Problem 98 - Chemistry The Central Science In Si Units

Question

The electron affinities, in \(\mathrm{kJ} / \mathrm{mol}\), for the group 11 and group 12 metals are as follows: $$ \begin{array}{|c|l|} \hline \mathrm{Cu} & \mathrm{Zn} \\ -119 & >0 \\ \hline \mathrm{Ag} & \mathrm{Cd} \\ -126 & >0 \\ \hline \mathrm{Au} & \mathrm{Hg} \\ -223 & >0 \\ \hline \end{array} $$ (a) Why are the electron affinities of the group 12 elements greater than zero? (b) Why do the electron affinities of the group 11 elements become more negative as we move down the group?

Step-by-Step Solution

Answer

Group 12 elements have full subshells, requiring energy to add electrons. Group 11 elements have increasing electron affinities (more negative) due to relativistic effects, despite larger atomic sizes.

Step 1: Analyze Group 12 Elements

The electron affinity greater than zero means that energy is required to add an electron to the atom. Group 12 elements like Zn, Cd, and Hg have filled d and s subshells, meaning they are stable and have low affinity to gain an extra electron, thus resulting in positive electron affinity values.

Step 2: Examine Atomic Structure of Group 11 Elements

Group 11 elements (Cu, Ag, Au) have incomplete d subshells which allow them to gain stability through accepting additional electrons, thus making their electron affinities negative. The negative sign indicates energy is released when they gain an additional electron.

Step 3: Explain Trend Down Group 11

As we move down group 11 from Cu to Au, the increasing atomic size and additional electron shielding reduce the effective nuclear charge experienced by added electrons, generally decreasing electron affinity. However, relativistic effects become significant for heavier atoms like gold, enhancing these atoms' ability to accept electrons, and making the affinities more negative.

Key Concepts

Group 11 Elements

Group 11 elements consist of copper (Cu), silver (Ag), and gold (Au). These elements are often referred to as coinage metals due to their historical use in minting coins. These metals possess unique properties as they have one electron in their outermost shell and an incomplete d subshell.

The electron configurations for these elements allow them to easily accept an additional electron, which provides them with extra stability by filling those d subshells. Because of this property, these metals exhibit negative electron affinities. This means that when they gain an electron, energy is released. As such, their ability to accept electrons is quite notable when compared to Group 12 elements. Understanding how this property affects their chemical behavior is crucial for many applications, including metallurgy and electronics.

Group 12 Elements

Group 12 elements include zinc (Zn), cadmium (Cd), and mercury (Hg). These metals are characterized by having fully filled s and d subshells which make them relatively stable.

Because their electron configurations are complete, adding an extra electron would disturb this stability. As a result, it takes energy to add another electron, leading to electron affinities greater than zero. This is why the electron affinities for these elements are positive, indicating that they are less likely to gain an electron spontaneously. The relative stability due to filled electronic subshells also explains why these elements are often found in their metallic state in nature.

Periodic Trends

Electron affinity, like many other properties of elements, shows periodic trends across the periodic table. In general, electron affinity tends to decrease as you move down a group in the periodic table.

However, for Group 11 elements, while decreased effective nuclear charge is observed due to greater atomic size and additional shielding, the trend is slightly different. Overall, the electron affinity becomes more negative down the group. This is largely attributable to the relativistic effects observed in heavier elements which can enhance the atom's capacity to attract additional electrons. These periodic trends help predict how elements will behave in various chemical situations, forming a backbone for chemical reactivity analysis.

Atomic Structure

The atomic structure of an element defines its properties and behavior, including its electron affinity. The configuration of electrons within an atom's shells and subshells determines how willing it is to accept additional electrons.

For both Group 11 and Group 12 elements, the differences in their electron affinity values are rooted in their atomic structure. Group 11 elements, with partially filled d subshells, are eager to accept electrons, therefore releasing energy. For Group 12 elements, the fully filled s and d subshells contribute to their reluctance to gain extra electrons, which requires additional energy, hence the positive electron affinities. Understanding these atomic structures is crucial to predicting metal reactivity and designing materials with desired properties.

Relativistic Effects

Relativistic effects become significant in heavier elements, affecting properties like electron affinity. In Group 11 elements, such as gold, these effects result from the inner electrons moving at velocities close to the speed of light, which influences the behavior of the electrons and the overall chemical properties of the element.

As a result, the electron affinity of gold is notably more negative compared to copper and silver. These effects enhance the gold atom’s ability to attract electrons more strongly despite the increase in atom size when moving down the group. Recognizing the role of relativistic effects is essential for understanding the subtle differences in electron affinity and overall chemical reactivity exhibited by heavier elements.