How do the following properties change in group - 1 and in the third period? Explain with example.
(b) IE

Explanation

Ionisation Energy: As we move from top to bottom of group-1 elements the atomic radius increases gradually. So the distance from the nucleus to the outer electrons increases. This results in the decreases in nuclear attraction on outer electrons. Hence ionisation energy decreases from Li to Cs as we move down the group.
While moving from left to right along 3rd period atomi radius decreases and effective nuclear charge increases gradually. So ionisation energy increases in the 3rd period form Na to Cl.