The modern periodic table was prepared by :

Which of the following statement about the modern periodic table is correct?

An element X forms an oxide X₂O₃. In which group of Mendeleev’s periodic table is this element placed ?

The atomic particle whose number in the atoms of an element always remains the same and which forms the real basis for the modern classification of elements is :

The atomic masses of three elements X, Y and Z having similar chemical properties are 7, 23 and 39 respectively.

(a) Calculate the average atomic mass of elements X and Z.

(b) How does the average atomic mass of elements X an Z compare with the atomic mass of element Y ?

(c) Which law of classification of elements is illustrated by this example ?

(d) What could the elements X, Y and Z be ?

(e) Give another example of a set of element which can be classified according to this law.