In the last topic we found that everything is built from 118 kinds of atom, and that ninety two of those occur in nature. That ought to worry you slightly. Look around the room and count the different substances you can see, and you will pass ninety two before very long. So how can so few kinds of building block produce so many different things? The answer is in this topic, and it is that atoms join together.
Molecule
Matter is made of extremely small particles joined together. In a molecule, two or more atoms of the same kind or of different kinds are chemically joined. The molecules of some elements, such as H₂, N₂ and O₂, are made of two or more atoms of that same element joined together.
In a molecule all the properties of the substance remain exactly as they are. For example, if one spoon of sugar tastes sweet, then a single particle of sugar is sweet as well.
The smallest particle of an element or a compound that can be found in a free state in nature, and in which all the properties of the substance remain as they are, is called a molecule. The molecules of an element are made of the same kind of atoms, for example a molecule of oxygen is made of two oxygen atoms joined together. The molecule of a compound, on the other hand, is made of two or more kinds of atoms: two hydrogen atoms and one oxygen atom together make one molecule of water.
Atom or molecule? One question separates them
| Atom | Molecule | |
|---|---|---|
| What it is the smallest piece of | The smallest particle that can take part in a chemical reaction | The smallest particle in which the properties of the substance remain |
| What it is made of | Protons, neutrons and electrons | Two or more atoms, chemically joined |
| Found free in nature? | Usually not. Most atoms are joined to something | Yes, this is the piece that exists on its own |
| Example | One O, one H, one Cl | O₂, H₂, Cl₂, H₂O, HCl |
Read the first row of that table again and notice that the two definitions are not competing. They answer different questions, and once you see which, you will never confuse them. An atom is the smallest piece that can react. A molecule is the smallest piece that still is the substance. Take water. A single oxygen atom can take part in a chemical reaction, so it counts as an atom, but nothing about it is watery: it is not wet, it does not put out fires, it is not water in any sense at all. Water begins to exist only when two hydrogen atoms and one oxygen atom are joined, and that joined group is the water molecule. Split it and the water is gone, even though every atom is still there. That is why the definition of a molecule uses the phrase in which all the properties of the substance remain as they are. The molecule is the smallest amount of a thing that still deserves the thing's name.
Compound
If there are 118 elements, guess how many substances in total there might be in nature. And how might so very many substances in nature have been made?
The process in which the atoms of two or more elements react with one another to make a substance that has new properties is called a chemical reaction. A substance formed in this way, by a chemical reaction between elements, is a compound.
A substance made by the atoms of two or more elements combining together in a definite proportion is called a compound. For example, a chemical change between hydrogen and oxygen makes water, and a chemical reaction between sodium and chlorine makes common salt. Water (H₂O) and common salt (NaCl) are examples of compounds.
A compound is not a mixture of its parts
The book gives two examples of compounds and moves straight on, but stop and look at what those two examples actually say, because it is remarkable. Hydrogen is a gas that burns fiercely. Oxygen is the gas that makes other things burn. Join them and you get water, which is what we throw on fires to put them out. Now the second one, which is even stranger. Sodium is a soft metal so violently reactive that a small piece dropped into water catches fire. Chlorine is a choking green gas that was once used as a poison. Join those two together and you get the ordinary white salt that you sprinkle on your food every day. So a compound does not inherit the properties of the elements that made it. It has entirely new properties of its own, and that is exactly what the definition of a chemical reaction says: a substance that has new properties. This is worth carrying with you, because it is the difference between a compound and a mixture. Mix sand and salt and you still have sand and salt, each behaving as it always did. React sodium and chlorine and both of them are gone, and something new is standing where they were.
| The elements | What each one is like on its own | The compound they make |
|---|---|---|
| Hydrogen and oxygen | One burns fiercely; the other makes things burn | Water, which puts fires out |
| Sodium and chlorine | A metal that catches fire in water; a poisonous green gas | Common salt, which you eat every day |
Why the words definite proportion matter
Look again at the definition of a compound and notice the phrase in a definite proportion. It is easy to read past and it is doing a great deal of work. Water is always two hydrogen atoms to one oxygen atom. Not sometimes three to one, not sometimes two to two. Always two to one, in every drop of water anywhere in the world, whether it came from a Himalayan spring or a laboratory. That fixed ratio is exactly why a compound can be written as a formula at all: H₂O means two of these and one of those, and the small numbers would be meaningless if the proportion could change.
Compare that with a cup of tea. You can make it with more sugar or less, with more milk or none at all, and it is still tea. There is no definite proportion, and that is the test that separates a mixture from a compound. A mixture can be made in any proportion you like, and its parts keep their own properties and can usually be separated again. A compound has one fixed proportion, its parts have lost their own properties completely, and getting them back needs a chemical reaction rather than a sieve.
Study that figure from the top down, because it tells the whole story of this topic in four rows. The first row is a single atom, standing alone. The second row shows molecules: two matching circles joined together make a molecule of an element, while circles of different colours joined together make a molecule of a compound. The third row shows a compound, which is many identical molecules of the same kind all together. And the fourth row shows a mixture, in which molecules of several different kinds are jumbled up but not joined to one another. The difference between the third row and the fourth row is the difference between something being combined and merely being stirred together.
How 92 elements make millions of substances
Now go back to the question at the start. If there are only 92 natural elements, how can there be so many substances in the world? The answer is the same as the answer to a question about language. There are far fewer than a hundred letters in any alphabet, and yet nobody has ever run out of words. The letters can be put together in different orders, in different numbers, in groups of different lengths, and the possibilities never end. Atoms do exactly the same thing. Two hydrogens and one oxygen give water. One hydrogen and one chlorine give hydrochloric acid. Carbon and hydrogen together, in different numbers and arrangements, give petrol, plastic, rubber, sugar and most of the substances in a living body. Change the ratio, change the arrangement, and you have a different substance with different properties. In nature elements combine to make compounds, and compounds break up or react to form new substances again, and this is what brings about change in matter.
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