Question

By giving suitable examples show that all the elements of a group show similar properties, while the properties of the elements in a period show a gradual change.

Answer 1

In the Modern or long form of the Periodic Table, the elements are arranged in the increasing order of their atomic numbers.

Similarities down the group:

1. In a group, the elements show similar properties due to the same number of electrons in their valence shells.
2. As the valence electrons are the ones involved in the electronic transfer, they are responsible for conferring the chemical properties to the elements.
3. For example, the Halogens of group $17$ readily form halides with hydrogen, due to their high electronegativities.

Variations across a period:

1. Across the period, the effective nuclear charge increases due to the increase in atomic number from left to right.
2. The number of valence electrons also increases on moving from left to right across the period.
3. Hence there is a gradual change in properties across a period.
4. For example, the electrical conductivity in the third period increases from Na to Al, due the increase in valence electrons from 1 to 3. But the electrical conductivity decreases from Al to Ar gradually due to Al being a metal and the rest of the elements being nonmetallic.