The values of ionzation enthalpies tend to increase within each period, expect for small drops in ionizarion enthalpy at group_____and______elements

Options

A. `14, 17`
B. `13, 16`
C. `16, 18`
D. `13, 15`

Answer

B

Explanation

The first ionization enthalpies for group `13` elements `(B, A1, Ga)` are exceptions to the general horizontal trends. They are lower than those of group `2` elements `(Be, Mg, Ca)` in the same period because an `ns` electron is on an average closer to the nucleus than and `np` electron, it is held more tightly, and is harder to remove. Altenernatively, the shielded by the inner electrons and the `ns^(2)` electrons. Therefore, less enegry is needed to remove a single `p` electron (which is at a higher enegry, i.e., less stable) than to remiove a paired `s` electron from the same principle enegry level.
The general left-to-right increase in `Delta_(i)H_(1)` is interrupted by a dip after group `15` elements `(N, P,As)` at group `16` elements `(O, S, Se)`. In group `15` elements `(ns^(2)np^(3))`, the `p` electrons are in three separate orbitals (according to Hund's rule). In paired with one of the three `p` electrons in the same orbital results in greater electrostatic repulsion (than it would in an orbital by itself), which makes it easier to ionize an atom of group `16` element, have though nuclear charge has increased by one unit.