 A covalent bond occurs between two non-metal atoms. Non-metal atoms have relatively high electronegativity, so in order to get a full outer shell of electrons, the preferred method is for them to gain electrons. If a non-metal atom bonds with a metal, the non-metal can take electrons from the metal and both become ions. The metal becomes a cation and the non-metal becomes an anion. However, if two non-metals bond together, neither wants to give up electrons, so the solution is to share them. Here each hydrogen atom has one valence electron and to have a full outer shell it needs two. If the atoms move close together, they can each share the other's electrons, so that effectively they both have two. The electrostatic attraction is then between the two positive nuclei of the atoms and the shared electrons. A covalent bond always consists of two electrons. When two atoms form a covalent bond, the usual situation is that each atom donates one electron to the bond, although there are examples where one atom will donate both. In the previous slide I drew the atoms using the old Bohr-style diagram of electrons orbiting like planets. Of course we know now that that's a great simplification and in fact the electrons exist as electron clouds or orbitals around the nuclei. So when we think of a covalent bond it's more correct to think about the two electron clouds joining up and becoming denser in the region between the two nuclei. There are a number of ways to represent a covalent bond. These two pictures from the Model 360 website show the hydrogen molecule in a ball and stick representation on the left and a space filling representation on the right, which is an approximation of what the electron cloud of the complete molecule looks like. As a shorthand we can draw what's called a Lewis structure. We draw the symbols for the two atoms and we draw two dots in the middle to represent the shared electrons. Better still we can condense the two dots into a line and use that to represent the bond. Unlike metallic and ionic bonds which occur in lattices and are non-directional, covalent bonds are directional, meaning that the location of the bond, or of the electrostatic attraction, is directly between the two atoms involved and nowhere else.