Why did Mendeleev leave some gaps in his periodic table of elements? Explain your answer with an example.

The atomic number of an element is more important to the chemist than its relative atomic mass. Why?

(a) Name an element whose properties were predicted on the basis of its position in Mendeleev's periodic table.

(b) Name two elements whose atomic weights were corrected on the basis of their positions in Mendeleev's periodic table.

(c) How many elements were known at the time of Mendeleev's classification of elements?

(a) State the modern periodic law.

(b) How many periods and groups are there in the modern periodic table?