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 :

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

The modern periodic table was prepared by :

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.

In the following set of elements, one element does not belong to the set. Select this element and explain why it does not belong :

Calcium, Magnesium, Sodium, Beryllium