Problem 82 - Chemistry: The Central Science

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?

Step-by-Step Solution

Answer

The sizes of transition elements change more gradually across a period compared to the representative elements because the additional d-electrons filling their d orbitals are less effective at shielding the outermost electrons from the increasing nuclear charge. This results in a slower decrease in atomic size compared to the regular trend observed in representative elements, which are primarily filling their s and p orbitals.

Step 1: Identifying the transition and representative elements

The representative elements are the elements found in groups 1A to 7A in the periodic table, while the transition elements are the ones located in group 3 through group 12, the d-block. Representative elements exhibit regular trends in atomic size, ionization energy, and electron affinity, while transition elements have more complicated trends owing to their electron configurations.

Step 2: Details about atomic size

Atomic size is generally measured by the atomic radius, the distance from the nucleus to the outermost electron shell. It's important to note that as we move across a period, the atomic number (number of protons) increases, leading to a higher effective nuclear charge on the electrons. This increased charge causes the electrons to be more strongly attracted to the nucleus, reducing the atomic size.

Step 3: Comparing atomic size trends in representative elements

Representative elements follow a predictable trend in atomic size across a period. As the atomic number increases, the atomic size decreases. This decrease in atomic size is due to the increase in effective nuclear charge experienced by the outermost electrons. These elements adhere closely to this trend since they are filling their s and p orbitals across the period, and there is limited shielding effect from inner electron shells.

Step 4: Exploring atomic size trends in transition elements

In transition elements, the size changes more gradually as we move across a period. The transition metals fill their d orbitals, which are located between the outermost s and p electrons. The d-orbitals' ability to shield the outer electrons from the nucleus's increasing positive charge is weaker compared to the s and p orbitals. Therefore, the increase in effective nuclear charge is only partially offset by the increased shielding provided by the additional d-electrons. Consequently, the atomic size does not reduce as drastically as it does in the representative elements.

Step 5: Concluding the explanation

In summary, the sizes of transition elements change more gradually across a period compared to the representative elements because the additional d-electrons filling their d orbitals are less effective at shielding the outermost electrons from the increasing nuclear charge. This results in a slower decrease in atomic size compared to the regular trend observed in representative elements, which are primarily filling their s and p orbitals.