Wikijunior:The Elements/Introduction

=Introduction=



To know what "Elements" are, you need to know about chemistry. So, what is chemistry? Chemistry is the study of matter and the changes that take place within that matter. Unfortunately, this does not tell you very much about chemistry, and it is also a very long way from telling you EVERYTHING about chemistry.

Everything on Earth, everything in our solar system, everything in our galaxy, and everything in the universe is made up of matter. Matter is the name that scientists have given to everything you can touch, or see, or feel, or smell. Matter takes up space, and matter has weight. To be really correct, we should say that matter has volume and mass. In other words matter has substance, and "Elements" (about 118 of them) are what gives matter its substance, even what makes it exist at all.

Most of the matter around you will have more than one element in it. But there are bits of matter that you have certainly seen and touched that are made up of just one element. If you have ever held a diamond, for example, it has been formed from just one element, the element Carbon. Surprisingly, the graphite in a pencil that you use for drawing or writing is also Carbon, just arranged differently.

Some other examples of matter you may have seen which have just one element are: an aluminium drink can; 24 carat gold jewelry; the helium gas in a balloon that floats upwards; cast iron garden railings; lead sheeting used by builders on the roofs of houses; a powder called "Flowers of Sulphur" that you can buy in a pharmacy.

In actual fact these are not perfectly true examples of elements, because an element, like gold or aluminium, will always have tiny amounts of other elements present as impurities.

Not many elements occur "naturally;" that is to say as pieces of nearly pure substance that you can pick from the ground. The elements Gold and Sulphur do occur naturally, as does Carbon when it is a diamond.

To purify them, most elements have to be pried apart from one another, because in most matter they are fixed together by strong "chemical bonds." Sometimes this can be done quite easily by using heat or electricity.

Ancient people knew how to make the element Iron. They did it by heating iron ore dug from the ground with charcoal they got from burning wood. The matter containing the iron was originally a "compound" of two elements, iron and oxygen; by heating with charcoal the oxygen was removed as a gas called carbon dioxide, leaving iron behind.

The elements Sodium and Chlorine can be separated from common salt by passing an electric current into salt that is melted at a very high temperature. The salt we started with is said to be a "compound" of sodium and chlorine. It is given the scientific name of sodium chloride.

One interesting source of elements is the air around us. It contains a mixture of elements that are all gases, such as Oxygen which you will have heard of already, as well as many gases which are not elements but are compounds — a bit like salt, only different!.

To get the elements from the air we need to cool it down to a very low temperature, much lower than even Siberia in winter! At around &minus;190°C it forms a liquid. It's a bit like cooling down steam to make water. Afterwards, by boiling this liquid, the gases are released one at a time. As well as elements Oxygen and Nitrogen, we can get what are called the "Noble Gas" elements Argon, Xenon and Krypton. Two other noble gases, Helium and Neon, are in the air and can be taken out by cooling to an even lower temperature.

You have probably wondered how we have got this far without using some words that you probably have heard, and which are very much part of the subject of chemistry: atom, molecule, electron, nuclear and so forth. In fact, up until about 150 years ago chemists didn't really know very much about any of these things, even though they did understand the difference between elements and compounds.

Today we can "see" atoms by using a special electron microscope, which magnifies things that are millions of times smaller than those things we look at with an ordinary microscope. Under a scanning electron microscope, which shows things in 3-D, atoms appear as fuzzy balls.

Atoms are the key to describing why there exists different elements and why these elements form just those compounds that they do.

Atoms also explain why the Periodic Table of Elements is the way it is, even though much of it was worked out by the old-time chemists like Dobereiner and Mendeleev (see Chapter 4). Understanding atoms can explain, for example, why there are just 81 elements that have atoms which can exist for ever.

Wikijunior Die Elemente/ Einführung Materie fr:Wikijunior:Les éléments/Introduction