As we saw in our last video, an atom is made up of a central nucleus which comprises protons and neutrons and then has electrons orbiting that nucleus. We also saw that it's the number of protons that tells us which element it is. The smallest element is hydrogen as its atoms have just one proton in their nucleus with one electron orbiting it. The next smallest is helium whose atoms have two protons and two neutrons in the nucleus and then two electrons orbiting them. In total, there are around 100 different elements and they've been organized into the periodic table with each box representing a different element. We call these boxes nuclear symbols as we saw in the last video and the number in the bottom left is the atomic number which is unique to that element as it represents the number of protons in the atoms of that element. And any atom with that many protons must be that element. So our atom here which has two protons must be helium because helium's atomic number is two. Or if we take carbon we can see that its atomic number is six. So all carbon atoms have six protons. So, if we found an atom with three protons, like this one here, then it couldn't be carbon, we'd have to find the element with an atomic number of three, which if we check the periodic table, is lithium. Another feature of nuclear symbols is the one or two letter symbol that represents the element name. So, C stands for carbon and LI means lithium. Some symbols though are a bit more confusing. For example, sodium is N A and iron is Fe. Most periodic tables have both a symbol and the name though. So if you're not sure what a certain symbol is, you can just find on the periodic table and check. So now that we know that it's the number of protons that determines which element it is, what about the number of neutrons? Well, the number of neutrons can actually vary between the individual atoms of an element. As long as the number of protons remains the same, it's still the same element. And we call these different forms of that element isotopes. The definition which you need to remember is isotopes are different forms of the same element that have the same number of protons but a different number of neutrons. Let's take a look at carbon to show you what we mean. The most common form of carbon is so-called carbon 12 which has six protons, six neutrons and six electrons. A rarer form is the isotope carbon 13, which of course still has six protons because it's carbon, but has seven neutrons and six electrons. This means that it has a mass number of 13 rather than 12, which is why we call it carbon 13. Because isotopes only vary in the number of neutrons they have, chemically, they all react in basically the same way. Now, because they all have different numbers of neutrons, the different isotopes will have different masses. And one of the things that you need to be able to do is calculate the average mass of all the isotopes that make up a particular element, which we call the relative atomic mass. Let's take a look at copper as an example. Now, copper has two stable isotopes. Copper 63, which has an abundance of 69.2%. and copper 65 which has an abundance of 30.8%. And when we say abundance, all we mean is how common or rare that isotope is. So if you were to pick up a handful of copper, then on average 69.2% of the atoms in your hand would be the copper 63 isotope and the other 30.8% would be the copper 65 form. Now in an exam you might get a question like calculate the relative atomic mass of copper to one decimal place. The equation for relative atomic mass is sum of isotope abundance times isotope mass divided by the sum of the abundances of all the isotopes. So the first thing we do is multiply each isotope's abundance by its mass. So for the copper 63 isotope this would be 69.2 which is its abundance time 63 which is its mass and for copper 65 it would be 30.8 * 65. Then we sum these two figures together which just means we add them together. Lastly, we divide this by the sum of the abundances of all the isotopes, which in this case would be 69.2 + 30.8. Now, you can either put this all straight into your calculator or simplify it down first. 69.2 * 63 gives us 4,359.6, while 30.8 * 65 gives us 2002. And then we add these together to get 6,361.6 on the bottom. 69.2 + 30.8 gives us 100, which makes sense because there are only two isotopes. So together they should add up to 100%. So that they represent all of the copper atoms. So we divide 6,361.6 by 100. And our answer is 63.616. 616. But as the question asks for one decimal place, we give the answer as 63.6. And this number that we found is the relative atomic mass, which is denoted by the symbol a r. So we could say that the average mass of a copper atom is 63.6. If you haven't heard yet, you can find all of our videos on our website, cognito.org. You'll also find questions, flashcards, exam style questions, and past papers. And we track all of your progress so that you always know what to study next. So, sign up for free by clicking here or browse our playlist here on YouTube.