Question

What is screening effect ? How does it govern the ionization enthalpy of an atom?

Answer 1

Hint: The screening effect is the phenomenon of shielding of valence electrons from the nuclear force of attraction by the inner shell electrons. Higher is the nuclear force of attractions, greater is the ionization enthalpy required. Thus, the screening effect is inversely proportional to ionization enthalpy.

Complete answer:
The Screening effect is also called the shielding effect.
We know that the nuclear charge attracts the valence electrons and binds them in an atom. The screening effect or shielding effect is the phenomenon of reduction of nuclear force of attraction due to inner shell electrons towards valence electrons.
The inner shell electrons protect the valence shell electrons from the nuclear force i.e. they shield them.
Now, the next thing is how does it govern the ionization enthalpy of an atom.
First, we will understand what ionization enthalpy is.
Ionization enthalpy is the energy required to remove an electron from the free gaseous atom. The ionization energy is directly proportional to the nuclear force because if a nuclear has higher force of attraction towards its valence electrons. So, it will bind electrons very well and thus, it will be difficult to remove them. So, higher will be the energy required for removal.
So, if nuclear force is decreased by screening effect, then the ionization energy required will be low. So, the screening effect will lead to decrease of ionization energy.

Note:
It must be noted that the s-shell is close to the nucleus. Its electrons experience most force from the nucleus. Then, comes electrons in the p-shell. The s-shell electrons shield or screen the p-shell electrons. Further, the s and p both shell electrons screen the electrons of d-shell.