Sunday, April 06, 2008

Chemistry Blast...

...From the past, that is.

Hubby gets to help his oldest daughter from his previous marriage with school work occasionally. Her most recent bump in the academic road has been chemistry in eighth grade. Her class is going over Lewis diagrams (also known as electron dot diagrams) and how bonds are formed. This was really hard to help with!

I found it just the kind of puzzle that I love. I know that with enough work we could find the answers and in going through some examples I saw some of the assumptions that she was making that were leading to wrong answers and I got a chance to learn something new myself.

Quick summary: as you recall, an atom has a number of protons (positive in charge) as indicated by the atomic number. A neutral atom has an equal number of electrons (negative in charge) as protons. The electrons orbit the nucleus in shells. The first shell is considered full with two electrons. The second shell is considered full with eight electrons. More electrons move into shells further and further out and the numbers of electrons that settle in the further out shells can vary, but if there are usually still eight in the outermost shell. Once a shell is full it is off the combination market (in our simplistic view) and is a stable atom. The periodic table is arranged so that most of the elements in a single column have the same number of valence electrons in the outer shell. The last column of the periodic table (the one that contains Helium, Neon, Krypton) is known as the Noble Gases because they are very stable--because of their full outer shell.

Electron dot diagrams are made by looking at the number of electrons that are available for bonding in the outermost shell of an atom. These diagrams can help you see how many electrons are left in the outer shell (the valence electrons). Let's remember that atoms are more stable with full outer shells. So atoms with outer shells that have less than eight electrons (or less than two in the case of helium) have space and are more stable by combining (bonding) with other atoms.

There are two types of bonds that we were learning about. Atoms can get stable, or full, outer shells by either taking an electron from another atom (ionic bonds) or by sharing pairs of electrons (covalent bonds). Ionic bonds occur when each atom will have a full outer shell because of giving the available electrons away. If an atom loses an electron to the other atom it is bonding with, it will have more protons (positive) than electrons (negative) and will develop a positive charge (a charged atom is called an ion--hence the name of the 'ionic' bond). If the atoms do not have the proper number of valence electrons that would lead to giving up the electrons, then both (or all) of the atoms need to use electrons simultaneously to have full shells: the covalent bond.
As an example, lets take one carbon atom and four hydrogen atoms (methane molecule). We know hydrogen has an atomic number of one, which mean one proton and, thus, a neutral atom will also have one electron. The electron dot diagram will have the element symbol (from the periodic chart) with the number of outer shell dots shown around it: H . (imagine that period a little higher, please). Thus we know the element and the number of valence electrons available. It is important to remember for hydrogen and helium that a full outer shell will only need TWO electrons. All other elements will need eight. Next we need to look up carbon. Carbon has an atomic number of 6; six protons and six electrons. The first two electron fill up our inner most shell (which only needs two to be full).

The last four are then in the outer shell just waiting for some company. So the electron dot diagram for carbon will have a C with four dots around it (as shown below). The electron dot diagram is also used to show the molecule bonds.

Here they are with just the dots. You'll note that each hydrogen now has two dots, representing electrons, next to it, indicating a full outer shell. The carbon atom now has eight dots around it(four shared with the four hydrogens) and now has a full outer shell. Everyone is stable and happy.

Some color coding and another representation using Xs to indicate which electrons come from the hydrogen.

The with the electron dot diagram it becomes much easier to see which atoms have shells to fill, which kinds of bonds are made, and how many bonds form between the atoms.

Which reminds me of a joke. An ion walks into a bar and looks really sad. The bartender says, "Hey, why are you so upset?" The ion sadly says, "I lost an electron." The bartender asks, "Are you sure?" and the ion answers, "I'm positive." Which is just a joke I love! If you're lucky you'll never hear me tell the one about the piece of string who walks into a bar.

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