Posted By: Jeremy Starr, Grad Student, Chemistry, California Institute of Technology
Area of science: Chemistry
ID: 892653756.Ch
Message:
Hi Paul, The water molecule is composed of two hydrogen atoms and one oxygen atom. Oxygen is the central atom and each hydrogen atom is bonded to the central oxygen with a single two- electron covalent bond. This arrangement is stable because the shared pairs of electrons between the two hydrogens and the oxygen simultaneously complete the outer valence shells of both hydrogens and the oxygen. It is generally true that structures which allow all their constituent atoms to have filled valence shells are stabilized and it is emperically true that structures that possess this property are found quite a bit more frequently in nature than the alternative. A number of physical truths conspire to make covalent bonds possible. I have already mentioned one of those truths: atoms exist which need electrons from an outside source in order to attain a filled valence shell and bonding is one of the ways this can be accomplished. Another truth is that opposite charges attract eachother. Nuclei (composed of protons and neutrons) are positively charged and electrons are negatively charged so they attract eachother. Electrons from one atom can be attracted to the nucleus of another atom if the atoms are close enough together. In terms of potential energy, this means that the lowest energy state (most stable) is achieved when electrons are as close as possible to a nucleus. When two nuclei are near eachother (as in a bond) they repel eachother because they are like-charged, increasing the potential energy of the molecule. Also, since electrons are not static particles there is kinetic energy in the molecule as well. It is a fact that as electrons come closer to a nucleus their kinetic energy must go up. So, what does this all mean?? Well... Electrons want to give up all their potential energy and crash into a nucleus except that they must gain a bunch of kinetic energy to do it, so there is a balance of forces within an atom that allows it to exist. However, if a second atom comes along and has space for more electrons (unfilled valence shell) then shared electrons between them can be stabilized by two nuclei rather than just one without having to take on so much kinetic energy. The repulsion of the nuclei then balances this tendency to stabilize by closing the gap between atoms, allowing there to be a discrete distance across which electrons can be shared by nuclei to acheive the lowest possible combined potential and kinetic energy of the molecule. This is a covalent bond. The fact is, the classical mechanics description I just gave is only an intuitively satisfying approximation of what is really going on (and would fall apart under scrutiny). Quantum mechanics is required in order to accurately describe how, and why, bonding occurs. The figure below is a picturesque way of saying what quantum mechanics has to say about bonding in water.A hydrogen atom, by itself, comes with a single electron in a spherical "orbital". An "obital" can be thought of as the space occupied by the electron and is defined by its energy. An orbital really is the 3-dimensional representation of the mathematical probability of finding an electron within the orbital. (most published pictures of orbitals represent >95% probability, meaning that the picture represents the space one would have to look in to find the electron 95% of the time. Hmmm, where is it the rest of the time?). The oxygen comes with six valence electrons dispersed in four identical oblong shaped orbitals arranged in a radially symmetric fashion around the nucleus. (by the way, these orbital shapes only occur in bonded oxygen. Truly free oxygen would have a different arrangement). The orbitals of two hydrogen atoms and one oxygen atom combine to form the "molecular orbitals" shown on the left of the diagram above. The six atomic orbitals (4 from oxygen, 1 from each hydrogen) form six molecular orbitals. Those are: one bonding orbital for each O-H bond, two non-bonding orbitals on the oxygen, and one "anti-bonding" orbital for each O-H bond. The electrons that occupied the atomic orbitals (8 total) re-dispurse themselves into the lowest energy molecular orbitals as possible (to maximize stability) with two electrons in each orbital. Thus, the two bonding orbitals are filled, providing the two O-H bonds, and the two non-bonding orbitals are filled, providing the lone pairs of electrons on the oxygen and the two anti-bonding orbitals remain unoccupied. The relative energies of these orbitals is the result of quantum mechanics as are the shapes and the final geometry of the water molecule. One more thing to note: the bonding molecular orbitals were drawn such that more of the electron density was near oxygen than near hydrogen because oxygen has a greater nuclear charge and a higher affinity for electrons than hydrogen so the bonding electrons "prefer" to be closer to the oxygen nucleus than to the hydrogen nucleus. I hope this info is helpful! Jeremy.