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Classification of Elements - Part IV |
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The periodic table thus gave a chart of elements
grouped in such a manner that elements showing similar properties occur in
the same vertical group. Mendeleev’s
periodic law states that the properties of elements are a periodic function
of their atomic mases. Figure below shows the chart of elements initially made by Mendeleev. The elements are arranged in such a manner that the vertical columns are called groups and horizontal columns are called periods.
Elements in each group have similar physical and chemical properties (valence, melting point). The periods are made with elements written in rows of increasing atomic masses. As one goes vertically downwards in a group, the elements show increase in atomic volume. The first two periods are similar to Newlands law of octaves. The feature within each group thus explained Lothar Meyer’s observations also. Mendeleev
did something quite ingenious. He placed elements that had similar
properties under vertical columns, even if other elements were not found.
For example he placed titanium (Ti) under silicon (Si) as he saw that Ti and
Si had similar properties. Thus there was a gap below aluminum (Al) in the
group and after calcium (Ca) in the horizontal period. As if this were not
enough, he also found it necessary to leave gaps altogether in his table.
Rather than considering these gaps as imperfections in the table, Mendeleev
said that they represented elements as yet undiscovered. In
1871, he pointed to three gaps in particular, those falling next to the
elements boron, aluminum, and silicon in the table. He named the unknown
elements as eka-aluminum, and eka-silicon ("eka" is the Sanskrit word
for "one"). He also predicted various properties of these missing
elements, such as density, boiling point, judging from what these must be
from the properties of the elements above and below the gaps in his table.
Much later on, Gallium (Ga) and Germanium (Ge) were found, which had
same properties as eka-aluminum, and eka-silicon respectively. This demonstrated the
success of Mendeleev's periodic table of elements. Although
very successful, Mendeleev’s periodic table had the following problems : 1. The positions of isotopes could not be accommodated within the table. As you well know, isotopes are elements having same properties but different atomic masses (same proton number Z, but different neutron number N. Thus atomic mass A = Z + N differs in isotopes). If one obeys Mendeleev’s periodic law then the variation in chemical properties vertically in a group is not followed strictly. For example, an isotope of carbon is 14C. This would have to be accommodated along with nitrogen (Fig 1). But 14C shows properties same as normal carbon (12C). Mendeleev was thus unable to place isotopes in his table. 2. In order to make the elements fit the requirements, that those in a particular column all have the same valence, Mendeleev was forced in one or two cases to put an element of slightly higher atomic weight ahead of one of slightly lower atomic weight. Thus, tellurium (Te) (atomic weight 127.6, valence 2) had to be put ahead of iodine (I) (atomic weight 126.9, valence 1) in order to keep tellurium in the valence-2 column and iodine in the valence-1 column. Success
of Mendeleev’s periodic table : 1.
When Mendeleev presented his periodic table,
inert gas elements like He, Ne, Ar were not discovered. When they
were discovered, they could be neatly put in as the last group of elements,
without disturbing the rest of the table. 2.
His predictions about unknown elements from gaps in his table were a
great success. Scientists who repeatedly discover newer and newer elements
follow this feature. Although the periodic table is now complete with about
109 different elements (naturally occurring and artificially made in the
lab), scientists still use Mendeleev’s method to predict properties of
elements by looking vertically across groups and horizontally across periods
of elements. |
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