Shell model (atomic physics)
The shell model is an atomic model in which the electrons surround the atomic nucleus in concentric shells. The location of an electron is modeled by a probability function. The amplitude of this function at a certain location is proportional to the probability of the electron being in that location. The shell model is thus a simplification of the orbital model .
According to this model, the electrons in the atomic shell are arranged in shells, vividly like the shells of an onion. Each shell is the spatial location of electrons with similar binding energies . The innermost shell closest to the atomic nucleus is called the K shell . It contains a maximum of two electrons. The next shell, the L shell , has room for a maximum of eight electrons.
There is space for 18 electrons in the M shell, 32 electrons in the N shell and 50 electrons in the O shell. However, about eight beyond electrons in these more outer shells play in the major groups - elements concerning the chemical properties virtually no role.
The number of bowls of an element corresponds to the number of the period in the periodic table of elements : elements of the 1st period have one bowl , those of the 2nd period two bowls, those of the 3rd period three bowls, etc.
Properties of the elements
With the shell model of the atoms, various properties of the elements can be explained well, since each atom “strives for” its noble gas configuration , eight electrons in the outer shell . Examples:
- Alkali metals only have a single external electron ( valence electron ) and can therefore give it off particularly easily; d. that is, its ionization energy is low. Therefore, alkali metals are particularly reactive .
- The halogens only lack one electron for a fully occupied outer shell, so they easily take electrons from other elements (e.g. from sodium ) and are therefore also very reactive.
- The noble gases already have a fully occupied outer shell and therefore show no “tendency” to enter into chemical reactions .
Names of the electron shells
1912 led Charles Glover Barkla , professor of physics at King's College of the University of London , the terms K and L for the inner electron shells one, because even the designations A, B, etc. to the absorption lines of sunlight had used. Because Barkla thought that many absorption lines would still be found, he started with the designation of the electron shells in the middle of the alphabet, with K.
Weaknesses of the atom shell model
There are phenomena that the atomic shell model cannot explain. Above all, this includes the spatial shape of the molecules . Why does z. B. Methane (CH 4 ) has a tetrahedral shape, or why is the water molecule angled? These properties of the molecules can be interpreted with the VSEPR model and / or via hybrid orbitals and explained with molecular orbitals .
- W. Finkelnburg: Introduction to atomic physics . Springer Verlag, Berlin Heidelberg 1976.
- Atombau - The shell model , explanation at the intermediate level