What are electron shells?
Thanks to Eshne_Dovyurak for this subject. Check out her great works: My Rants, Dangerous, Helping Hansel and When the Clock Never Strikes Twelve.
Here we go again with the atomic stuff, but this is actually a chemistry thing. As I've said before, an atom has a very tiny nucleus composed of protons and neutrons with electrons orbiting. It turns out in the case of higher atomic weight elements that these electrons are found in specific orbits that chemists call shells. These shells represent quantum energy levels, which means that each shell has a specific energy associated with it and can only hold a specific number of electrons.
Each shell can be further divided into subshells and the further away the shell is from the nucleus the more subshells it can have. Nit picking chemists have labeled these main shells using letters. The lowest shell is K, the next is L and so on up to Q. The K shell can hold two electrons, the L shell can hold eight and the M shell can hold 18 and so forth. Physicists prefer numbers from 0 to 7 for these shells.
Although elements must fill up their lower shells first, it is possible to have incomplete outer shells. Subshells can hold a specific number of electrons and are labeled with lowercase letters: s, p, d, f and g. A s subshell can old 2 electrons; a p subshell can hold 6; and a d subshell can hold 10 electrons.
The thing to keep in mind about these electrons being in specific shells is that when the atom is stimulated by radiation or heat the electrons can move up and down in the shells and produce energy that is seen as discrete (a line at a specific frequency) spectral lines. That's because the electrons that jump can only jump in discrete quantum steps or energy levels.
All this is interesting, but for a chemist the only shells that matter are the so-called valance shells. These outer shells are where chemical boding happens. These valance electrons make it possible for elements to form bonds with other elements by sharing them or by giving (taking) them. Remember when I made chemical bonding equivalent to a marriage. Well, it seems that an element with all of its valance shells filled up with the permitted number of electrons is usually inert, which means that it won't react with other atoms easily. An atom has to have needs, essentially an urge for electrons and these electrons can come from a willing atomic partner.
The degree of need (the number of electrons in its valance shell) for an atom determines how willing it is to mate with another atom. For example an atom like chlorine needs only one electron in its valance shell to be fulfilled. It likes an element with a free electron or electrons in its valance shell. It says sock it to me. Give me those nice electrons. In other words the atom with a need for electrons is attracted to the atom with electrons to offer. For example a halogen atom like chlorine only needs one electron in its valence shell to be complete, so it really loves an alkali atom.
Chemists call this the valence of an element, and they use it to predict how well certain atoms can react (have affinity) with other atoms. What this boils down to is how many other atoms can attach to a given atom. For example, a carbon atom can react with four other atoms. Methane is an example of this. One carbon atom is bonded with four hydrogen atoms, so a carbon atom has a valence of 4. Hydrogen, on the other hand, has a valence of 1.
Chemists love chemical bonds because they make the creation of molecules (collection of atoms) possible. A chemical bond is essentially an attraction between atoms, which I like to think of as a marriage. In some cases these marriages are polygamist affairs with one element attaching to several other elements.
The valance bond theory proposes that the reason that chemical bonds form is because the valance electrons from two atoms are shared and this is what keeps the marriage together.
However, this isn't the only chemical bonding theory. The Molecular Orbital theory says that the boding electrons are moving under the influence of the combined nuclei of the molecule not the individual atoms that make up the molecule. In other words the molecule has electrons in orbit around it. This theory is the result of a quantum physics influence on chemistry, and it reflects the idea that the exact positions and energies of the electrons can't be determined at the same time. However, by using the Schrodinger equation, chemists try to predict the shape and size of chemical bonds. Here is where we get into the idea, for example, of 2p orbitals combing to form a pi bond. This approach is only reasonable with simple bonds involving hydrogen atoms. Despite that, quantum physicists are using this concept to come up with a method of predicting chemical reactions by calculating what bonds between various atoms would look like and how the electrons would be distributed. This is really advanced stuff.
So, what sort of bonds can form between atoms? There are only two major types: ionic and covalent. An ionic bond is formed when one atom gives up its valence electrons to another and a covalent bond is formed when two atoms share electrons.
An example of ionic bonding is when a sodium atom loses its only valence electron to a chlorine atom. What you end up with is a sodium positive ion, which is smaller, and a chlorine negative ion, which is larger. The result is sodium chloride or salt.
With covalent boding, atoms form molecules with bonds that share electrons, and some atoms can form multiple bonds. A carbon atom, for example, can form single, double and triple bonds with another carbon atom.
Covalent bonds can be polar in nature. A dipole covalent bond is where the electronic charge on a molecule is polarized so that one end is more positive and the other is more negative. This happens with the water molecule. A single oxygen atom shares valence electrons with two hydrogen atoms, but the oxygen atom tends to hold on to these valence electrons more than the two hydrogen atoms. In other words the oxygen acts like a bully and makes the shared electrons hang around near it more, giving it a more negative charge, resulting in the hydrogen end of the water becoming more positive. This is how a dipole works. This dipolar nature of a water molecule is responsible for the interesting properties of water, such as how it crystalizes when frozen and how it behaves as a liquid.
The bottom line is that chemists are attempting to move chemistry from a descriptive science to one that is predictive by using these quantum physics concepts. More power to them!
Thanks for reading.