[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe the structure of the giant calent molecules diamond and silicon dioxide you should then be able to describe the properties of these molecules and finally you should be able to link these properties to the structures of the molecules in the last video we looked at the properties of small calent molecules and I've put a link to the video in the description below remember that small calent molecules only have a small number of calent bonds I'm showing you here the hydrogen molecule H2 and hydrogen is a small Cove valent molecule small calent substances have weak intermolecular forces between the molecules when we melt or boil a small coent substance we need to break these weak intermolecular forces breaking the weak inter molecular forces does not take a lot of energy and because of this small calent substances have low melting and boiling points this means that most small calent substances are gases at room temperature now in this video we're looking at a different type of calent substance these are called giant calent substances unlike small calent substances giant calent substances contain millions of calent bonds we're going to look at three giant covalent substances these are diamond silicon dioxide or silica and graphite in this video we're looking at Diamond and silicon dioxide and in the next video we're looking at graphite now a key fact you need to learn is that giant Cove valent substances are always solids at room temperature that's because all giant covalent substances have high melting and boiling points as we've seen giant calent molecules have millions of strong calent bonds in order to melt or boil these substances we have to break all of these calent bonds and that takes a great deal of energy so giant calent substances all have high melting and boiling points okay we're going to start by looking at the giant coent substance diamond diamond is formed from the element carbon and it's really important that you learn that I'm showing you a carbon atom here carbon atoms have four electrons in their outer energy level in order to achieve a full outter energy level each carbon atom forms Cove valent bonds to four other carbon atoms and I'm showing you that here now another way to represent diamond is by using a diagram like this in this diagram the carbon atoms are shown as circles and the Cove valent bonds are shown as sticks now a diamond contains a huge number of carbon atoms joined by calent bonds and I'm showing you that here even a tiny diamond contains millions and millions of carbon atoms joined by Cove valent bonds this makes Diamond an extremely hard substance now if we wanted to melt a diamond then we need to break all of the Cove valent bonds and that takes a huge amount of energy so that means that diamond has a very high melting and boiling point in fact the melting point of diamond is over 3,700 de C now there's one other key property of diamond that you need to learn Diamond cannot conduct electricity that's because in Diamond all of the outer electrons are in calent bonds this means that diamond has no free electrons to carry electrical charge and again it's really important that you learn that okay here's our next G covalent substance this is called silicon dioxide or silica silicon dioxide contains the elements silicon and oxygen covalently bonded together because this is a giant calent molecule it contains a huge number of calent bonds and in fact I'm only showing you a tiny part of the molecule here just like diamond silicon dioxide has a very high melting and boiling point and again this is because silicon dioxide has a huge number of strong coent bonds in order to melt silicon dioxide these calent bonds must be broken and that takes a great deal of energy you'll find plenty of questions on this topic in my vision workbook which you can get by clicking on the link [Music] above