All of matter is made up of smaller particles. At the smallest level scientists understand, there are tiny particles called "subatomic" particles. They are the smallest particles because we don't know how to break them up into something smaller. And there are a lot of them. One of these particles, called an "electron", is what electricity is made from. Some of the other particles combine to form objects called "protons" and "neutrons". These protons and neutrons combine with electrons to make atoms.

Different kinds of atoms have different numbers of protons, neutrons, and electrons. For example, oxygen, which we all need to breathe, is made from 8 protons, 8 electrons, and 8 neutrons.

Atoms can also combine together to make things called molecules - water is made from two hydrogen atoms and one oxygen atom. And molecules combine together to make everything - you, me, this computer, your school, your house, the sun - everything. If you understand about atoms, you understand the most important thing to know about science.

Actually (like many simple questions) your question is difficult to answer. Many years (about 2000) ago, the Greek philosopher, Democritus realized that if you keep breaking down something (say a piece of metal such as gold) into smaller and smaller parts you get smaller and smaller pieces of gold. But if you go on in this manner, you will finally reach a stage when the pieces that you have are no longer gold. He used this idea to say that all materials are made up of small "things" called atoms, and anything smaller than an atom will not have any resemblance to the material.

In more modern terms, we know that there are a certain number(about a 100) of chemical elements (can you name some ?) and all of these are made up of unique atoms --- gold atoms are different from carbon atoms. But if you break up a gold atom,what you have is no longer gold.

Atoms are very small. 1 g of gold has approximately 3 000 000 000 000 000 000 000 atoms.

Scientists and philosophers wondered for many years about the smallest components of all material things. The Greeks thought that all matter was made up of small particles of four elements: Earth, Air, Fire and Water. By the end of the 18th century, experiments with simple chemicals led some scientists (Priestly, Lavoisier, Dalton) to postulate that all matter was made up of a fairly small set of chemical 'atoms' which combined to make all kinds of matter, but could not be broken down.

There are about 100 known elements, each having a particular atom. Everything is made of such atoms, including the air, water, fire and earth of the ancients. The only things you are likely to have seen which are not composed of atoms is light and magnetic fields.

Most simply, atoms can be though of as having a core (nucleus) which is massive and positively changed, and a cloud of much lighter electrons which are negative and balance the positive charge. Although atoms are not broken down in known chemical reactions, they can be broken or fused at much higher energies such as in the core of the sun or a nuclear reactor. So, although they are not the simplest nor the smallest particles of matter, atoms are the smallest particles for which chemical reactions such as fire have meaning.