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 (Fig. . ).
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 elements were discovered later. Some of the properties predicted by Mendeleev for these elements and those found experimentally are listed in Table . . The boldness of Mendeleev’s quantitative predictions and their eventual success made him and his Periodic Table famous.
Mendeleev’s Periodic Table published in is shown in Fig. . . Table .
Mendeleev’s Predictions for the Elements Eka-aluminium (Gallium) and Eka-silicon (Germanium) Property Eka-aluminium (predicted) Gallium (found) Eka-silicon (predicted) Germanium (found) Atomic weight . Density/(g/cm ) . . .
. Melting point/K Low . High Formula of oxide E O Ga O EO GeO Formula of chloride E Cl GaCl ECl GeCl PERIODIC SYSTEM OF THE ELEMENTS IN GROUPS AND SERIES Fig. .
Mendeleev’s Periodic Table published earlier . MODERN PERIODIC LAW AND THE PRESENT FORM OF THE PERIODIC TABLE We must bear in mind that when Mendeleev developed his Periodic Table , chemists knew nothing about the internal structure of atom. However, the beginning of the th century witnessed profound developments in theories about sub-atomic particles. In , the English physicist, Henry Moseley observed regularities in the characteristic X-ray spectra of the elements.
A plot of (where is frequency of X -rays emitted) against atomic number ( Z ) gave a straight line and not the plot