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Atoms are joined to form compounds and molecules.
- A compound contains 2 or more different
kinds of atoms chemically bonded in particular proportions.
Water is one of the simplest compounds. It contains
atoms of oxygen and hydrogen. The ratio of hydrogen
atoms to oxygen atoms is 2 to 1. So we could call
it dihydrogen oxide. Carbon dioxide is another familiar
chemical substance. Although we cannot see or smell
it you are exhaling it right now. As its name suggests
there are twice as many oxygen atoms as carbon atoms.
We have all heard of carbon monoxide, the poisonous
gas released by faulty burners. Carbon monoxide contains
carbon and oxygen atoms in equal numbers, one for
one.
- Molecules consist of atoms joined
together by chemical bonds. Molecules can consist
of atoms of the same element. Oxygen, for example,
usually exists in the molecule 02, which
is two oxygen atoms bonded together. Molecules can
also consist of the atoms of different elements. These
types of molecules are the components of compound
substances. For example a water molecule (H2O)
consists of 2 hydrogen atoms and an oxygen atom bonded
together.
- A mixture is 2 or more substances
which have been mixed together but which have not
chemically bonded. Air is a mixture. It contains nitrogen,
oxygen, carbon dioxide, a small amount of inert gases
and some moisture. The proportions of these substances
are not fixed. The air we breathe out contains rather
more carbon dioxide and moisture than the air we breathe
in. Seawater is also a mixture. It is mostly water,
but has a lot of salt dissolved in it.
Atoms bond together by sharing electrons from their
outer shells in order to get a full or empty outer shell
of electrons. What occurs in a chemical bond is actually
an electrostatic force of attraction between 2 atoms
or ions. There are 3 main ways that atoms bond:
- Ionic bonding is when atoms transfer
electrons to another atom to form ions. Ions are formed
when atoms lose or gain electrons and become charged:
atoms that lose electrons form a positive ion (usually
metal e.g. Fe2+); atoms that gain electrons
form a negative ion (usually a non-metal element e.g.
Cl-).
- Covalent bonding is when a pair
of electrons are shared between 2 atoms. The electron
pair is usually one electron from each atom. Electrostatic
forces between the shared pair of electrons and the
nuclei of the 2 atoms hold the atoms together. This
type of bond is usually formed between 2 non-metal
elements. The atoms may be from one single element
or from different elements chemically combining to
form a compound. Most of the compounds we come across
are covalent compounds – wood, plastic, clothing,
food - the atoms of the majority of these kinds of
materials are joined by covalent bonds. Sometimes
atoms share more than one pair of electrons, for example
a double covalent bond can occur, as in the carbon
dioxide molecule where carbon shares 2 pairs of electrons
with each oxygen atom: O=C=O (where = represents a
double bond).
- Another kind of covalent bonding is called co-ordinate
or dative covalent. This occurs when both
electrons in a covalent bond come from just one
of the atoms. Carbon monoxide is bonded in this
way: the oxygen atom donates 4 electrons, whilst
carbon donates 2.
- Metallic bonding occurs only between
pure metals. Metalic bonds are giant structures of
atoms held together by neither ionic nor covalent
bonding. The outer shell of metal atoms has either
1, 2 or 3 electrons which actually move around from
one atom to another, in a kind of sea of electrons,
and the structure is all held together by the positive
attraction in the nucleus.
All chemical substances consist of particles. These
may be individual atoms as in the case of neon gas,
small individual molecules as in the case of hydrogen
or oxygen gas, larger molecules of compounds ranging
from methane CH4 or glucose C6H12O6,
very large molecules such as proteins and DNA, or giant
structures such as diamonds. In every case the individual
atoms vibrate and the atomic or molecular particles
will move around. How much these particles vibrate and
move depends on how much energy they have. Cooling substances
down involves removing energy and the more energy is
removed the less the particles will move. In theory,
at absolute zero, i.e. minus 273 degrees Celsius, the
particles will have absolutely no energy and will stop
moving, although in practice this is impossible to achieve.
In contrast, heating a substance will make the particles
vibrate and move more rapidly.
There are 3 main states
of matter – solid,
liquid and gas, and
2 other curious states – super-cooled
liquid and plasma. The main
states of matter are obvious to everyone. We all know
what solids, liquids and gases look like though we may
not appreciate how the particles in them are behaving
or how they change from one state to another.
In a gas the particles have so much energy they move
around freely: each particle is free to vibrate and
move around. Although there is an attraction between
individual particles in a gas, it is not strong enough
to hold the particles together. When a gas is cooled
down, the particles have less energy and start to stick
together: individual particles are attracted to one
another. The gas is said to condense as it turns into
a liquid. In a liquid, the particles
are still free to move around, but the attractions between
them prevent them from moving so freely. Squeezing a
gas also helps it to condense. In fact, turning oxygen
gas into liquid oxygen is only possible if it is cooled
and squeezed. The opposite of condensation is evaporation.
When a liquid is heated, individual particles have enough
energy to escape from the attraction of other particles.
When this happens the liquid evaporates. Liquids will
evaporate at lower temperatures if the pressure is reduced.
The temperature at which a gas condenses or the liquid
evaporates is its boiling point. The change is a change
of phase.
When a liquid is cooled sufficiently it will become
a solid. The particles are no longer free to move around.
