[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe what's meant by an element a compound a mixture and a molecule these ideas are important in chemistry so it's essential that you understand them let's start by looking at elements now there are around 100 different elements and the elements are shown in the periodic table I'm showing you the periodic table here and you are given a copy of this in your exam each of the symbols in the periodic table represents a different element so for example we've got magnesium on the left and we have sulfur on the right okay now this brings us to the definition of an element and you need to learn this definition in an element all of the atoms are the same so for example this shows the atoms in a sample of magnesium and we can see that all of the Magnesium atoms are the same here are the items in a sample of sulfur and again all of the sulfur atoms are the same so this means that both magnesium and sulfur are two different elements every element has a symbol and every symbol starts with a capital letter so the symbol for magnesium is capital M followed by lowercase G and the symbol for sulfur is simply capital s now if we take atoms of magnesium and atoms of sulfur and chemically combine them then we have a compound this compound is called magnesium sulfide compounds contain two or more different elements chemically combined in fixed proportions and again you need to learn that definition so what's meant by fixed proportions well if we look at magnesium sulfide we can see that for every one atom of magnesium we have one atom of sulfur so the proportion of magnesium atoms and sulfur atoms in magnesium sulfide is fixed another key fact is that compounds usually have totally different properties to the elements that they're made from as you can see magnesium is a shiny metal and sulfur is a yellow solid however magnesium sulfide forms white crystals now if we want to separate a compound back into its elements then we have to use a chemical reaction and that's not always easy to do okay let's take a look now at mixtures in a mixture we have different elements or compounds but they're not chemically combined I'm showing you here a mixture of magnesium atoms sulfur atoms and magnesium sulfide if we want to separate a mixture then we can use a physical separation technique rather than a chemical reaction for example we could use filtration distillation crystallization or chromatography and we're going to look at all of these physical separation techniques in later videos now one word that's often used in chemistry is molecule and some students get confused about this however it's not as difficult as it sounds remember how a compound has different elements chemically combined while a molecule has any elements chemically combined even if they're the same element so all of these are examples of molecules methane water and ammonia are also compounds because they contain different elements chemically combined for example methane contains the elements carbon and hydrogen water contains the elements hydrogen and oxygen and ammonia contains the element nitrogen and hydrogen the bottom two molecules are not compounds these are elements the chlorine molecule contains two chlorine atoms chemically combined and the oxygen molecule contains two oxygen atoms chemically combined so both chlorine and oxygen are molecules but not compounds you'll find plenty of questions on this topic in my revision workbook which you can get by clicking on the link above [Music] foreign [Music]