Atoms, elements and compounds

You put salt on your chips without a second thought. Salt is sodium chloride - and it is made from two things you would never eat. Sodium is a soft, silvery metal so reactive that it is stored under oil, away from air and water. Chlorine is a choking yellow-green gas that was used as a weapon in the First World War. Nothing was added to make them safe, and nothing was taken away. The two were joined - and what they made is something you eat.

Right, start with the smallest thing in the story. An atom is the smallest particle of an element that can exist. The salt, the chips and you are all built from atoms. An element is a substance made of only one kind of atom. There are about a hundred elements, and every one of them is listed on the periodic table you are given in the exam. Those two words get swapped, so keep them apart from the start. An atom is one particle you could count. An element is a substance, something you could name and hold a jar of. Every element has a symbol. It is one capital letter, like O for oxygen, or a capital followed by a small letter, like N a for sodium. A capital N with a capital A, though, is not a symbol at all. Now, oxygen in the air does not float about as loose single atoms. Each little unit of it is two oxygen atoms joined, and it is written O two. Two oxygen atoms joined: is O two still an element? Yes, O two is an element. Both of its atoms are oxygen, so there is only one kind of atom in it. It could be two atoms or two million; the kind decides, not the number.

Now, join atoms of different elements, and you have a compound. A compound is a substance made of two or more elements chemically combined, in fixed proportions. Chemically combined means joined by a chemical reaction, not stirred together. Fixed proportions means the recipe never changes. So salt always has one sodium for every chlorine. Now put the three substances side by side. Sodium is a soft metal. Drop a piece in water, and it fizzes across the surface and can burst into flame. Nobody eats a pinch of pure sodium. Chlorine is a pale green gas with a sharp smell. It dissolves in water to make a bleaching solution, and breathing it damages your lungs. Sodium chloride, though, is white crystals. It dissolves in water quietly, with no fizz and no smell, and you eat some every day. Why is salt not somewhere halfway between sodium and chlorine? Because a compound is a new substance. Its elements are chemically combined, so it has properties of its own. It is not a blend of the two it came from. A formula tells you which atoms are in a compound, and how many. H two O is water. The small two belongs to the H in front of it, so that is two hydrogen atoms and one oxygen atom. From any formula, you can count two things. There are the kinds of atom, and there are the atoms. Water has two kinds, hydrogen and oxygen, and three atoms in all. And two kinds makes it a compound. Here are six formulae: F e, C O two, A r, M g O, O two and N H three. The test is the same every time. One kind of atom means an element, and two or more kinds means a compound. Count the kind of atom in each of the six: element or compound? F e is iron and A r is argon. Each is one kind of atom, so both are elements. O two goes with them, because it is still one kind, twice over. C O two, M g O and N H three each hold two kinds of atom. Carbon and oxygen, magnesium and oxygen, nitrogen and hydrogen. All three are compounds. Here is your handle for this video, and it is the test you used six times: count the kinds, not the atoms.

Those two words, atom and element, also decide marks when you write. Here is a student's sentence about water. The student wrote that water is made of one element of oxygen joined to two elements of hydrogen. One word in it is used wrongly - and more than once. Which word is wrong in that sentence, and what should it be? Every element in it should be atom. A water particle is one oxygen atom joined to two hydrogen atoms. Hydrogen and oxygen, then, are the two elements in water. One examiner's report records the same slip. Some students "referred to 'elements'" when they were describing atoms, and that "did not gain the first marking point." So what saves the mark is a habit. Before you write atom or element, ask whether you mean a particle you could count, or a substance you could name. One more test. Two students want the sodium back out of those white crystals. One says use a magnet, and the other says dissolve it and filter it. Why will neither the magnet nor the filter get the sodium back? Because the sodium is not sitting loose in the salt. It is chemically combined with the chlorine, so there is no sodium metal in there to pick out. Only a chemical reaction can split a compound into its elements. That is the line between a compound and a mixture. A mixture is substances side by side, not joined, so it can be separated without a reaction. How you do that has its own video, on separating mixtures.

To finish, four quick questions, and the first two use substances this video never mentioned. Neon is written N e. Is neon an element or a compound? An element. N e is one symbol, so neon is one kind of atom. Next one: C two H six. Element or compound, and why? A compound. There are eight atoms in it, but only two kinds, carbon and hydrogen, and two kinds is what decides it. Try this: can a magnet pull the iron back out of rust? No. Rust is a compound of iron and oxygen. The iron is chemically combined, so getting it back takes a chemical reaction. One more, back to your chips: why is salt safe to eat? Because sodium chloride is a new substance. The sodium and the chlorine are chemically combined, not mixed, so the salt on your chips behaves like neither of them.

This is video one of seven in Atomic structure, and the first teaching video of the whole chemistry series, so nothing comes before it. If you could teach this one to a friend, press the thumb and it comes off your list. If atom and element still blur, save it; almost nobody separates them first time.

Next in the chapter: Protons, neutrons and electrons.

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