 Okay, so let's consider a simple covalent bond between two identical atoms, as happens with diatomic elements like bromine, oxygen, hydrogen, and so on. Remember Brinkelhoff. In a bond like this, since the atoms are identical, they have the same electronegativity. This means that they both exert an equal pull on the electron density, so it's evenly shared between them. This picture from the model's 360 database illustrates this by showing a whole fluorine molecule colored green, which indicates that there is even electron density around the molecule. However, if a covalent bond forms between two different atoms, then the atom with the higher electronegativity will exert a greater pull on the electron density, and it will not be evenly shared. This is a picture of hydrogen fluoride. The red indicates high electron density, and the blue indicates low. You can see that the very electronegative fluorine atom, the larger one on the left, has taken the lion's share of the electron density. When you get uneven electron density in a bond, the bond is called polar. Now if we're going to talk about polar bonds, I need to introduce some new terms. Firstly, if we refer to something as polar, we generally mean that it has two opposite ends. These could be geographic poles, or magnetic poles, or philosophical poles. Whereas in this case, electrostatic. A polar bond is a bond that has a slight separation of charge. By this I mean that there's a small excess of negative electrostatic charge at one end, that's where the greater electron density is, and a small excess of positive electrostatic charge at the other end. These two ends represent the two poles of the polar bond, and we can also refer to them as the bond dipole, which is just a fancy way of saying it has two opposite ends.