Katie: Time for some science. And today it's a chemistry classic, the periodic table. And here to explain H to the Cs to the Ns and the Os, it's our super scientist extraordinaire, Dr Tim Gregory.

CHEERING

Dr. Tim: Hello Katie.

Katie: Hello, Dr Tim. Now, we've got part of the periodic table here. I'm seeing lots of letters. It's a little bit like a giant game of Scrabble. What do they all mean? And what is the table actually used for?

Dr. Tim: So this is the periodic table and each letter on it represents a different element. We call these letters symbols. And an element is a pure substance made from only one type of atom. And there are quite a lot of elements. There are something like 100, and each has a unique symbol made up of a letter and sometimes two letters.

The table is used to arrange the elements and we put chemically similar elements together in groups.

Katie: Amazing! Thank you very much, Dr Tim. Well, before we move on, here's some more information to help us understand the basics of chemical symbols and formulae.

VOICEOVER: Chemical symbols and formulas.

Every element in the periodic table is represented by a unique chemical symbol, like O for Oxygen or Mg for Magnesium. Chemical formulas represent compounds or elements where more than one atom are bonded together. They often, but not always, have little numbers called subscripts that show how many atoms of each element are present in one molecule of a compound.

The chemical formula for Magnesium Oxide, MgO, tells us that there is one atom of magnesium for every one atom of oxygen. Butane, which has the chemical formula C₄H₁₀, has four carbon atoms and ten hydrogen atoms. And the formula of sulfuric acid is H₂SO4.

And this tells us that one molecule of sulfuric acid contains two hydrogen atoms, one sulfur atom and four oxygen atoms.

Katie: OK, Dr Tim, what's next then?

Dr. Tim: Alright. Take a look at the periodic table there. Are there any symbols that you recognise? Any elements that you might know?

Katie: Erm, how about O for oxygen?

Dr. Tim: Yep, perfect. How about any others?

Katie: I recognise Fe but if it's got two letters like that, does that mean that it's two elements combined?

Dr. Tim: Not quite. Now, remember, on the periodic table we only put elements, not compounds. Fe is actually iron and you can tell this is an element because it's got an uppercase F followed by a lowercase e.

Some elements have two letters in their symbol, because there are more than 100 elements, but there are only 26 letters in the alphabet, so we have to double some of them up.

Katie: OK, so why is it Fe and not Ir like a shortened version of Iron. That would be much easier to remember, wouldn't it?

Dr. Tim: It would be easier, but you know the history of who discovered these elements and when and where is really long and complicated and interesting and the symbols for some of the elements are left over from their old names.

For example, the symbol for iron, Fe, comes from the Latin name for iron, ferrum.

Katie: Oh, wow. I'm learning lots here, sir. How about W? Is that one water?

Dr. Tim: Not quite. W is actually the element tungsten, and the W symbol comes from the mineral in which tungsten is found called wolframite. And tungsten is actually what the filament of some light bulbs is made from, the little spiral filament that lights up when you switch it on.

Dr. Tim: Water isn't an element, it's a compound. And, remember, we only show pure elements on the periodic table.

Katie: So what's a compound then?

Dr. Tim: A compound is a pure substance which is made from more than one element. And atoms, they chemically bond together in a fixed ratio to make a compound, and this means we can describe these compounds with a chemical formula.

So, since water is a compound, it means its chemical formula is made up from elements that we can pick from the periodic table.

Katie: So which chemicals is water made up of?

Dr. Tim: It's made of two elements. Oxygen, which you've already found, and hydrogen.

Katie: OK, let me find hydrogen. OK, there it is. So H is H₂O! Water is H₂O. Of course it is. What's the number two all about?

Dr. Tim: Let me write it down to explain.

We've got water with the chemical formula H₂O. Just like that. Now, we always put these numbers down at the bottom when we're writing chemical formula and this number, it shows how many of each type of atom we have in the compound. And the two is because for every one oxygen atom there are two hydrogen atoms in water. And if we were to draw the molecule, it would look a little bit like this.

So we've got our oxygen atom and then we've got our two hydrogen atoms. H₂O, water. When a compound forms there isn't always an equal number of elements. For example, water, there are two hydrogens and just one oxygen. We use the numbers to show how many of each atom there are in a compound.

Now, another one you'll have heard of is carbon dioxide.

Katie: Yes, so that's carbon and dioxide. So is oxide oxygen?

Dr. Tim: Yes, perfect.

Katie: And 'di' means two, so dioxide means two oxygens?

Dr. Tim: Absolutely spot on, exactly right.

Katie: So, carbon dioxide means a carbon plus two oxygens. That's CO₂.

Dr. Tim: Excellent. So if we write this down, we've got CO₂. We've got one carbon atom and two oxygen atoms. And if we draw it, it would look a little bit like this. So we've got our one carbon atom, we've got one oxygen there and another oxygen there. CO₂, carbon dioxide.