MadSci Network: Chemistry

Re: What holds the atoms together in a real water molecule??

Date: Wed May 13 17:34:47 1998
Posted By: Jeremy Starr, Grad Student, Chemistry, California Institute of Technology
Area of science: Chemistry
ID: 892653756.Ch

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 
	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!


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