formula for the molecule.
    3. Is the molecule a simple or giant molecule?
    Click here for the solution
  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
    Click here for the solution

Summary

  • The smallest unit of matter is the atom. Atoms can combine to form molecules.
  • A compound is a group of two or more atoms that are attracted to each other by chemical bonds.
  • A small molecule consists of a few atoms per molecule. A giant molecule consists of millions of atoms per molecule, for example metals and diamonds.
  • The structure of a molecule can be represented in a number of ways.
  • The chemical formula of a molecule is an abbreviated way of showing a molecule, using the symbols for the elements in the molecule. There are two types of chemical formulae: molecular and empirical formula.
  • The molecular formula of a molecule gives the exact number of atoms of each element that are in the molecule.
  • The empirical formula of a molecule gives the relative number of atoms of each element in the molecule.
  • Molecules can also be represented using diagrams.
  • A ball and stick diagram is a 3-dimensional molecular model that uses 'balls' to represent atoms and 'sticks' to represent the bonds between them.
  • A space-filling model is also a 3-dimensional molecular model. The atoms are represented by spheres.
  • In a molecule, atoms are held together by chemical bonds or intramolecular forces. Covalent bonds, ionic bonds and metallic bonds are examples of chemical bonds.
  • A covalent bond exists between non-metal atoms. An ionic bond exists between non-metal and metal atoms and a metallic bond exists between metal atoms.
  • Intermolecular forces are the bonds that hold molecules together.

End of chapter exercises

  1. Give one word or term for each of the following descriptions.
    1. A composition of two or more atoms that act as a unit.
    2. Chemical formula that gives the relative number of atoms of each element that are in a molecule.
    Click here for the solution
  2. Give a definition for each of the following terms: descriptions.
    1. molecule
    2. Ionic compound
    3. Covalent network structure
    4. Empirical formula
    5. Ball-and-stick model
    Click here for the solution
  3. Ammonia, an ingredient in household cleaners, can be broken down to form one part nitrogen (NN) and three parts hydrogen (HH). This means that ammonia...
    1. is a colourless gas
    2. is not a compound
    3. cannot be an element
    4. has the formula N3HN3H
    Click here for the solution
  4. Represent each of the following molecules using its chemical formula, its structural formula and the ball-and-stick model:
    1. nitrogen
    2. carbon dioxide
    3. methane
    4. argon
    Click here for the solution

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