Term scheme
A term scheme , also level scheme or Grotrian diagram (after its inventor Walter Grotrian ), is the compilation of all energy levels (terms) of an atom , ion , molecule or atomic nucleus in a clear graphic representation.
construction
The main and secondary shells of the system are plotted as energetic platforms on the ordinate (Y-axis) and the number of electrons on the abscissa (X-axis) . The main shells are counted starting at 1 from the core (alternatively, the shells can also be designated with letters). The highest shell of an energy level scheme corresponds to the period of the respective element . Each atom has its own electron configuration . The further a shell is from the nucleus, the higher the energy level of an electron that is on it.
The term scheme can only be represented for a very simple system with only one electron by a single "rung ladder". With more complicated systems, several ladders have to be placed in or next to each other.
regulate
The energies of the individual terms are scaled as horizontal lines so that the line representing the energetically most favorable or basic state is furthest down. When occupying the orbitals , Hund's rule must be followed, according to which several orbitals with the same energy must first be occupied with electrons of the same spin before electrons with opposite spins are distributed.
Atoms whose last occupied electron is on the P level and which have a fully occupied S and P shell belong to the class of noble gases . Atoms that have their outer electrons at the 4f level belong to the group of lanthanoids , whereas at 5f they belong to the actinides .
Transitions
In the case of a transition from a higher to a lower term, the distance between the starting and the ending term corresponds to the energy of the emitted photon ; this process therefore generates a corresponding spectral line . However, not all transitions are allowed, some are forbidden for quantum mechanical reasons and are therefore not observed in the spectrum (see selection rules ).
See also
Web links
- Energy levels in the atom explained at student level ( LEIFI )
- Energy level scheme of the elements - automatic display of any scheme after entering the ordinal number