Chapter 11, Problem 55A

Interpretation Introduction

Interpretation: Reason due to which size of the atoms of the elements in a periodic table decreases from left to right in a period is to be given.

Concept introduction: A period is a horizontal row consisting of elements in the increasing order of their atomic number where atomic number is defined as the number of electrons or protons present in an atom. Effective nuclear charge is defined as the net nuclear charge that holds the electrons of an atom. Shielding effect in an atom arises because the electrons closer to nucleus shield the outermost electrons from the nuclear pull of the nucleus.

Answer to Problem 55A

It happens because effective nuclear charge increases as we go from left to right in a period.

Explanation of Solution

Shielding effect increases as the number of shells increase in an atom. When going from left to right in a period, no new shell gets added up due to which neither the size nor the shielding effect of electrons increases. However an electron get added up at each step in the same type of orbital and also the number of protons in the nucleus increases due to which the effective nuclear charge increases that pulls the electrons towards itself thereby decreasing the size of atoms.

Conclusion

The size of atoms decreases as we move from left to right in a period because effective nuclear charge increases as we go from left to right in a period.