Write the number of periods the modern periodic table has.

There are 7 periods in the modern periodic table. Metallic character tends to decrease on going across a period whereas the number of valence electrons increases in a period and valency first increases from 1 to 4 and then decreases from 4 to 0. Atomic radius increases from top to bottom within a group. This is caused by the addition of a new shell. In a group, the number of valence electrons does not change. Hence, all the elements of one group have the same valency.