Student Questions and Instructor Answers
Last revised: Wednesday, September 8, 1999
Note: student questions of potential interest to others will be edited and posted here anonymously, along with my comments. Most recent comments are at the top of the list.
Q. I'm having trouble with Ionic, hydrogen, Van der waals, and Hydrophobic bonds. I get confused on what each one does and I'm having trouble understanding when you draw diagrams of water, carbon and hydorgen molecules and their electronegativity. I just don't understand. Unfortunately I'm unable to come see you at your office hours today or tomorrow, and I can't not attend the SI meetings either. I'm really trying to understand by going back to the readings and the notes but it's just not sinking in. I would really appreciate it if you could help me out because I don't want to get behind in any of the work assigned. (9/9/99)
A. Let's see if this helps ....
Strong bonds are no problem, right? Just the sharing of electrons between two atoms to create a stable pair (or 2 or 3) of bonded atoms.
Some covalent bonds are polar -- the electrons are not equally shared. As a result, the molecule of which they are a part "cheats" a bit by forming temporary weak bonds with other polar molecules. Think of an analogy: John and Mary form a couple, but John is always "on the make" for other women, whereas Mary is content. They walk around holding hands (covalent bond), but he's always touching other women as he flirts (forming temporary weak bonds). But they remain a couple. That would be an analogy for a H-bond.
Ionic bonds are easy. Elements in columns 1 and 2 (on the left) like to give up their 1 or 2 outer electrons to get a complete shell. Elements in column 7 (next to right) like to add one electron to get a complete shell. That leaves them charged: Na+ or Cl-, for example. So one of the basic "laws of chemistry" is that elements try to get complete electron shells. But there's another basic law, which is that + and - charges attract. So as soon as ions are formed, they seek to surround themselves with other ions of the opposite charge. This differs from covalent bonding in that ions don't form unique partners -- unlike H and C, which will share a pair of electrons and form a stable bond, Na+ and Cl- will pair, but each will also form similar bonds with as many ions of the opposite charge as will fit on other sides. Since such pairing occurs in 3-D space, one ion can be surrounded by 6 ions of the opposite charge - one above, one below, one to the L, one to the R, one in front, and one in back. Analogy: think extremely promiscuous Rick and Jane; Rick likes to have as many girlfriends hanging around him at any time as possible, and Jane likes many boyfriends. Unlike the handholding stable pair of John & Mary, Rick's multiple partnerships are all equally strong (or all equally weak). Different kind of bonding.
Van der Waals bonds are stranger. The basic fact is this: when any two atoms come very close to each other, they attract each other VERY weakly. That attraction only occurs at a certain critical distance, the "Van der Waals radius" -- closer away or farther away it disappears. Don't worry about understanding WHY this happens, if the words "induced dipole" send your brain to sleep - just think of it happening. Analogy: any two people, no matter what age or sex, will warm each other if their skins touch. Not a strong bond, no sexual overtones, just a weak attraction. In cold countries without central heating, people will sometimes sleep huddled together for warmth. A different kind of bond than hydrogen or ionic.
Finally, hydrophobic bonds are easy. The analogy here is oil and water. Why does oil float on water? We know water molecules are attracted to each other -- maybe the oil molecules are equally attracted to each other? NO! The only thing that "holds" the oil molecules together is their inability to penetrate the water lattice, made up of so many H-bonds holding water molecules together. Whenever oil tries to penetrate, it would mean breaking up many H-bonds -- but that is not likely to happen, so the oils are forced out of water and "stuck together" by a measurable energy, even though there's no direct attraction. Analogy: think of a clique of friends who live on the same dorm floor and who share years of common history and experience and are really tight. A few new roommates who have nothing in common with each other or the clique are added to the mix. The clique totally ignores the newbies, who find themselves thrown together since it's the only social life they have -- but they don't have much interest or attraction to each other, it's just that there is even less possibility of them mixing with the clique. That's what hydrophobic "bonds" are like.
Q. I have a copy of the previous Campbell text, 4th Ed. Is it OK if I use this text instead of buying a new copy of the 5th Ed.? Frankly, I really can't afford the $$. (8/30/99)
A. Yes, save your money and use the previous edition text, with the following cautions:
- There's maybe 10% difference in content -- for example, the 5th Ed. has a new chapter on Cell Communication, which is part of our coverage this semester. You are responsible for assigned reading, so be sure you make arrangements to get a copy of material that is substantially different.
- Note that the book is used in both Biology 107 and 108, so you may have similar need to stay abreast of differences with the Bio 108 course if you take that next semester.
- The new 5th Ed. of Campbell includes a free subscription to The Biology Place, which we will be using extensively this semester. You should plan on purchasing a 3 month subscription for $10 as soon as possible, so you can do the assigned work at that web site.
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