Question

As we move across a period of the periodic table, why do the sizes of the transition elements change more gradually than those of the representative elements?

Short answer

The more gradual change in atomic size for transition elements across a period compared to representative elements is mainly due to greater electron shielding experienced by transition elements. This occurs because transition elements have their outer electrons in more diffuse d orbitals, which partially neutralize the increased nuclear charge, allowing for a more gradual change in atomic size as we move across a period.

Step-by-step solution

Understanding Atomic Size

Atomic size refers to the distance between the nucleus and the outermost electrons of an atom. As we move across a period in the periodic table, the atomic size typically decreases. This decrease is due to an increase in the nuclear charge (number of protons), which pulls the electrons closer to the nucleus.

Differences between Representative and Transition Elements

Representative elements, also called main group elements, belong to groups 1, 2, and 13 to 18 of the periodic table. In contrast, transition elements are found in groups 3 to 12. The key difference between these two types of elements lies in their electron configurations. Representative elements gain or lose electrons to achieve a stable, full outer electron shell, while transition elements tend to fill their inner d subshells.

Electron Shielding in Transition Elements

As we move across a period, the electrons occupy the same energy level but in different orbitals for transition elements. This means that these electrons are in a d subshell apart from the outermost s orbital, while the representative elements' outermost electrons occupy the s or p orbitals in the same energy level. Since the d subshell has more diffused and less tightly held electrons than the s and p orbitals, transition elements experience greater electron shielding. This shielding effect partially neutralizes the increase in nuclear charge, as it keeps the outer electrons from being pulled in as tightly.

Gradual Change in Atomic Size for Transition Elements

Due to the greater electron shielding experienced by transition elements, their atomic size does not shrink as rapidly as representative elements as we move across a period. The more diffuse d orbitals mitigate the increased nuclear charge's effect, which causes a more gradual change in atomic size among transition elements. In summary, the sizes of transition elements change more gradually than those of the representative elements across a period because of the greater electron shielding experienced by transition elements due to their d subshell electron configurations.