we've already seen in other videos that non-metals can join together by covalent bonds in which they share electrons so that all of the atoms have full outer shells sometimes this results in small molecules such as chlorine or ammonia and we call these simple molecular substances in other cases though non-metals bond covalently to form giant covalent structures like diamond graphite or silicon dioxide and in today's video we're going to look at the respective properties of each of these and then finish up by taking a closer look at the structure of silicon dioxide now the first thing to know is that covalent bonds are really strong which means that a lot of energy is going to be needed to break apart any atoms that conveniently bonded to each other so if we consider a simple molecular substance like chlorine the atoms within each molecule will be strongly bonded together however in order to melt or boil chlorine we actually don't break these strong covalent bonds instead we only need to break the weak forces that exist between different molecules which we call intermolecular forces because of this we only need very low temperatures to melt or boil simple molecular substances for example chlorine boils at -34 degrees celsius although these intermolecular forces are individually quite weak the more of them that a molecule has the stronger the overall attraction is going to be for example let's compare the halogens chlorine bromine and iodine because we're going down the group the atoms and thus molecules are getting bigger so there'll be more intermolecular forces between them this in turn means that more energy will be required to break them all so the melting and boiling points should increase as you go down the group which they do as bromine has a boiling point of 59 degrees and iodine doesn't boil until 184 degrees there's no need to remember these specific numbers you just need to understand that the boiling point increases as you go down the group because the molecules get larger so there are more intermolecular forces between them another property to note is that simple molecular substances don't conduct electricity because there are no free electrons and the molecules themselves have no electric charge you'll see this point again and again in chemistry in order to conduct electricity or heat substances have to have some electrons or ions that are free to move about so moving on to giant covalent structures these are made of huge numbers of non-metal atoms that are all bonded to each other by covalent bonds and they're generally arranged into regular repeating lattices which just means that their structure kind of repeats over and over the three important examples are diamond graphite and silicon dioxide the main things to remember about these structures is that they're very strong and they have high melting and boiling points because we'd have to break all of these strong covalent bonds in order to melt them the other property to know is that they generally don't conduct electricity because they don't contain any charged particles even when they're molten an exception to this though is graphite which we take a closer look at in another video along with diamond which are both made of carbon atoms silicon dioxide on the other hand is made of silicon and oxygen atoms in a ratio of one to two it's also known as silica and is the main component of sand you won't have to draw it but you do need to be able to recognize giant covalent structures like this now the key thing to take away from this video is that simple molecular substances are small molecules that are made up of just a few conveniently bonded atoms and the separate molecules are only joined together by weak intermolecular forces meanwhile in giant covalent structures all of the atoms are covalently bonded in regular repeating lattices which makes them much stronger and gives them much greater melting and boiling points anyway that's it for now so hope you enjoyed it and we'll see you next time