 So far we've said that a single covalent bond consists of one pair of shared electrons. However it is possible to form double and triple bonds as well. Let's look at fluorine, oxygen and nitrogen molecules. Fluorine has seven valence electrons. It requires one more for a full outer shell. When a diatomic fluorine molecule forms each atom requires one more electron so they form a single bond, each sharing one electron and getting one from the other atom. The remaining valence electrons are called non-bonding electrons because they don't participate in the bond. Oxygen has six valence electrons. It requires two more for a full outer shell. When a diatomic oxygen molecule forms each atom requires two more electrons. So when the atoms get together rather than simply sharing one electron each they share two each. This means two pairs of electrons are involved in the bond and we call this a double bond. For convenience we draw it as two parallel lines with each line representing a pair of electrons. Nitrogen has five valence electrons. It requires three more for a full outer shell. So in a similar fashion to the oxygen it shares three electrons which means the bond is made of three pairs of electrons. This is a triple bond, the strongest kind of covalent bond. As we've said the electrostatic attraction that causes a covalent bond arises because the nucleus of each atom is attracted by the bonding electrons. The atoms have to be very close for this to happen. But remember that the two nuclei of the atoms are both positively charged. So if the atoms get too close the nuclei will start to repel each other. We can explore the balance between the electrostatic attraction and repulsion using a diagram like this. Vertical axis of this graph represents the force between the two atoms. When it's above the middle line it's representing a repulsive force. When it's below it's an attractive force. The x-axis represents the distance between the two nuclei. If we start with two atoms a long way away they don't feel each other. Their electrostatic fields aren't interacting and the force between them is at zero. If we bring them closer the nucleus of each atom starts to interact with the electric field from the electrons of the other atom. A slight attraction is experienced. This pulls the atom in closer and the attraction grows. You can see our line is dropping further below the middle of the graph. At this point the nuclei are not repelling each other because they're shielded from each other by their electron clouds. The atoms continue to get closer until the point of maximum attraction is reached. The atoms are now at their preferred distance and the attraction is strong. We would say at this point that a bond has formed. If we force the two atoms closer together the two nuclei come within range of each other. The electron clouds are so far overlapped that they can't shield the positive nuclei from each other and so they start to repel. So the attraction turns into a repulsion. This forces the atoms apart again until they reach their preferred distance. This preferred distance, the length of the bond, is different for different atoms. A rule of thumbs is that the shorter a bond is the stronger it is. This makes sense. If the electrostatic attraction is able to pull the atoms closer together then it must be able to cancel out a greater level of repulsion from the nuclei. Double and triple bonds are shorter than single bonds because there are more electrons involved and so a greater level of electrostatic attraction.