Covalent Bonding

In a covalent compound the electrons are shared between the atoms in the compounds.  This "sharing" is not always equal, but it does not result in full excess electron charges residing on one atom compared to another.  Instead the electrons are distributed about the entire molecule in such a way the minimizes their energy compared to their energies in the separate atoms.  This is the key idea.   The energy of the molecule is lower (more stable) than the energy of the separated atoms.  We typically discuss this in terms of the potential energy.  Often this is represented graphically where we look at the potential energy as a function of distance between the two atoms that are "bonding".  If there is a distance at which the energy is lower than the two separated atoms we say these two atoms form a chemical bond.  The bond length is the distance at which the potential energy is a minimum.  The bond strength is the difference in the energy at the minimum compared to the separated atoms.  This is shown on the diagram below.

This diagram show the energy for two hydrogen atoms as as function of the distance between them.   There is a clear minimum at 74 pm (0.74 Å). We would say the bond length is 74 pm.  The bond strength is the depth of this "well" which is 436 kJ mol^-1.  That is the amount of energy the H₂ molecule would need to gain to break the bond and end up as two separate H atoms.  Note the that while the two H atoms are attracted to each other at long distances (there energy is lower as the distance gets shorter), they are repulsive at very short distances (the energy is lower if the distance increases). This is due to the overlapping of the electrons in the two system (and repulsion of the two nuclei).