 This video is about counting atoms. Atoms can be hard to count because they're quite small. So let's back up and just think about counting things first. Mostly we count in the ordinary way using integers. One, two, three, four, five, six apples in this picture for instance. Sometimes it's convenient to skip count. For instance shoes usually come in pairs. So if you have a lot of shoes to count you would count by twos. Two, four, six, eight. And rather than saying there are 40 shoes in this picture here you could say there are 20 pairs. In other situations we might need a larger number to skip count with. Eggs and donuts are often measured in dozens. We understand that one dozen means 12 so when someone talks about say four dozen donuts we can calculate that they mean four times 12 which is 48 donuts. This picture shows six eggs. Six is half of 12 so this is half a dozen eggs. Then there are products that come in even larger groups. A standard packet of printing paper contains 500 sheets of paper. 500 is known as a ream. This is because it's easier for someone to say they want 20 reams of paper than it is to say 10 000 sheets of paper although it means the same thing. It's also easier for us to imagine what 20 reams of paper looks like, how much it actually is. Our brains don't imagine very large numbers particularly well. But what about atoms? What size of number do we need to conveniently count atoms? Atoms are so small that even if you had a ream of them you would need some seriously good equipment to detect that they were even there let alone count that you had the full 500. This picture of silicon atoms in a silicon nanowire was taken using a transmission electron microscope. You can see the scale bar showing one nanometer that's one times ten to the minus nine meters a billionth of a meter so it gives you an idea of just how small the atoms are. So we need a big number in order to skip count atoms because they're so small but how big? Well it should be a number that relates to convenient amounts of chemicals that humans are likely to use. To get an idea of where we're heading think about the head of a steel pin. Steel is predominantly iron so how many iron atoms make up that pinhead? A million? A billion? Well it's about six times ten to the 19. That's 60 quintillion atoms in a pinhead. Okay so clearly we need a really big number if we're going to skip count atoms because atoms are so small. Now a lot of work was done on this in the 19th and early 20th centuries and it was eventually determined to use the isotope carbon-12 as a standard. It was decided that if you weighed exactly 12 grams of carbon-12 then the number of atoms in that sample would be our standard number of atoms and the name of that standard number is the mole. Think back to reams of paper. It's like saying well however many pieces of paper are in one packet that will be our standard number of pieces of paper and it turns out to be 500 and we're going to call 500 a ream. So a mole is a really large number of atoms and we can skip count atoms by counting one mole of atoms, two moles of atoms and so on. But how many is a mole? A dozen is 12, a ream is 500. What's a mole? The number is this 6.022 times 10 to the 23 and if we write it out it looks like this. It's a big big number but that makes sense because since atoms are so small it's convenient to count them in large groups. Remember that although this is a massive number this number of atoms of carbon is still only a small pile in the palm of your hand. Now apart from being the number of things in one mole this number is also known as the Avogadro constant or Avogadro's number. Amadeo Avogadro was an Italian lawyer in the early 1800s. He became interested in mathematics and physics and in 1820 he became the first professor of mathematical physics in Italy. He came up with a hypothesis that equal volumes of different gases at the same temperature and pressure contain the same number of particles and this laid the basis for the gas laws that we'll look at in another part of the course. However it also set scientists on the right track to being able to compare quantities of different substances in terms of the number of atoms or molecules they contained rather than just how much they weighed. So this constant this number although it was determined long after his death is named in his honor.