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Johann Dobereiner in early 1800’s was the first to consider the idea of trends among properties of

elements.John Alexander Newlands in 1865 profounded the Law of Octaves. He arranged the

elements in increasing order of their atomic weights and noted that every eighth element had

properties similar to the first element The relationship was just like every eighth note that resembles

the first in octaves of music. Newlands’s Law of Octaves seemed to be true only for elements up to

calcium.The Periodic Law, as we know it today owes its development to the Russian chemist,

Dmitri Mendeleev (1834-1907) and the German chemist, Lothar Meyer (1830-1895). Working

independently, both the chemists in 1869 proposed that on arranging elements in the increasing

order of their atomic weights, similarities appear in physical and chemical properties at regular

intervals. Lothar Meyer plotted the physical properties such as atomic volume, melting point and

boiling point against atomic weight and obtained a periodically repeated pattern. Unlike Newlands,

Lothar Meyer observed a change in length of that repeating pattern By 1868, he had developed a

table of the elements that closely resembles the Modern Periodic Table. However, his work was not

published until after the work of Dmitri Mendeleev, the scientist who is generally credited with the

development of the Modern Periodic Table.

Accordin to mandeleev the periodic law states like this

The properties of the elements are a periodic function of their atomic weights.

He fully recognized the significance of periodicity and used broader range of physical

and chemical properties to classify the elements. In particular, Mendeleev relied on the similarities

in the empirical formulas and properties of the compounds formed by the elements. He realized that

some of the elements did not fit in with his scheme of classification if the order of atomic weight

was strictly followed. He ignored the order of atomic weights, thinking that the atomic

measurements might be incorrect, and placed the elements with similar properties together. For

example, iodine with lower atomic weight than that of tellurium (Group VI) was placed

in Group VII along with fluorine, chlorine, bromine because of similarities in properties At the

same time, keeping his primary aim of arranging the elements of similar properties in the same

group, he proposed that some of the elements were still undiscovered and, therefore, left several

gaps in the table. For example, both gallium and germanium were unknown at the time Mendeleev

published his Periodic Table. He left the gap under aluminium and a gap under silicon, and called

these elements Eka- Aluminium and Eka-Silicon. Mendeleev predicted not only the existence of

gallium and germanium, but also described some of their general physical properties. These

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elements were discovered later. Some of the properties predicted by Mendeleev for these elements

and those found experimentally are listed in Table 3.3. his Periodic Table published in 1905

is shown in Fig.

Mandeleevs periodic table

3.3 MODERN PERIODIC LAW AND THE PRESENT FORM OF THE PERIODIC TABLE

In 1913, the

English physicist, Henry Moseley observed regularities in the characteristic X-ray

spectra of the elements. A plot of root of (where is frequency of X-rays emitted) ν ν

against atomic number (Z ) gave a straight line and not the plot of vs atomic mass. ν

He thereby showed that the atomic number is a more fundamental property of an

element than its atomic mass.Mendeleev’s Periodic Law was, therefore, accordingly

modified. This is known as the Modern Periodic Law and can be stated as :

“The physical and chemical properties of the elements are periodic functions of their

atomic numbers.”

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The Periodic Law revealed important analogies among the 94 naturally occurring

elements (neptunium and plutonium like actinium and protoactinium are also found

in pitch blende – an ore of uranium).

In the modern periodic table Elements having similar outer electronic configurations in their

atoms are arranged in vertical columns, referred to as groups or families. According to the

recommendation of International Union of Pure and Applied Chemistry (IUPAC), the groups are

numbered from 1 to 18 replacing the older notation of groups IA ... VIIA, VIII, IB ... VIIB and 0.

There are altogether seven periods. The period number corresponds to the highest principal

quantum number (n) of the elements in the period. The first period contains 2

elements. The

subsequent periods consists of 8, 8, 18, 18 and 32

elements, respectively. The seventh period is

incomplete and like the sixth period would have a theoretical maximum (on the basis of quantum

numbers) of 32

elements. In this form of the Periodic Table, 14 elements of both sixth and seventh

periods (lanthanoids and actinoids, respectively) are placed in separate panels at the bottom.

3.4 NOMENCLATURE OF ELEMENTS WITH ATOMIC NUMBERS > 100