Copyright © 2007 Dorling Kindersley
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.
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.