Explain why atomic radius decreases as we move to the right across a period for main-group elements but not for transition elements.
The Correct Answer and Explanation is:
Correct Answer:
Atomic radius decreases across a period for main-group elements because more protons are added to the nucleus, increasing the nuclear charge, which pulls electrons closer to the nucleus. In transition elements, however, electrons are added to inner d orbitals, which provide shielding that counteracts the increasing nuclear charge. As a result, the atomic radius remains relatively constant across transition elements.
300-Word Explanation:
As we move from left to right across a period in the periodic table, the atomic radius of main-group elements decreases. This trend occurs because each successive element has one more proton in its nucleus and one more electron in the same energy level. The added protons increase the nuclear charge, which strengthens the attraction between the nucleus and the electrons. Since the electrons are added to the same outer shell and no new energy levels are introduced, there is minimal increase in shielding. Therefore, the stronger pull draws the electrons closer to the nucleus, resulting in a smaller atomic radius.
In contrast, the trend is different for transition elements, which occupy the d-block of the periodic table. As we move across a period in the transition series, electrons are added to the inner d orbitals, not to the outermost shell. While the number of protons still increases, the additional electrons in the d orbitals do not effectively shield the outer electrons from the nuclear charge. However, the d electrons themselves add a small shielding effect and are located closer to the nucleus than the outermost s electrons. This causes the increase in nuclear attraction to be somewhat balanced by the slight increase in electron repulsion and shielding. As a result, the contraction in atomic size is much less significant for transition elements compared to main-group elements.
In summary, main-group elements experience a strong decrease in atomic radius across a period due to increased nuclear charge with minimal shielding. Transition elements, on the other hand, show a more stable atomic radius across a period due to the electron configuration involving inner d orbitals.
