 In the last few videos, we looked at predicting the shapes of molecules. The next step is to have a look at what are called molecular dipoles. When you understand the idea of molecular dipoles, you'll be able to see how and why the shape and type of atoms in a molecule directly influence the properties of the substance. Why, for instance, water has a massively higher boiling point than methane, or why oil and water don't mix. Before we look at molecular dipoles, however, we have to take a step back and look at individual bonds. Like in Unit 1, you were introduced to a number of periodic trends, one of which was electronegativity. This was defined as the tendency of an atom in a bond to draw electron density towards itself. Note that the atom has to form a bond for the concept of electronegativity to have any meaning, and this is why some atoms on the periodic table don't have an electronegativity value. Either they don't form bonds, like the noble gases here, or they're so short-lived that no one has ever been able to do the right measurements, like the radioactive elements here. So fluorine is the element that has the highest electronegativity, and down the bottom left, cesium and francium, are the elements with the lowest electronegativity. And the trend in general is that the electronegativity increases as you go across a period, and decreases as you go down a group. You may also remember that when two atoms get together, it's their electronegativities that influence what kind of bond they'll form. For instance, if a pair of atoms with widely differing electronegativities meet, they'll form an ionic bond. The atom of low electronegativity, which will be the metal, will donate one or more of its valence electrons to an atom of high electronegativity, which will be the non-metal, and the resulting ions attract to form an ionic bond. However, if both atoms have relatively high electronegativity, it's not energetically favourable for either of them to give up an electron to get a full outer shell. So instead, they share electrons, and they complete their outer shells in that way. And this is called the covalent bond.