Chemistry Reference
In-Depth Information
Crystal structure symbols
Molecular symbols
OH
{Ca
2+
[8c + 12c]
F
-
2
[4t + 6o]}
{Ca
2+
F
-
2
8/4 }G
H
PARTHÉ symbol and
NIGGLI symbol
for the ionic structure
Stereo symbol for
the molecular
structure
C
C
HO
H
H
H
{(Ca
2+
)
4
(F
-
)
8
}
HH
HCC
Symbol for the
unit cell
Constitutional
symbol
H
OH
OH
{(Ca
2+
)
1
(F
-
)
2
}
HO
CH
2
CH
2
OH
Half structural
symbol
Symbol for the
ion ratio
C
2
H
4
(OH)
2
Ca
2+
(F
-
)
2
CaF
2
CH
2
OH
CH
3
O
Empirical formula
Empirical formula
Fig. 7.1 Different types of chemical symbols (examples: calcium fluoride and ethane diol)
both particles of matter. But as soon as the term “particle” is used for salts or salt
solutions, one gets to the limits of this model. According to the convention, “one ball
in the particle model of matter is used for one particle of a pure substance,” there are
problems with salts: in the case of sodium chloride, one ball would have to be used for
one ion pair Na
+
Cl
or even for the unit cell {(Na
+
)
4
(Cl
)
4
}. Because salts and salt
solutions are built of at least two kinds of ions, any correct model should use two
different balls. Due to this problem, the term “particle” in the sense of the particle
model of matter is normally not used for solid salt crystals or for salt solutions.
The second problem is the observation of powder mixtures, suspensions or
aerosols. Students often describe a mixture of iron powder and sulfur powder
with “iron particles and sulfur particles.” They use the term “particles” for visible
crystals, although they know that one crystal consists of billions of particles. If one
attaches importance to the consequent use of the term “smallest particle” for the
submicroscopic level of atoms, ions, and molecules, small amounts of substances
should be called “crystal, grain, droplet” or similar. Properties should only be
assigned to those terms, not to particles: the sulfur particle is not yellow.
Valence.
We still use the term “valence” in a way that was historically helpful
for hypotheses of the composition of substances. In the nineteenth century, chemists
believed that all compounds are built of molecules. “Aluminium is trivalent” was
understood as “Al atoms have three valences according to Kekul´'s valance theory
from 1865.” Corresponding to these ideas the symbols AlF
3
or Al
2
O
3
were written
like for molecular compounds - in 1865, the ions were unknown.
