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.
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] ).
Diagram showing (a) the molecular, (b)
the empirical and (c) the structural formula of isobutane
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] .
A ball and stick model of a water
molecule
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.
A space-filling model and structural
formula of a water molecule. Each molecule is made up of two hydrogen atoms thatare attached to one oxygen atom. This is a simple molecule.A space-filling model and structural
formula of a molecule of ammonia. Each molecule is made up of one nitrogen atomand three hydrogen atoms. This is a simple molecule.
[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.
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 thelattice structure. Diamond is a giant molecule.
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 useas 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 ofrepresenting molecules.
Ball-and-stick view of ethanol from
(External Link)
Atoms and molecules
In each of the following, say whether the chemical
substance is made up of single atoms, simple molecules or giant molecules.
ammonia gas (NH
3 )
zinc metal (Zn)
graphite (C)
nitric acid (HNO
3 )
neon gas (Ne)
Refer to the diagram below and then answer the
questions that follow:
Identify
the molecule.
Write the molecular formula for the molecule.
Is the molecule a simple or giant molecule?
Represent each of the following molecules using its
chemical formula ,
structural formula and
ball and stick model .
