Introduction to Electron Configuration
Electron configurations are a simple way of writing down the locations of all of the electrons in an atom. In all atoms electrons and protons tend to attract each other, while electrons repel other electrons. Becuase of these attractions and repulsions, the electrons stay in very distinct spots in the atom. This results in beautiful geometric structures called orbitals that represent the distinct regions around the nucleus that each electron traces out.
The orbitals represent identifiable “addresses” for each electron around an atom. Think of the electrons as being tenants in one of several blocks of studio apartments located near a nice park. The electrons all want to live near the park (nucleus), but they can’t all live in the same place. Instead, some electrons get to live in the apartments closest to the nucleus, but as the number of electrons that want to live near a given nucleus increases, the further out some of them need to move since apartments closer to the nucleus fill up. This describes a trend observed in the periodic table: elements with small atomic number (and thus fewer electrons) tend to have most of their electrons living in orbitals near the nucleus. As we move further down the periodic table, orbitals and energy levels further out from the nucleus begin to fill up with electrons.
In order to track down where a given electron lives in an atom, you need to know not only how far from the nucleus it is found (which determines its energy level, since electrons further out from the nucleus tend to have higher energy) but also the type of orbital that it can be found in. Think of this as knowing not only which apartment building (energy level) the electron lives in, but also its apartment number.
Because chemists are really interested in keeping track of where all the electrons in a given atom live, they write down a series of symbols called an electron configuration that keeps track of all of this information for a given atom.