[Music] hi and welcome back to freeze lessons by the end of this video you should be able to describe and explain how the reactivity of the halogens changes Down group seven you should then be able to describe how a more reactive hogen can displace a less reactive hogen from an aquous solution of its salt so far on this topic we've been looking at the elements in group seven which are also called the halogens in the last video we saw that when a hogen reacts with a metal the hogen atom gains one electron to achieve a full outer energy level here's Florine reacting with lithium you can see that the Florine atom gains one electron from the lithium atom now here's a really important fact that you need to learn the negative electron is attracted to the positive charge in the nucleus of the fline atom if we look at group seven we can see that Florine is at the top of the group now now Florine is the most reactive element in group 7 and the halogens get less reactive as we move down group s now you need to learn that and you also need to learn why to understand this we're going to compare Florine with chlorine which is the element below Florine in group seven chlorine is less reactive than Florine as we said when a hogen atom reacts with a metal the hogen atom gains one electron from the metal atom now chlorine gains an electron on less easily than Florine and there are two reasons for this firstly in a chlorine atom there's a greater distance between the outer energy level and the nucleus compared to a fline atom this means that the outer electrons are less attracted to the positive nucleus in a chlorine atom compared to a fline atom secondly in a chlorine atom we have more electrons in internal energy levels remember that electrons in internal energy levels repel the outer electrons and scientists is called this shielding so in a chlorine atom the level of shielding is greater than in a Florine atom so both increased distance and increased shielding mean that the attraction between the outer electrons in the nucleus is lower in a chlorine atom compared to a fline atom this makes it harder for a chlorine atom to attract an electron into its outer energy level than for a fline atom and this makes chlorine less reactive than Florine so moving down group seven The increased distance and the increased shielding mean that the elements get less reactive okay now this difference in reactivity affects the reactions of the halogens here's one final key fact that you need to learn a more reactive hogen can displace a less reactive hallogen from an aquous solution of its salt we're going to look at the reaction between the compound sodium bromide and the element Florine the sodium bromide is an acous solution and this means that we've dissolved the sodium bromide in water now in order to work out what happens we need to look at the reactivity of the halogens involved remember that the halogens get less reactive as we move down group seven fline is higher than bromine in group seven so this tells us that Florine is more reactive than bromine this means that the Florine can push out or displace the bromine like this scientists call this a displacement reaction as the fline has displaced the bromine you need to be able to work out the products of displacement reactions here's one for you to try potassium iodide plus chlorine so pause the video now and try this yourself okay we can see that chlorine is higher than iodine in group seven which means that chlorine is more reactive than iodine this means that the chlorine displaces the iodine and we make potassium chloride plus iodine here's another one for you lithium chloride plus bromine so pause the video now and try this yourself we can see that bromine is lower down group seven than chlorine this tells us that bromine is less reactive than chlorine this means that bromine cannot displace chlorine so we get no reaction you'll find plenty of questions on this topic in my vision workbook which you can get by clicking on the link [Music] above