[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe how ionic compounds form giant ionic luses you should then be able to explain why ionic compounds have high melting and boiling points and finally you should be able to explain why ionic compounds cannot conduct electricity when solids but can conduct electricity when molten or dissolved in water in the last two videos we've been looking at ionic bonding remember that ionic bonding takes place when a metal reacts with a non-metal a good example is the reaction between sodium and chlorine in this reaction one electron is transferred from the outer energy level of the sodium atom to the outer energy level of the chlorine atom at the end the sodium atom has a one positive charge and we now call this a sodium ion and remember that an ion is an atom with an overall charge the chlorine atom now has a one negative charge and we call this the chloride ion both the sodium ion and the chloride ion now have a full outer energy level in other words they have the stable electronic structure of a group zero noble gas now in this reaction we're making the compound sodium chloride and I'm showing you sodium chloride here as you can see sodium chloride is a crystal and in fact many ionic compound compounds form crystals like this so in this video we're looking at how the ions are arranged in a crystal and how this determines the properties of ionic compounds the first key fact that you need to learn is that ionic compounds form giant structures and scientists call this a giant ionic lce in a giant ionic lattice every positive ion is surrounded by negative ions and every negative ion is surrounded by positive ions I'm showing you here the giant ionic lattice for sodium chloride in this lattice we have positive sodium ions surrounded by negative chloride ions and you need to remember that giant ionic lates are three-dimensional structures in a giant ionic latice there are very strong forces of attraction between the positive and negative ions scientists call these electrostatic forces of attraction and I'm showing the electrostatic forces of attraction here these strong electrostatic forces of attraction hold the positive and negative ions in place the electrostatic forces of attraction between positive and negative ions are also called ionic bonds and ionic bonds act in all directions now we can also represent giant ionic luses like this here we're not showing the electrostatic forces of attraction but we know that they're there now there are two Key properties of ionic compounds that you need to learn firstly ionic compounds have very high melting and boiling points that's because the strong electrostatic forces of attraction require a great deal of heat energy to break I'm showing you that here as we heat the ionic solid the particles vibrate when the particles vibrate with enough energy the electrostatic forces of attraction break and the solid melts now because the electrostatic forces of attraction between the ions are very strong it takes a great deal of heat energy for these to break for example the melting point of sodium chloride is around 800° the second key property of ionic compounds is that they cannot conduct electricity when they're solids in a solid the ions are locked in place by the strong electrostatic forces of attraction so in a solid the ions can vibrate but they cannot move however ionic compounds can conduct electricity when they're melted or dissolved in water that's because the ions can now move and carry the electrical charge now this often comes up in the exams and students sometimes get this wrong remember that when ion a compounds conduct electricity it's the ions that are moving not electrons you'll find plenty of questions on this topic in my vision workbook which you can get by clicking on the link [Music] above