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Combustion means the burning of a substance. It is a process that is highly
exothermic, i.e., produces a lot of heat. The products of combustion of carbon and its
compounds are heat energy, carbon dioxide and water (vapor).
When a fuel undergoes combustion, the basic requirements should be present.
- A combustible substance: All carbon compounds are combustible except
carbon as diamond.
- A supporter of combustion: Atmospheric air or oxygen gas is a supporter of
combustion. Combustion does not take place in their absence. Carbon dioxide or
nitrogen gases do not support combustion.
- Heating to ignition temperature: A minimum amount of temperature or heat is
required to enable a fuel to catch fire. Coal has a high ignition temperature; a
matchstick cannot produce enough heat to ignite it. However, a matchstick can ignite
paper or LPG gas as it has low ignition temperature.
- When the above conditions are present in any combustion process, proper
combustion (energy production) takes place with minimum wastage and pollution.
- For example, if an ideal fuel like LPG (high calorific value and relatively
high amounts of branched hydrocarbons) is available, a sufficient and continuous
supply of oxygen should be maintained to burn it. If the ignition spark or flame is
sufficient then the combustion is smooth and completes as follows.
Most of the carbon compounds like the hydrocarbons when burnt in air or
oxygen produce large amounts of heat, carbon dioxide and water vapor. Hence they
are used as fuels. For example, methane burns with a blue flame in air.
In a very limited supply of air methane gives carbon black.
Some carbon compounds are very combustible and have an explosive reaction
with air, e.g., alkenes. They burn with a luminous flame to produce carbon dioxide
and water vapor.
Some hydrocarbon compounds undergo cracking or thermal decomposition. In
this process, substances are heated to high temperatures of (500 - 8000C) in the
absence of air, and they decompose into a mixture of saturated and unsaturated
hydrocarbons and hydrogen.
Allotrope of Carbon
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An element, in different forms, having different physical properties but similar
chemical properties is known as allotropy. Carbon shows allotropy. Such different
forms are called 'allotrope' of an element or allotropic forms. There are three well
known allotropic forms of carbon and they are amorphous carbon, diamond and
graphite. The fourth allotropic form of carbon is buckminsterfullerenes which is
basically an artificial form of carbon and is made up of 60 C atoms.
A few examples of pure carbons are as follows:
Coal, Coke, Charcoal (or wood charcoal), Animal Charcoal (or bone black),
Lamp black, Carbon black,
Gas carbon and Petroleum coke
Diamonds and graphite are two crystalline allotropes of carbon. Diamond and
graphite both are covalent crystals. But, they differ considerably in their properties.
It occurs naturally in free state.
It occurs naturally and is manufactured
It is the hardest natural substance known. It is soft and greasy to touch.
It has high relative density (about 3.5).
Its relative density is 2.3.
It is transparent and has high refractive
It is black in color and opaque.
It is non-conductor of heat and electricity.
Graphite is a good conductor of heat and
It burns in air at 900°C to give CO
It burns in air at 700-800°C to give CO
It occurs as octahedral crystals.
It is insoluble in all solvents.
It is insoluble in all ordinary solvents
These differences in the properties of diamond and graphite are due to the difference
covalent bonds. This leads to a three-dimensional network of covalent bonds. In
graphite, the carbon atoms are arranged in flat parallel layers as regular hexagons.
Each carbon in these layers is bonded to three others by covalent bonds.
Graphite thus acquires some double bond character. Each layer is bonded to adjacent
layers by weak van der Waals forces. This allows each layer to slide over the other
easily. Due to this type of structure graphite is soft and slippery, and can act as a
Biochemistry is the branch of science that explores the chemical processes
within and related to living organisms. It is a laboratory based science that brings
together biology and chemistry. By using chemical knowledge and techniques,
biochemists can understand and solve biological problems.
on what’s happening inside our cells, studying components like proteins, lipids and
organelles. It also looks at how cells communicate with each other, for example
during growth or fighting illness. Biochemists need to understand how the structure
of a molecule relates to its function, allowing them to predict how molecules will
Biochemistry covers a range of scientific disciplines, including genetics,
microbiology, forensics, plant science and medicine. Because of its breadth,
biochemistry is very important and advances in this field of science over the past 100
years have been staggering. It’s a very exciting time to be part of this fascinating area
What do biochemists do?
