 In this video we're going to look at what an allotrope is and specifically at the allotropes of carbon. So let's start by recalling some things about carbon. We know that it's in group number 4 on the periodic table which means that it has 4 valence electrons. It's got 6 electrons overall. It's got intermediate electronegativity which means it forms covalent bonds. And because it's got 4 valence electrons it generally forms 4 covalent bonds. Now since like charges repel electrons in neighbouring bonds in a molecule like to be as far away from each other as possible. So molecules arrange themselves so that their bonds are positioned as far apart as possible. But for the case of carbon with 4 bonds the way to arrange 4 bonds so that they are as far apart as possible is to put them in a tetrahedron. The simplest example of this structure is the methane molecule. One carbon bonded to 4 hydrogens arranged in a tetrahedron. This ball and stick model here shows you the 3D arrangement of the bonds more clearly. However another way for carbon to distribute its 4 electrons is to form 2 single bonds and 1 double bond. For instance in the compound ethene with 2 carbons and 4 hydrogens the 2 carbons are joined by a double bond and each is then bonded to 2 extra hydrogens to make up its total of 4 bonds. In this configuration with bonds to 3 different atoms the best way for the bonds to be arranged is in a flat 2D arrangement called trigonal planar. You can see that each carbon is arranged in a sort of a triangular arrangement and it's also 2 dimensional so planar. And there's 120 degrees between the bonds. So this is what happens here in ethene with each carbon atom having a trigonal planar arrangement. So if we compare these two, methane and ethene, you can see that carbon is able to form at least 2 different bond arrangements. What's the significance of that? Well, methane and ethene are 2 different compounds. They're made up of more than one element and they have different numbers of atoms making up their molecules. But what if we only use carbon atoms? Can we get different bonding then?