Heating the solid will cause it to melt. The extra energy
of the particles allows them to escape the attraction
of other particles. Individual particles in a solid
still have some energy and continue to vibrate, but
they cannot escape from their neighbours. The change
from liquid to solid and solid to liquid is another
change of phase and the temperature at which this happens
is referred to as the melting point, or sometimes freezing
point.
Boiling points of substances are affected by pressure.
Increasing pressure will increase the temperature at
which the substance boils. Tables of boiling points
give figures for standard pressure. The temperature
at which substances melt and freeze is not affected
by pressure in the same way. However, impurities will
affect both melting and boiling points.
The general tendency is for smaller molecules to change
phase at lower temperatures and larger molecules to
change phase at much higher temperatures. For example,
the hydrocarbons methane CH4, ethane C2H6,
propane C3H8, and butane C4H10
are all gases at ordinary room temperature. Pentane
C5H12 is a liquid at room temperature.
Hydrocarbons (molecules made up of only hydrogen and
carbon atoms) with chains of more than about 20 carbon
atoms are solids at room temperature. The longer the
chains are, the more difficult it is to melt them.
For every day purposes a substance like glass is obviously
a solid. However, glass does not behave like other solids.
The convenient division of matter into solid, liquid
and gas does not fit every situation. Although glass
is hard, and it feels and looks like other solids, the
molecules actually move around rather like the particles
in a liquid. For this reason it is sometimes called
a super-cooled liquid. Our perception of solids, liquids
and gases is not sufficient to explain what the matter
in a star is really like. The Sun consists of a mixture
of elements, but is mainly hydrogen and helium. The
nuclear process converting hydrogen into helium only
happens at exceedingly high temperatures and pressures.
Individual particles in the Sun (atoms of hydrogen and
helium) have an enormous amount of energy. Scientists
refer to this material as plasma rather than gas. The
particles are very close to each other as in a liquid,
but moving around as in a gas.
Some elements can exist in different forms but in the
same physical state. This is known as allotropy. Take
carbon, for example. There are 2 types of solid carbon:
diamond and graphite, so it is said to exist in allotropic
forms. Diamond and graphite are the most commonly known
allotropes of carbon, but there are actually more: fullerene
is also a solid form of carbon. This is crystalline
carbon which is made up of clusters of carbon atoms.
Why are there different versions of the same element
in the same state?
The answer to this question lies with an understanding
of how the atoms have bonded. In a diamond, the carbon
atoms are strongly bonded (covalent bonds) to 4 other
carbon atoms. This makes up a tetrahedral giant structure.
The structure is uniform and extremely strong, making
it a tough, resistant material.
Graphite, on the other hand, is bonded in layers rather
than in a tetrahedral structure as in the diamond. Each
layer in graphite is bonded covalently, and bonds within
the layers are strong, but the bonds between those layers
are weak, which means the layers can slide over each
other. This explains why graphite is a fairly brittle,
flaky substance.
When substances are heated or cooled there are usually
changes in volume. Heating a gas causes it to expand
and if it is in an enclosed container the pressure will
increase. Cooling a gas has the opposite effect. Refrigerators
use a similar process to keep the food inside cool.
Heating and cooling solids and liquids also results
in changing volume. Heating metals makes them expand
and cooling them makes them contract. Large metal structures
must have special expansion joints. As the metal heats
up and expands on a hot summer’s day or cools
down and shrinks on a cold winter’s night, the
expansion joints compensate. Metal bridges don’t
collapse and railway lines don’t buckle from season
to season.
Water has some very interesting properties. As water
is cooled it contracts. Cold water is denser than hot
water. Water molecules attract and repel each other
at the same time: the hotter they are, the more they
repel each other. If a given mass of water is heated
and expands, it will become less dense. As water at
the surface of the sea warms up in summer, it becomes
less dense and floats at the top of the sea. The surface
of the ocean in the summer may rise to 20 degrees celsius
or more whilst the water beneath is still hovering above
freezing point. This phenomenon is important in central
heating systems. Hot water rises to the top of radiators.
As it cools and heats the room, it will sink to the
bottom of the radiator, return to the boiler and be
reheated. This is known as a convection current. The
same thing happens to the air in the room. Hot air rises
and cold air sinks and air circulates around the room.
In fact, on a much larger scale, convection currents
are involved in producing our weather.
So why then, if cold water falls to the bottom of a
lake, does ice float on top of it? When water is cooled
below 4 degrees Celsius, the hydrogen bonds connecting
different the water molecules begin to form a crystalline
structure, and as the temperature drops further they
join together to form ice. The resulting structure is
less dense than water as a liquid - approxiamately 9%
less dense. In other words, if a container with a volume
of 1 litre was filled with ice, it would weigh less
than if it was filled with liquid water, and since objects
lighter than water will float, when you place a block
of ice into water, it floats.
 Until
very recently it was impossible to see molecules. Now
scientists are manipulating molecules at the level of
a billionth of a metre. This is called nanotechnology.
Research has focused on molecular manufacturing - the
creation of tools and machines (called nanobots) that
will eventually enable scientists to snap together the
fundamental building blocks of nature. The uses of nanotechnology
are endless, particularly in medicine where it could
be used to fight viruses or even repair genetic diseases.
All materials, from metals to wood to food, could be
replicated using nanotechnology, or it could be used
to halt pollution. However, nanotechnology is still
very much in its infancy.
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