Problem 11 - Chemistry: The Central Science in SI Units, Global Edition

Question

The lanthanide contraction explains which of the following periodic trends? (a) The atomic radii of the transition metals first decrease and then increase when moving horizontally across each period. (b) When forming ions the period 4 transition metals lose their \(4 s\) electrons before their \(3 d\) electrons. (c) The radii of the period 5 transition metals (Y-Cd) are very similar to the radii of the period 6 transition metals (Lu-Hg).

Step-by-Step Solution

Answer

The lanthanide contraction explains the periodic trend given in option (c), where the radii of the period 5 transition metals (Y-Cd) are very similar to the radii of the period 6 transition metals (Lu-Hg).

Step 1: Option (a) Analysis

Here, we are given a trend regarding the atomic radii of transition metals decreasing and then increasing when moving across each period. The lanthanide contraction specifically affects elements in the lanthanide series and, as a result, neighboring elements in the d-block. This trend doesn't appear to directly involve the lanthanide contraction.

Step 2: Option (b) Analysis

This option states that when forming ions, period 4 transition metals lose their 4s electrons before their 3d electrons. This trend is related to electron configurations and ionization energies, not the lanthanide contraction. Thus, this trend cannot be explained by the lanthanide contraction.

Step 3: Option (c) Analysis

This option presents a trend where the radii of period 5 transition metals (Y-Cd) are very similar to the radii of the period 6 transition metals (Lu-Hg). We know that the lanthanide contraction causes a decrease in atomic radius across the lanthanide series. Consequently, the elements immediately following the lanthanides (in period 6) are expected to have a smaller atomic radius than one would expect without the lanthanide contraction. This trend involves the decrease in atomic radii across periods due to the lanthanide contraction and seems to be directly related.

Step 4: Conclusion

Based on the analysis, we can conclude that the lanthanide contraction best explains the periodic trend given in option (c), where the radii of the period 5 transition metals (Y-Cd) are very similar to the radii of the period 6 transition metals (Lu-Hg).

Key Concepts

Periodic Trends

Periodic trends are a set of patterns observed in the periodic table as we move across periods (rows) and down groups (columns). These trends help predict the behaviors and properties of elements based on their position in the table. Such trends include:

• Atomic size or radii
• Ionization energy
• Electronegativity
• Electron affinity

For example, ionization energy generally increases across a period as the atomic number rises, making it harder to remove an electron. Likewise, atomic radii tend to decrease across a period due to the increase in positive charge in the nucleus, which pulls electrons closer. Understanding these key trends is crucial for recognizing how elements will interact chemically.

Atomic Radii

Atomic radii refer to the size of an atom, specifically the distance from the nucleus to the outermost shell of electrons. This size can be influenced by several factors:

• Number of electron shells (energy levels)
• Nuclear charge (more protons pull electrons closer)

The lanthanide contraction plays a significant role in the atomic radii of transition metals. It causes a reduction in size across the lanthanide series, which subsequently impacts the surrounding elements. Due to the lanthanide contraction, elements in period 6 have anomalously small atomic radii, making them quite similar to elements in the preceding period.

Transition Metals

Transition metals, found in the d-block of the periodic table, have some unique properties due to their electron configurations. These metals include familiar elements like iron, copper, and gold. Key characteristics include:

• Variable oxidation states
• Formation of colorful compounds
• High melting and boiling points

In the context of lanthanide contraction, the similarities in atomic radii between period 5 and period 6 transition metals become apparent. Without the contraction, period 6 metals would be significantly larger, but their size is compressed due to the shrinking effect seen across the lanthanide series. As a result, understanding these details about transition metals helps clarify trends in chemical reactivity and bonding.