Bonding

Atoms form bonds to other atoms by transferring or sharing electrons

There is a wonderful rule in chemistry called the “octet rule,” which basically says that an atom will do whatever it takes to keep its outer shell filled up with eight electrons. As with any rule, there are exceptions. For example, hydrogen has one electron and is looking for one more. But for the biologically relevant atoms the rule works like a charm, and it explains the bonding patterns we see in molecules such as water (H2O).

We just used the word “bond,” and now we are in the position to start talking about bonding between atoms. Generally speaking there are two types of bonds formed between atoms: the ionic bond and the covalent bond.

Ionic bonding


A 3-D representation of salt

Sodium chloride, or salt, is a good example of an ionic bond. Remember, atoms can link to other atoms depending on their valence electrons. In the Periodic Table, sodium (Na) has one electron in its outer shell, while chlorine (Cl) has seven. Using the Octet Rule, we know that these two elements are made for each other and together they make their outer electron total eight. Sodium is looking for a place for the one electron in its outer shell, and chlorine has one spot available.

 

Covalent bonding

So what are covalent bonds? Covalent bonds occur when two atoms share two or more electrons equally. It is kind of like two kindergartners on the playground fighting over a toy. Each one wants the toy, but neither will let go. So the two kindergartners are attached together. The kindergartners are the atoms, and the toy is the covalent bond.


Carbon has four valence electrons

We know from the periodic table that carbon has six protons and six electrons. Now take a look at the “electronic” configuration about carbon. It has two electrons in its first shell and four electrons in the second shell. These four are the valence electrons. Carbon doesn’t like having only four electrons in its outer shell. It wants eight electrons (according to the octet rule). So what can it do? It can find other atoms that will share their electrons.

 


The compound methane

Ever heard of methane? It is a major component of natural gas, is highly flammable, and is a relatively potent green house gas. Its chemical formula is CH4. By sharing electrons with four hydrogens, the carbon in methane is filling up its outer shell with eight electrons. Go ahead and count the electrons in carbon’s outer shell. The bonds between carbon and hydrogen are covalent bonds because they are two atoms sharing two or more electrons equally. Next consider oxygen. It has six valence electrons in its outer shell. It wants eight. So what does it do? Well, one way to get eight electrons would be for it to share electrons from two other atoms. If it shares or bonds with two hydrogens, you get water.

 


The compound ammonia

How about nitrogen? It has five valence electrons. According to the octet rule, it wants eight. So how many bonds do you think nitrogen will make with other atoms? It makes three. Let’s consider ammonia as our example, which is the stuff that stinks in household cleaning supplies such as Windex. It has a molecular formula of NH3 and its electronic configuration is shown to the right. Go ahead and count the number of electrons in nitrogen’s outer shell. It has a total of eight electrons, three of which are shared with the hydrogens. Again, the octet rule can help explain a lot about the electron configuration in the first two to three shells of an atom. So, you can think of the octet rule the next time you are washing the windows!

Double and triple bonds

So far we have simply talked about single covalent bonds where two electrons are shared between two atoms. But so-called double and triple bonds also occur. Consider the molecule carbon dioxide. It is a very important molecule in both biology and in industry. For example, it is used to produce carbonated soft drinks, and it is found in some fire extinguishers. Ever heard of Pop Rocks candy? The popping that occurs when you place the candy in your mouth results from carbon dioxide being released. What’s the chemical formula of carbon dioxide? It is CO2.


Carbon dioxide and double bonding Does this make sense according to the octet rule? It sure does. Count the number of electrons in the outer shells for both the carbon and the two oxygens in the figure on the right. Each oxygen shares four electrons with the carbon, and the bonds in CO2 are double rather than single bonds.


Molecular nitrogen and triple bonding

Let’s try one more example about bonding before we move on to how chemists draw out molecules and that will be nitrogen bonding with another nitrogen atom. For each nitrogen atom to get eight electrons in its outer shell, it has to share a total of six electrons.  This is a triple bond, and this bond is probably the strongest bond observed in Nature. Again, each nitrogen in N2 has a total of eight valence electrons in its outer shell.

 

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