Provide new ideas and experiments to understand how life works
Contribute innovative information to the technology revolution
Work alongside chemists, physicists, healthcare professionals, policy makers,
engineers and many more professionals
Biochemistry, study of the chemical substances and processes that occur in
plants, animals, and microorganisms and of the changes they undergo during
development and life. It deals with the chemistry of life, and as such it draws on the
techniques of analytical, organic, and physical chemistry, as well as those of
physiologists concerned with the molecular basis of vital processes. All chemical
changes within the organism—either the degradation of substances, generally to gain
necessary energy, or the buildup of complex molecules necessary for life processes—
are collectively termed metabolism. These chemical changes depend on the action of
organic catalysts known as enzymes, and enzymes, in turn, depend for their existence
on the genetic apparatus of the cell. It is not surprising, therefore, that biochemistry
enters into the investigation of chemical changes in disease, drug action, and other
aspects of medicine, as well as in nutrition, genetics, and agriculture.
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
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 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 the reasons for this in a later chapter.
We have now looked at many examples of the types of matter and materials
that exist around us, and we have investigated some of the ways that materials are
classified. But what is it that makes up these materials? And what makes one material
different from another? In order to understand this, we need to take a closer look at
the building block of matter, the atom. Atoms are the basis of all the structures and
organisms in the universe. The planets, the sun, grass and trees, the air we breathe,
and people are all made up of different combinations of atoms.
Models of the Atom
It is important to realise that a lot of what we know about the structure of atoms
developed over a long period of time. This is often how scientific knowledge
develops, with one person building on the ideas of someone else. We are going to
look at how our modern understanding of the atom has evolved over time.
The idea of atoms was invented by two Greek philosophers, Democritus and
Leucippus in the fifth century BC. The Greek word __o
μo_ (atom) means indivisible
because they believed that atoms could not be broken into smaller pieces.
Nowadays, we know that atoms are made up of a positively charged nucleus in
surrounded by negatively charged electrons. However, in the past, before the
structure of the atom was properly understood, scientists came up with lots of
different models or pictures to describe what atoms look like.
How big is an atom?
It is difficult sometimes to imagine the size of an atom, or its mass, because we
cannot see them, and also because we are not used to working with such small
measurements. How heavy is an atom?
It is possible to determine the mass of a single atom in kilograms. But to do
this, you would need very modern mass spectrometers, and the values you would get
would be very clumsy and difficult to use. The mass of a carbon atom, for example, is
about 1.99 x
10−26kg, while the mass of an atom of hydrogen is about 1.67 x
10−27kg. Looking at these very small numbers makes it difficult to compare how
much bigger the mass of one atom is when compared to another.
A molecule is a group of two or more atoms that are attracted to each other by
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 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.
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
molecule, or some
other chemical substance. In chapter 1, we saw how chemical compounds can
sented using element symbols from the Periodic Table. A chemical formula can
us the number of atoms of each element that are in a molecule, and their ratio
For example, the chemical formula for a molecule of carbon dioxide is: CO2
The formula above is called the molecular formula of that compound. The
us that in one molecule of carbon dioxide, there is one atom of carbon and two
oxygen. The ratio of carbon atoms to oxygen atoms is 1:2.
Definition: Molecular formula
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: C6H12O6
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 CH2O. This is called the empirical formula of the
Definition: Empirical formula
This is a way of expressing the relative number of each type of atom in a
In most cases, the empirical formula does not show the exact number of atoms,
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
don’t give us
any idea of what the molecule actually looks like, in other words its shape.
