Why does the first ionization energy generally increase across a period for the main group elements?
A. The charge of the nucleus decreases across a row and so there is an increase in the attraction between the nucleus and the surrounding electrons.
B. The charge of the nucleus increases across a row and so there is an increase in the attraction between the nucleus and the surrounding electrons.
C. There is an increase in effective nuclear charge and an increase in shielding across a row, so there is a decrease in the attraction between the nucleus and the surrounding electrons.
D. There is a decrease in effective nuclear charge and a decrease in shielding across a row, so there is a decrease in the attraction between the nucleus and the surrounding electrons.
The Correct Answer is (B) The charge of the nucleus increases across a row and so there is an increase in the attraction between the nucleus and the surrounding electrons.
As we move across a period from left to right, the nuclear charge increases and the atomic radius decreases though the principal quantum number of the valence shell remains same. As a result of increased nuclear charge and simulataneous decrease in atomic radii, the valence electrons are more and more tightly held by the nucleus as we move from left to right in a period. Consequently more and more energy is needed to remove an electron and hence, ionization energy keeps on increasing.