What does water look like?

         At birth, 80% of a baby’s weight is due to water; as an adult, 70% of the body is water.  ~ 70% of the earth’s surface is covered with water; all of the fresh water underground will add up to more than is available on the surface.

                Water’s cool.  In the Soap! post and the Hand Sanitizers vs. Soap post, I discussed how water can hang on to certain other kinds of molecules and pull them places.  In the case of soap, water pulls the soap from our hands and flushes it down the drain.  

                Water’s necessary.  Some of the enzymes in our bodies require water to function.  Dehydration is a serious problem that can lead to death.  A human can survive for a few weeks without food, but only a few days without water.

                But, what does water look like?  Many people know that a water molecule is comprised of one oxygen atom and two hydrogen atoms, but how are they arranged?  Are they just stuck together arbitrarily?  Where are the electrons?  What’s going on there?

                Okay.  Let’s start with a guided tour of an atom.

                We’ll start deep in the center of an atom at the nucleus (Figure 56.1).  Here is where we will find protons, neutrons, some forces called the strong nuclear force and the weak nuclear force and just a lot of stuff.  The nucleus of an atom is quite dense – meaning, there’s a whole lot of mass in a really small space.  Let’s get out of here.

                Upon leaving the nucleus, you will now encounter mostly empty space.  Electrons are flying around (not exactly willy-nilly, but also not in “orbits” like planets around the sun), but the vast majority of the space is empty.  Think of an atom as the fenced in yard of a home and two kids as the electrons.  The fence determines the size of the property and the kids are allowed to run anywhere within the fence.  If you just stand there, a kid will eventually run by you, but you’re mostly standing there alone.  That’s what it’s like to stand among the electrons.

                The left of Figure 56.2 gives you a closer look at the atom oxygen.  It has eight electrons roaming around its nucleus.  However, two of these eight are too close to the nucleus to really be bothered by anything so we are only going to consider six of its electrons; these six are called oxygen’s valence electrons.  Valence electrons are those that are involved in forming bonds with other atoms.

                The right of Figure 56.2 shows hydrogen.  It only has one lonely electron floating around, so hydrogen has one valence electron.

                So how does all this jazz fit together?  Bonding is all about sharing of valence electrons.  In the case of water, one oxygen atom offers six electrons and each hydrogen atom offers one electron each.  So, we have three atoms and eight electrons that need to be arranged between them.

                Let’s start with what we know: the oxygen binds both hydrogens and the hydrogens do not bind each other.  This makes oxygen the central atom.  So… let’s draw it in the middle and place each hydrogen on either side to make a schematic of one water molecule (Figure 56.3).

                Now we have to fill in those eight electrons.  In chemistry, electrons hang out in areas called orbitals.  Orbitals are areas of space that electrons are likely to be found.  I’m not going to go in depth about them because, quite frankly, they are a smidge complicated.  I’m going to keep things straightforward here!  Each orbital can hold two electrons.  

                How many electrons do we have between the oxygen and hydrogens?  Eight.  

                Since each orbital can hold two electrons and we have eight, we need four orbitals around our oxygen*.

                Electrons all have a negative charge.  Like charges repel so these orbitals need to be situated around the oxygen as far from each other as possible.  The best way to do this is to have a tripod + camera situation, as shown in Figure 56.4.  The center of the tripod is the oxygen and the three legs plus the arm sticking up with the camera represent the orbitals.  I’ve redrawn the schematic with this orbital arrangement in mind (Figure 56.5).

               

                Now we fill in our electrons; two per orbital. Look at Figure 56.6.

                Ta-da!  That’s water!  That’s three dimensional water! 

                People tend to draw water like a bent line or refer to water as “bent.”  Looking at Figure 56.7 helps explain why.  If you are looking strictly at the atoms, you will see one oxygen atom with two hydrogen atoms placed slightly lower.  You will not see the orbitals just containing electrons because they are considered part of the oxygen atom. 

                In the Soap! post, I specifically drew water as a large center circle with two smaller circle slightly lower on either side.  I was trying to accurately represent how water looked in real life. 

 

No, you can’t see water molecules with your eyes, but now you know what it would look like if you could.  Interestingly, the bent shape of water lends itself to many of water’s wonderful properties, but that is definitely a topic for another post.

*  For the more science-y among us: I realize that this sentence is very very simplified.  It’s a backhanded and not entirely accurate explanation of VSEPR Theory, but I did not want to get into that here, especially since I’ve never explained electron configuration, Lewis dot structures or orbital shape.  To get through all that just to explain water would be overkill.  If I ever do explain those topics, I will certainly link back to this post and offer a more in depth explanation.  Until then – this is what I thought would get the point across as simply as possible.

* - For any chemistry students who stumbled upon this, read your text book to get a handle on VSEPR Theory!  You should know and understand electron configuration, Lewis Dot structures, orbital shape, the Pauli Exclusion Principle and Aufbau's law before looking at VSEPR or MO Theory.   Or taking a test.

Valence electrons: electrons that are involved in bonding with other atoms

Orbitals: areas of space where electrons are likely to be found.  They are three dimensional areas (like a sphere) and not two dimensional circular orbits (like the plants going around the sun).

REFERENCES

http://www.allaboutwater.org/water-facts.html

 

Zumdahl, Steven S. “Chemical Principles, 4^th Edition” (2002) Houghton Mifflin Company, Boston, MA.