Answer:
The effective nucleus charge increases on going from left to right.
Explanation:
There are two factors that determine the atomic size of the atom.
1. Shielding constant :It is the shielding of the outershell electron from the nucleus by the inner shell electron. Thses are repulsive forces.If the value of shielding contant is more , then the size of the element increases.
2.The Effective Nuclear Charge : The net attraction of the nucleus felt by the electron present in the outermost shell . It is represented by Zeff.
This is attractive force.
If the Zeff is more then the nucleus of the element will pull the electron cloud more strongly.This leads to decrease in the size of the element.
On going left to right ,each time one electron is added to the element .This electron attracts the nucleus more strongly(the nucleus is Positive charge and the electron is negative charge, opposite charge attracts each other). Hence , the size decreases.
Thus, in period two , there is decrease in the atomic radius with increasing atomic number because the effective nuclear charge increases on going from left to right.
