Look at the room around you. What do you see? The walls, your desk, and your computer are all made of matter. The trees outside the window and the water in a drinking glass are all matter. Everything you see and many things you can’t see are made up of matter.

Matter has mass and takes up space. Just as a building may be made of bricks, matter is made of atoms. Atoms are the building blocks of matter. They are so tiny that you cannot see them with your eyes. Atoms cannot be broken apart by ordinary means. To understand nature of atoms a little more, we can do a thought experiment – this is an experiment that we do in our head. Imagine that you have a piece of aluminum foil. You cut the piece in half and then that piece in half and then that. If you keep doing this, you would eventually have a piece so small, that you could not cut it in half. This piece would be an atom. (Of course you could only do this experiment in your head, because your scissors could not cut pieces that small).

Some matter is made up of only one kind of atom. When a substance is made up of only one kind of atom, we call it an element. Examples of elements are gold, oxygen and aluminum. The properties of each element (hardness, color, specific gravity etc.) are all dependent on the kind of atoms that make up the element. There are about 112 known kinds of atoms and therefore about 112 known elements. Some matter is made up of different kinds of atoms.

Scientists have discovered three particles present in atoms:
Electrons, which have a negative (-) charge
Protons, which have a positive (+) charge
Neutrons, which have no charge

All the protons and neutrons are inside the nucleus of the atom. The nucleus is the center of the atom and contains almost all the mass of the atom. Electrons are outside the nucleus. In every atom there are an equal number of protons as electrons. The kind of atom is determined by the number of protons. The number of neutrons is usually not the same as the number of protons, but these two numbers are usually quite close. One of the most surprising findings about atoms is that almost all the space within an atom is empty space. The nucleus takes up only a tiny portion of the space of the atom.

As we have said before, we cannot see atoms, though we can make images of some atoms with very powerful microscopes. Because we cannot see atoms, let alone see inside the nucleus, we have to make models inside our heads and on paper, to help us understand them. In the past two hundred years, there have been many different models of atoms. These models have been based on what scientists knew about atoms at that time. As new evidence was found, our models had to change and will probably change more in the future as more is learned.

There is no perfect way of representing atoms. The image shown below is helpful in many ways – it shows the protons and neutrons in the nucleus and the presence of electrons outside the nucleus. However, scientists believe that electrons occupy space around the nucleus in quite different ways from that shown. As you learn more about chemistry, you will learn more about the nature of electrons. (And the more you learn, the more you will discover that they are very strangely behaving particles, stranger than any science fiction!)

This is a picture of a model of a helium atom:

Do you see the clump in the middle? It is called the nucleus of the atom. It is made up of two protons (shown in red) and two neutrons (shown in green). The electrons of an atom occupy space around the nucleus. In this atom, you can see that there are two electrons (shown in yellow).

Matter would not be very interesting if atoms just existed as individual atoms, but most atoms join with other atoms. We call this chemical bonding. Chemical bonding happens through changes in how the electrons are arranged. There are a few elements, such as helium and neon in which the atoms do not bond with other atoms, but this is unusual.

There are some arrangements of electrons that are very stable and some that are unstable. When atoms bond, they usually do so in a way that makes their electron arrangements stable. There are two ways to do this; atoms can transfer electrons from one to another when they are bonding, or they can share electrons when they are bonding. If they donate or receive electrons, then they form substances composed of a regular pattern of positively charged atoms and negatively charged atoms. (We call charged atoms ions).

An example of a substance that is made by atoms transferring electrons is table salt – those tiny white grains people sprinkle over their fries! Table salt is made of sodium ions and chlorine ions bonded together. The sodium atoms give up an electron each and the chlorine atoms receive an electron each. As we learned in the grade four science, objects with positive charges are attracted to objects with negative charges. To learn more about the nature of salts such as table salt, go to: http://www.sciencebyjones.com/salts.htm 

When atoms bond by sharing electrons, they form structures called molecules. Some molecules are made of atoms that are all the same and some are made of atoms that are different. Water is an example of a substance that is made up of molecules and each molecule is made up of different atoms. Each molecule is made up of two hydrogen atoms and one oxygen atom joined together by sharing electrons. Sometimes we represent this as H₂O, where H stands for hydrogen and O stands for oxygen. Elemental oxygen is also formed of molecules. Each oxygen molecule is made from two oxygen atoms joined together by sharing electrons.

Many compounds are composed of a regular arrangement of atoms or ions (as in table salt). How do we know this? One way is that scientists use a machine called an x ray crystallography machine that sends x rays into a piece of the compound. The x rays are bounced by the atoms that make up the compound and the patterns made by the bounced x rays can be recorded on x ray film. One of the most exciting discoveries made using x ray crystallography was the structure of the enormous molecule that makes up our genes – DNA. This discovery was made by four scientists, Francis Crick, James Watson, Maurice Wilkinson and Rosalind Franklin. Rosalind Franklin was both the crystallographer and played a very important part in solving the puzzle.