Question

How did Mendeleev predict the existence of some yet to be discovered elements in his periodic table?
A.Gaps
B.Atomic number
C.Mass number
D.None of the above

Answer 1

Hint: All the known elements of his time were arranged by Mendeleev in horizontal rows and vertical columns in the order of their increasing atomic weights. He placed elements with similar properties beneath each other. In order to achieve this, Mendeleev left some slots in his table and predicted that new elements would appear in these slots.

Complete step by step answer:
The Mendeleev’s periodic law states that the physical as well as the chemical properties of the elements are the periodic function of their atomic weights. When Mendeleev published his periodic table, only 63 elements were known at that time. He arranged these 63 known elements according to their similarities in chemical properties. He left some gaps for elements that were yet to be discovered.
For example, at that time, the element known to exist after zinc was arsenic. The properties of arsenic were similar to that of phosphorus and so he left two gaps in his table and placed arsenic in group V. This meant that there were two elements missing, one like aluminium and other like silicon.
Mendeleev predicted that after zinc, there are two elements yet to be discovered and the properties of these elements should be similar to aluminium of group III and silicon of group IV respectively. These missing elements were named as eka-aluminium and eka-silicon. Later on, these missing elements were discovered and were named as gallium and germanium respectively. The properties of these elements were found to be similar to those predicted by Mendeleev.

Hence, the correct option is A.

Note:
Mendeleev’s periodic table also has some disadvantages as mentioned below:
There was no attempt to separate the metals from the non-metals in his table.
Isotopes of the same element should be given different positions in the table on the basis of atomic weights. But, isotopes of an element possess similar chemical properties which mean that they should be placed at the same place.
Some elements like copper and mercury which have similar chemical properties were placed in different groups and some elements with different chemical properties were grouped together.