Assertion (A) : The first ionization enthalpy of alkali metals decreases down the group. Reason (R) : Increase in number of orbitals increases the shielding effect which outweighs the increasing nuclear charge, therefore, the removal of outermost electron requires less energy on moving down the group. A. A and R both are correct but R is not the explanation of A. B. A is false but R is correct. C. A and R both are correct and R is the correct explanation of A. D. A and R both are incorrect.

Question

Philoid · Accepted Answer

C. A and R both are correct and R is the correct explanation of A.

The first ionization enthalpies of the alkali metals are considerably low and further decrease down the group.

This is because the effect of increasing size due to increase in no. of orbitals outweighs the increasing nuclear charge, and the outermost electron is very well screened from the nuclear charge. Thus, it becomes easy to remove an electron from outermost orbital while moving down the group and this is the reason of decreasing ionisation enthalpy.