Biomolecules and the Chemistry of Life
Topics in Depth
The Theme of Bonds in Biomolecules and the Chemistry of Life
Kinds of Bonds and the Laws of Attraction
You may ask yourself, what are all these atoms and ions doing with themselves? As it turns out, a lot of them are bonding with other atoms and ions. Get a room, guys. Seriously.
Sometimes, bonds form between two of the same kind of atom, like in O2. Other times, bonds form between different kinds of atoms or ions, as in NaCl or H2O. If a substance is made of different kinds of atoms or ions that are in some way chemically bound together, it is called a compound. Just as an atom is the smallest unit of an element that has all the properties of that element, a molecule is the smallest unit of a compound that has all the properties of that compound.
There are three kinds of chemical bonds you should be familiar with.
- Ionic bonds
- Covalent bonds
- Hydrogen bonds
Oh wait, there's a fourth kind of bond we'd like you to be familiar with: the James bond. (Those bonds actually come in six varieties: Connery, Lazenby, Moore, Dalton, Brosnan, and Craig.)
OK, for you sticklers out there, a James bond isn't a real thing. We repeat: There is no such thing as a James bond. Except in the classic, Moneypenny-affiliated way, of course. Just so we're clear.
Ionic bonds form between ions that have opposite electrical charges. Let’s take an atom of sodium (Na) and an atom of chlorine (Cl). Sodium only has one "lonely" electron in its outer shell, and you know how electrons feel about being lonely. Chlorine has seven electrons in its outer shell – one short of a full house. It’s a match made in heaven! This type of compound is an ionic compound.
Sodium gives up its lonely electron and in doing so becomes a positively charged sodium ion (+1), or Na+, and chlorine fills its outer electron shell and acquires a negative charge (-1), or Cl-. You might be thinking that chlorine stole its electron from sodium, and that is true, but sodium does not mind because it was making him very grumpy, or what we like to call "reactive."
Since the +1 charge of the sodium ion exactly balances the -1 charge on the chlorine ion, they are held together by their mutual electrical attraction in a 1:1 ratio. The result is a crystalline structure, or sodium chloride (NaCl). You may be familiar with its common name, table salt. We like NaCl better. It makes us sound smart. Right? Right.
In this case, sodium actually gave its electron to the chlorine, but even if sodium had lost its electron to someone else in the past, and if the chlorine had already filled its outer shell, they would still be attracted to each other and form an ionic bond. Our minds are wandering into weird places thanks to that sentence. Let's...move on.
Basically, it’s not the exchange of electrons that matters; it’s the fact that they have opposite charges and are therefore attracted to each other.
Pictures always make everything better. Presenting both the ionic bond between NaCl and NaCl in its lattice structure with other NaCls:
Covalent bonds form between atoms that are willing to share. Let’s say we have two hydrogen atoms; each one has one electron in its outer shell, but each wants two. Greedy hydrogens. If they merge their electron shells, each contributing their one electron, then its like they each have two. It's a win-win situation! This is a single covalent bond – each atom is contributing one electron to the shared supply of electrons.
You can also have a double or triple covalent bonds. Pretty wild, huh? For example, oxygen has six electrons in its outer shell but wants eight. If we have two oxygen atoms, neither one would be happy giving two electrons to the other. Why? Although one oxygen would be content, the other would be left with a highly unsatisfactory four electrons. Unpaired electrons give off a bad vibe. However, if they each contribute two electrons to the common good for a total of 4 shared electrons, then it’s like they each have eight. Hooray! This is a double covalent bond since the atoms are sharing two pairs of electrons.
Covalent bonds are really strong – even stronger than ionic bonds. Sharing is always better than stealing. When two or more atoms covalently bond, the resulting structure is called a molecule. A molecule can be made of two or more of the same kind of atom (like O2), or there can be a bunch of different kinds of atoms (like C6H12O6, also known as glucose). Remember: If the molecules are made of at least two kinds of atoms, they are compounds.
Now, let’s backtrack a little: we told you that atoms whose moms taught them how to share form covalent bonds. And that’s true. But, some atoms are inherently a little more possessive than others, and they don’t really want to share equally (looks like we have another scandal on our hands).
Let’s take water, or H2O, for example. We have two hydrogen atoms, each of which needs one electron to feel complete, and oxygen, which needs two. They can help each other out: each hydrogen can share its one electron with the oxygen, and the oxygen shares an electron with each hydrogen. In other words, a single covalent bond forms between each hydrogen and the central oxygen.
At this point, oxygen, which has 8 protons (a lot compared to hydrogen’s measly 1), is in a much better position to hog the shared electrons by wooing their negative charges with its big positively charged nucleus. The little hydrogens can’t compete. They stay covalently bonded because they still are technically sharing the electrons. Are these electrons pulling at your heartstrings yet? This is not what they signed up for. It would be like if you went out for lunch with your best friend and agreed to share a salad and sandwich, but when the food came, your friend gave you a crouton (without salad dressing) and ate the rest him- or herself. Jerk.
A covalent bond that results in an uneven distribution of electrical charges is called a polar covalent bond. If we revisit our water molecule, the result is that the oxygen side of the molecule will have a slight negative charge, and the two hydrogens will have a slight positive charge, even though the molecule as a whole is electrically neutral.
Time for single, double, and triple the fun with our friends hydrogen (H2), oxygen (O2), and nitrogen (N3):
This brings us to the third major kind of bond you need to know about: hydrogen bonds. A hydrogen bond is the attraction between a hydrogen in a polar covalent relationship and another atom in a different polar covalent relationship. If we have a bunch of water molecules, for instance, a hydrogen bond forms between each slightly positive side of hydrogen in one molecule and the slightly negative side of oxygen in the other water molecules. Not exactly a model of fidelity, huh? An important difference between hydrogen bonds and other kinds of bonds is that hydrogen bonds are much weaker than either ionic or covalent bonds.
Want to see this tug of war at work? Us too.
Water tugging at itself:
Not satisfied with internal polarization, water molecules love tugging at each other as well:
The weakest of the bonds, the hydrogen bond, is also arguably the most important in biology. Because the hydrogen bond is involved in protein folding, it has a role in many genetic disorders, including cystic fibrosis, Creutzfeldt-Jakob disease (CJD), and cancers. Hydrogen bonds are also responsible for the mighty *thwack* you feel when you belly flop into a pool.
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