Another useful way of representing molecules is to use 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 2.1).
Using diagrams to show the structure of a molecule
Diagrams of molecules are very useful because they give us an idea of the
space that is
occupied by the molecule, and they also help us to picture how the atoms are
the molecule. There are two types of diagrams that are commonly used:
Figure 2.1: Diagram showing (a) the molecular, (b) the empirical and (c) the
• 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 2.2.
• Space-filling model
This is also a 3-dimensional molecular model. The atoms are represented by
Figures 2.3 and 2.4 are some examples of simple molecules that are
represented in different ways.
Figure 2.3: A space-filling model and structural formula of a water molecule.
atom. This is a simple molecule.
In the previous section, we focused our attention on the electron configuration
of neutral atoms.
In a neutral atom, the number of protons is the same as the number of
electrons. But what
happens if an atom gains or loses electrons? Does it mean that the atom will
still be part of the same element?
A change in the number of electrons of an atom does not change the type of
atom that it is. However, the charge of the atom will change. If electrons are added,
then the atom will become more negative. If electrons are taken away, then the atom
will become more positive. The atom that is formed in either of these cases is called
an ion. Put simply, an ion is a charged atom.
An ion is a charged atom. A positively charged ion is called a cation e.g. Na+,
negatively charged ion is called an anion e.g.
Look at the following examples. Notice the number of valence electrons in the
neutral atom, the number of electrons that are lost or gained, and the final charge of
the ion that is formed.
A lithium atoms loses one electrons to form a positive ion (figure 3.11).
The arrangement of electrons in a lithium ion.
A fluorine atom gains one electron to form a negative ion
The arrangement of electrons in a fluorine ion.
As a result of the models that we discussed in section 3.1, scientists now have a
good idea of what an atom looks like. This knowledge is important because it helps
us to understand things like why materials have different properties and why some
materials bond with others. Let us now take a closer look at the microscopic structure
of the atom.
So far, we have discussed that atoms are made up of a positively charged
nucleus surrounded by one or more negatively charged electrons. These electrons
orbit the nucleus.
The electron is a very light particle. It has a mass of 9.11 x
10−31 kg. Scientists
believe that the electron can be treated as a point particle or elementary particle
meaning that it can’t be broken down into anything smaller. The electron also carries
one unit of negative electric charge which is the same as 1.6 x 1
0−19 C (Coulombs).
Unlike the electron, the nucleus can be broken up into smaller building blocks
called protons and neutrons. Together, the protons and neutrons are called nucleons.
Each proton carries one unit of positive electric charge. Since we know that
atoms are electrically neutral, i.e. do not carry any extra charge, then the number of
protons in an atom has to be the same as the number of electrons to balance out the
positive and negative charge to zero. The total positive charge of a nucleus is equal to
the number of protons in the nucleus. The proton is much heavier than the electron
(10 000 times heavier!) and has a mass of 1.6726 x
When we talk about the atomic mass of an atom, we are mostly referring to the
combined mass of the protons and neutrons, i.e. the nucleons.
The neutron is electrically neutral i.e. it carries no charge at all. Like the
proton, it is much heavier than the electron and its mass is 1.6749 x
(slightly heavier than the proton).
Rutherford predicted (in 1920) that another kind of particle must be present in
positively charged protons in the nucleus, then it should break into bits because of the
repulsive forces between the like-charged protons! Also, if protons were the only
particles in the nucleus, then a helium nucleus (atomic number 2) would have two
protons and therefore only twice the mass of hydrogen. However, it is actually four
times heavier than hydrogen. This suggested that there must be something else inside
the nucleus as well as the protons. To make sure that the atom stays electrically
neutral, this particle would have to be neutral itself. In 1932 James Chadwick
discovered the neutron and measured its mass.
broken up into smaller pieces, the proton and neutron can be divided. Protons
and neutrons are built up of smaller particles called quarks. The proton and
neutron are made up of 3 quarks each.