The Greek word for indivisible is 'atomos', which is where we get the word atom. About 2500 years ago, ancient Greek philosophers argued about what the Universe was made of. One philosopher, Democritus, argued that there was a final particle which could not be cut or broken into any smaller pieces, that is, it was indivisible.
Like Democritus, scientists believe that atoms are the building blocks of matter, although the idea of what atoms actually are has changed a great deal. For instance, anyone who has ever heard of nuclear power will know that atoms can be split!
Imagine you had a big boulder, too big to get your arms around, say a diameter of about 1.2 metres. Imagine too that you had some magical way of cutting through rock very quickly and cleanly. Suppose instead of running off and making millions of dollars with your magical gizmo, you decide to cut the boulder in half. While you’re in the mood you cut one of the halves in half, then cut one of the quarters in half and so on and so on. How many cuts would you need to be left with an atom? After about 35 cuts you would have a grain of sand. On your 100th cut, you would have separated the last two atoms and be left with two individual atoms. These may well be silicon atoms as this element is a common component of rocks. (Remember, an element is a pure substance made up of a single kind of atom.)
Atoms contain three different kinds of particles: protons, neutrons, and electrons.
The protons, which are positively charged, and neutrons, which are neutral, are joined together in what is called the nucleus. The negatively charged electrons orbit the positive nucleus, surrounding it like a shell or cloud of negative charge. For convenience, atoms are often drawn as if they are mini solar systems, but it is not really accurate to draw the electrons in a single spot or a definite orbit.
Even though electrons, protons, and neutrons are much smaller than atoms, atoms themselves are the smallest pieces of ‘ordinary’ matter. If you took your magic gizmo and made cut number 101, cutting one of your silicon atoms in half, then you wouldn’t have two smaller silicon atoms, instead you’d have two nitrogen atoms! This is because what makes a silicon atom a silicon atom is how many protons it has in its nucleus. It will still be a silicon atom no matter how many neutrons or electrons there are. In this case there are 14 protons, so by cutting the nucleus in half you would get two atoms with 7 protons each, and the only substance that has 7 protons is nitrogen.
If you cut a nitrogen atom in half you would have to make two different kinds of atoms, lithium (with 3 protons) and beryllium (with 4 protons). The smallest atoms of all, those with just 1 proton, are called hydrogen.
Scientists have theories about smaller things that make up protons and neutrons, such as quarks. You might like to find out more about these subatomic particles.
Although very special equipment is needed to see atoms, it is very easy to see their effects—just as you can’t see the wind but can see its effects when leaves are blown around.