[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe how Group Seven Elements consist of molecules with two atoms you should then be able to describe how the melting and boiling points of the group seven elements change down the group okay I'm showing you group seven here group seven is also called the halogens and you could be asked that in your exam all the elements in group seven on non-metals now a key fact is that group seven elements have seven electrons in their outer energy level and I'm showing you that here with Florine and with chlorine I'm using crosses to represent electrons now remember that atoms react in order to achieve a full outer energy level if we take two atoms of a group seven element then they react with each other and I'm showing you that here with fline each fline atom has seven out to electrons I'm shown the electrons as two different colors to make it easier to see these two atoms now overlap their outer energy levels and they share a pair of electrons we call this shared pair of electrons a calent bond and we've produced a Florine molecule F2 every group seven element forms a molecule consisting of two atoms joined by calent Bond I'm showing you here the chlorine molecule CL two and again we can see the shared pair of electrons in the calent bond now we can represent the calent Bond as a stick between the two atoms so here are the halogens forming molecules consisting of two atoms joined by a Cove valent Bond now I should point out that we'll be seeing Cove valent bonding again in much more detail in a later topic okay now the melting and boiling points of group seven elements change as we move down the group and I'm showing the melting and boiling points of the group seven elements here we can use the melting and boiling points to work out the physical state of each hogen in other words whether it's a solid liquid or gas at room temperature and remember that room temperature is around 20° cus as you can see the melting and boiling points increase as we move down group seven fline and chlorine are both gases at room temperature as their boiling points are lower than 20° C bromine is a liquid at room temperature that's because the melting point of bromine is lower than 20° C but the boiling point of bromine is higher than 20° C finally iodine is a solid at room temperature that's because both the melting point and boiling point of iodine are higher than 20° C now there's one final thing that we need to look at the second column shows you the relative molecular mass of the hogen relative molecular mass gives us an idea of the size of a molecule and we'll be looking at relative molecular mass in a later video you can see that as we move down group seven the relative molecular mass increases in other words the molecules get bigger moving down group seven and you need to learn that you'll find plenty of questions on this topic in my vision workbook which you can get by clicking on the link [Music] above