Problem 88 - Chemistry 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 than those of representative elements across a period because the electron shielding by the inner d and f electrons is less effective than the shielding by s and p electrons. This results in a small increase in effective nuclear charge, leading to a more gradual decrease in atomic size for transition elements. In contrast, representative elements experience a significant increase in effective nuclear charge due to constant shielding effect and an increase in the number of protons in the nucleus, resulting in a more noticeable decrease in their atomic size.

Step 1: Understanding Atomic Size

Atomic size generally refers to the distance between an atom's nucleus and its outermost electrons. Moving across a period in the periodic table, atomic size usually decreases. This is because of the increase in effective nuclear charge (the net positive charge experienced by the outermost electrons) due to the addition of protons in the nucleus.

Step 2: Factors Affecting Atomic Size

There are two main factors affecting the atomic size of an element: 1. Effective nuclear charge (Z_eff): As the number of protons in the nucleus increases, the attractive force between the nucleus and the electron increases, pulling the electrons closer to the nucleus and decreasing the atomic size. 2. Electron shielding: Inner shell electrons repel outer shell electrons, reducing the effective nuclear charge experienced by the outer shell electrons. This leads to a decrease in the force of attraction between the nucleus and the outer electrons and increases the atomic size.

Step 3: Electron Configurations of Transition Elements

Transition elements are located in the d-block of the periodic table and have partially filled d-subshells in their electron configuration. The electron shielding by the inner f and d electrons is less effective than the shielding by the s and p electrons. As a result, the effective nuclear charge experienced by the outermost electrons increases marginally, leading to a small decrease in atomic size.

Step 4: Electron Configurations of Representative Elements

Representative elements are located in the s and p blocks of the periodic table. When moving across a period, electrons are added to the same energy level. The shielding effect remains almost constant while the effective nuclear charge increases considerably due to the increase in the number of protons in the nucleus. This results in a significant decrease in atomic size.

Step 5: Conclusion

The sizes of transition elements change more gradually than those of representative elements when moving across a period of the periodic table because the electron shielding by the inner d and f electrons is less effective than the shielding by s and p electrons. This leads to a small increase in effective nuclear charge, resulting in a more gradual decrease in atomic size for transition elements. On the other hand, representative elements experience a significant increase in effective nuclear charge due to constant shielding effect and increase in the number of protons in the nucleus, leading to a more noticeable decrease in their atomic size.