Most atoms join up with other atoms through chemical BONDS to form larger particles called molecules. They can join up with
atoms of the same element or with atoms of different elements. Substances whose
molecules contain different types of atom are called compounds. Chemical
reactions can CHANGE MOLECULES and when this happens, new
molecules and therefore new compounds are formed.
Molecules can be simple or complex. They can even be made up of just
one atom. The element argon is a one-atom molecule. Other molecules can consist
of two atoms of the same element. The oxygen molecule is made up of two oxygen
atoms bonded together. However, in certain circumstances, three oxygen atoms
bond together, forming a molecule called ozone.
Water molecules (H20) are very simple. They are made of
two hydrogen (H) atoms bonded to one oxygen (O) atom. All water molecules are
the same, but they are different from the molecules of any other substance. A
water molecule is the smallest possible piece of water. You can break it up
into smaller pieces, but they wouldn’t be water anymore. The symbols
that scientists use to represent molecules are called chemical formulae.
Some molecules, such as the plastic in a snorkel, contain hundreds
or even thousands of carbon, hydrogen, and chlorine atoms joined together in
long, winding chains. Such complex molecules are called polymers. They are
possible because carbon atoms are able to form very stable bonds with other
carbon atoms. Most of the molecules that make up living things are made of
complex polymers.
When atoms join together to form molecules, they are held
together by chemical bonds. These bonds form as a result of the sharing or
exchange of electrons between the atoms. It is only the electrons in the
outermost shell that ever get involved in bonding. Different atoms use these
electrons to form one of three different types of bond: ionic bonds, covalent
bonds, or metallic bonds.
DIFFERENT KINDS OF BONDS BETWEEN ATOMS
In ionic bonds, electrons are transferred from one atom to
another. When sodium and chlorine combine to form sodium chloride (salt),
sodium loses an electron and becomes positively charged; chlorine takes that
electron and becomes negatively charged. Ionic bonds are difficult to break.
Ionic compounds are usually solids with high melting points.
In a covalent bond, electrons are shared between two atoms. When
two oxygen atoms bond together to form an oxygen molecule, they share four
electrons – two from each oxygen atom. Other examples of covalent
bonding are water (H2O), and carbon dioxide (CO2).
Covalent compounds are usually liquids or gases with low melting points.
Metal atoms are bonded to each other through metallic bonding.
In this type of bonding, all the atoms lose electrons, which float around in a
common pool. The electrons in this pool can move around freely, which is why
metals can transfer heat or electricity so well. If one part of the metal is
heated, the electrons carry the heat quickly to other parts.
All around you, molecules are changing and rearranging
their atoms in chemical reactions to form new molecules and new compounds. When
you breathe in oxygen, it goes through a chemical change inside your body and
forms a new compound, carbon dioxide, which you breathe out. Catalysts are
special types of molecules that speed up chemical reactions, but do not
actually change themselves. They are used, for example, in catalytic converters
in cars.
A special effects explosion is a chemical reaction that releases
energy. Pyrotechnic experts want each explosion to be unique, so they use
different types and amounts of explosives. In every chemical reaction, some
bonds between atoms are broken and new ones are made. Energy is needed to break
a bond, but energy is released when a bond is made. Depending on the number and
type of bonds broken and made, a reaction may take in or give out energy.
When a car engine burns petrol, it releases harmful gases. Cars
fitted with a catalytic converter change the harmful gases into safer gases.
When they enter the catalytic converter, the gases form temporary bonds with
the surface of the catalyst. This brings them into close contact with each
other and allows new, safer gases to form.
Enzymes are catalysts found in nature. For example, it is the
enzymes in yeast that cause bread dough to rise. When yeast is mixed with warm
water and sugar it starts to grow and bubbles of carbon dioxide gas are
produced. When the yeast mixture is added to flour and water to make a dough,
the dough rises. Heating bakes the bread and kills the yeast. Scientists use
chemical equations to show how molecules change in a chemical reaction.
