Chemistry Reference
In-Depth Information
remain ions. Redox reactions of metals with nonmetals or electrolyses of salt
solutions cannot be described with this model, because atoms transform into ions
or vice versa. These changes can only be explained after the introduction of the
nucleus-shell model, including the knowledge about electrons in subshells.
Symbols on the level of the nucleus-shell model of the atom.
As long as the main
emphasis is on chemical structures involving atoms, ions and molecules, they can
be illustrated by concrete models on the level of Dalton's model - a sophisticated
atomic model is not necessary. Once we are not only dealing with spatially directed
and undirected bonds, but also with differentiated chemical bonding, Rutherford's
atomic model including the shell model or the energy level model cannot be
avoided. On the one hand terms like nucleus, proton, neutron and isotope have to
be introduced, on the other hand also the idea of electrons, electron clouds, electron
pair and the electron pair repulsion. The discussion of chemical bonds leads to the
concept of ionic and covalent bond, may be connected to the term electronegativity.
Also polarized bonds, hydrogen bonds or van-der-Waals bonds can be illustrated,
and dipolar molecules should be introduced.
On the basis of the nucleus-shell model it is possible to describe structure and
bonding of simple chemical structures derived from the first three periods of the
periodic table, as well as chemical reactions in which particle changes can be
explained:
Acid-base reactions
!
transition of protons
Redox reactions
!
transition of electrons
Complex reactions
!
transition of ligands
Terms on the level of the atomic orbital model.
Simple nucleus-shell models are
sufficient as long as the structure of substances with elements from the first three
periods is being discussed: successive insertion of electrons from shell to shell can
be explained until the element calcium. The differentiation of the main shells into
subshells becomes necessary as soon as the build-up of atoms from the subgroups or
transitional metals should be explained: s-, p-, d- and f-levels and the distribution of
electrons to these subshells.
The theory of wave mechanics needs to be discussed by the wave-particle
duality of electrons and by understanding electrons as standing waves. In this
sense a certain energy state can be assigned to a special standing wave and
described with quantum numbers: principal quantum number
n
, subsidiary quantum
number
l
, magnetic quantum number
m
, spin quantum number
s
.
Starting from the quantum number special energy states can be calculated with
the help of wave functions. These calculations lead to specific probability densities
or probabilities of finding electrons:
s
-,
p
-,
d
-, and
f
-electron clouds or orbitals,
atomic and molecular orbitals as well as bonding or antibonding orbitals need to be
differentiated. It needs to be pointed out that these orbitals are only obtained by
mathematical combinations of wave functions and compared to measurements of
the laboratory: densities of single electron clouds cannot be measured; only the
overall electron density of a molecule or of an atom. On the other hand structure
and bonding of electron systems can be predicted by mathematical combinations
