Valence Shells
The valence shell is the outermost shell of an atom. It is usually (and misleadingly) said that the electrons in this shell make up its valence electrons, that is, the electrons that determine how the atom behaves in chemical reactions. Just as atoms with complete valence shells (noble gases) are the most chemically non-reactive, those with only one electron in their valence shells (alkali metals) or just missing one electron from having a complete shell (halogens) are the most reactive.
However, this is a simplification of the truth. The electrons that determine how an atom reacts chemically are those that travel farthest from the nucleus, that is, those with the highest energy. For the transition elements, the partially filled (n − 1)d energy level is very close in energy to the ns level and hence the d electrons in transition metals behave as valence electrons although they are not in the so-called valence shell.
Read more about this topic: Electron Shell
Famous quotes containing the word shells:
“The etymologist finds the deadest word to have been once a brilliant picture. Language is fossil poetry. As the limestone of the continent consists of infinite masses of the shells of animalcules, so language is made up of images or tropes, which now, in their secondary use, have long ceased to remind us of their poetic origin.”
—Ralph Waldo Emerson (1803–1882)