Mendeleev periodic table appeared in his work “On the Relationship of the Properties of the Elements to
their Atomic Weights” in 1869. Mendeleev placed many elements out of order based on their accepted atomic weights at the time.
Mendeleev predicted the existence and properties of unknown elements which he called eka–aluminum, eka–boron, and
eka–silicon. The elements gallium, scandium and germanium were found later to fit his predictions quite well.
Working on the research done by Newlands, in 1869 a 35 year old Russian chemist named Dmitri Mendeleev presented a much bolder and more scientifically useful table of elements. In it, the periodic relationship between chemical groups is clearly illustrated. Mendeleev saw that there is a periodicity occurring in the physical and chemical properties, if the elements were arranged in order of their atomic weights. The periodic table gave order to the large amount of data available for all the elements. In Mendeleev's time there were about 60 to 70 known elements.
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 the periodic functions of their atomic masses. Elements in each group have similar physical and chemical properties (valency, 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.
Original table published by Mendeleev in 1869. Picture 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.
Although very successful, Mendeleev's periodic table had the following problems:
- The positions of isotopes could not be accommodated within the table. As you know well, 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. But 14C shows properties same as normal carbon (12C). Mendeleev was thus unable to place isotopes in his table.
- In order to make the elements fit the requirements, that those in a particular column all have the same valency, 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, valency 2) had to be put ahead of iodine (I) (atomic weight 126.9, valence 1) in order to keep tellurium in the valency – 2 column and iodine in the valency – 1 column.
Mendeleev foresight of leaving gaps made his table widely acceptable. Prediction of elements were proved core to be
correct in later findings.
Success of Mendeleev's periodic table:
- 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.
- 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 116 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.