|
3.
Lothar Meyer’s atomic volume
curves
A
German chemist Julius Lothar Meyer in 1869 was looking at physical
properties of elements along with their valence states. He made a table that
contained a preliminary tabulation of 28 elements. The table showed how the
integral valence changed as the atomic weight of elements increased.
Meyer
considered the volume taken up by fixed weights of the various elements.
Under such conditions, each weight contained the same number of atoms of its
particular element (Avogadro’s
number). This meant that the ratio of the volumes of
|
 |
|
Lothar
Meyer |
| the
various elements was equal to the ratio of the volumes of single
atoms of the various elements. Thus Lothar Meyer could determine the
atomic volumes of elements. |
If
the atomic volumes of the elements were plotted against the atomic weight, a
series of peaks were produced. The peaks had alkali metals: sodium,
potassium, rubidium, and cesium. Each fall and rise to a peak, corresponded
to a period like the waves. In each period a number of physical
properties other than atomic volume also fell and rose, such as valence and
melting point. Figure below shows the curve obtained by Lothar Meyer when he
plotted the atomic masses and the respective atomic volumes.
Hydrogen,
the first in the list of elements is a special case and can be considered as
making up the first period all by itself. The second and third period in
Meyer's table included seven elements each, and duplicated Newlands's law of
octaves.
Li Be
B
C
N
O
F
Na Mg
Al
Si
P
S Cl
However,
the next wave had more than seven elements. The third wave had about 17 to
18 elements. This clearly showed where Newlands’ law had failed. One could
not force the law of octaves to hold strictly throughout the table of
elements, with seven elements in each row. After the first two periods, the
length of the period had to be longer.
If
we see the atomic mass versus atomic volume curve, we will notice the
following features :
-
Alkali
metals such as Na. K, Rb that have similar properties, occur as peaks of
the curve.
-
Halogen
elements like F, Cl, Br, that have similar properties, occur at the
rising or the ascending part of the curve.
-
Noble
gasses such as Ne, Ar, Kr , that have similar properties, occur just
before the alkali elements.
-
H, He seem to be exception to the rule.
Meyer
published his work in 1870, but before that Dimitrii Medeleev has shown a
neat version of how elements can be arranged in periods and the periods
increased with atomic masses. Mendeleev retained Mayer’s observation that
H is an exception to the periodic behaviour.
|
