Chapter 6.3, Problem 20SSC

Interpretation Introduction

Interpretation:

Association of trend of atomic radii in period and group with electronic configuration is to be explained.

Concept introduction:

Electronic configuration can be assigned to any elements in ground when they follow certain rules like Hund rule, Pauli Exclusion Principle and Aufbau rule. If atomic number of an element is $Z$ then $Z$ numbers of electrons are filled into the orbitals which are arranged in increasing order of energy.

No two electrons in an atom can have same group of four quantum numbers and this is Pauli Exclusion Principle.

While filling of orbital’s, electron first enters to each energy level with degenerate energy before paring of electron begins and this is Hund’s rules.

Answer to Problem 20SSC

Atomic size increases down the group as electron enter into higher principal energy level and the trend of atomic size decreases across period as number of electron adds up into same principal energy level with rise in nuclear charge.

Explanation of Solution

The atomic size decreases across period from left to right and increases down the group. Atomic size is radius of an atom that is the distance from nucleus till valence shell. The above trend across period is due the increase in nuclear charge over the valence shell electron. Higher the atomic number higher the nuclear charge. The trend seen down the group is due to increase in distance from nucleus and lesser nuclear pull experienced by valence shell due to increase in number of intervening shell or electrons.

Conclusion

Atomic size increases down the group as electron enter into higher principal energy level and the trend of atomic size decreases across period as number of electron adds up into same principal energy level with rise in nuclear charge.