Problem 97 - Chemistry The Central Science

Question

The electron affinities, in \(\mathrm{kJ} / \mathrm{mol}\), for the group \(1 \mathrm{~B}\) and group \(2 \mathrm{~B}\) metals are $$ \begin{array}{|c|c|} \hline \mathrm{Cu} & \mathrm{Zn} \\ -119 & >0 \\\ \hline \mathrm{Ag} & \mathrm{Cd} \\ -126 & >0 \\ \hline \mathrm{Au} & \mathrm{Hg} \\ -223 & >0 \\ \hline \end{array} $$ (a) Why are the electron affinities of the group \(2 \mathrm{~B}\) elements greater than zero? (b) Why do the electron affinities of the group \(1 \mathrm{~B}\) elements become more negative as we move down the group? [Hint: Examine the trends in the electron affinity of other groups as we proceed down the periodic table. \(]\)

Step-by-Step Solution

Answer

In summary, (a) the electron affinities of Group 2B elements are greater than zero because adding an electron increases electron-electron repulsion within the d subshell due to the completely filled outer s subshell and partially filled d subshell. (b) The electron affinities of Group 1B elements become more negative as we move down the group because the increase in atomic size and electron shielding prevails over the electron-electron repulsion, resulting in a stronger attraction between the nucleus and the added electron.

Step 1: Understanding Electron Affinity

Electron affinity is defined as the energy change that occurs when an electron is added to a neutral atom to form a negatively charged ion. It is an exothermic process, which means energy is released when an electron is added to an atom. However, in some cases, energy must be added to force an electron to join an atom, and that makes the electron affinity of those elements positive.

Step 2: Explaining Positive Electron Affinities for Group 2B Metals

Group 2B elements have a completely filled outer s subshell and a partially filled d subshell. When adding an electron to a group 2B element, the additional electron has to enter the same d subshell, as per Hund's rule. This increases electron-electron repulsion because the electron-electron distance within the d subshell is relatively short, making it harder for the added electron to stay within. This repulsion causes the energy change to be positive when an electron is added to a Group 2B element. Therefore, the electron affinities of Group 2B elements are greater than zero because adding an electron increases electron-electron repulsion within the d subshell.

Step 3: Examining Periodic Trends in Electron Affinities

Similar to ionization energies, electron affinities also exhibit periodic trends in the periodic table. However, the trends for electron affinity do not have a continuous progression down a group. It is generally observed that electron affinity becomes more negative as we move across a period from left to right and tends to be less negative as we move down a group. This is due to the increased nuclear charge and smaller atomic size across a period and the increased atomic size and electron shielding down a group.

Step 4: Explaining the Trend for Group 1B Elements

Group 1B elements have a completely filled s subshell and a partially filled d subshell with a single unpaired electron. When an electron is added to a group 1B element, it occupies the same d subshell, but it occupies a different d orbital, resulting in a relatively weaker electron-electron repulsion compared to Group 2B elements. While moving down the periodic table, the trend of less negative electron affinity reverses within some groups, including group 1B. In this specific case, the increase in atomic size and electron shielding dominates over the positive features leading to more negative electron affinities down the group. This corresponds to the stronger attraction between the nucleus and the added electron and hence, less energy is required to attach the electron to the atom. To sum up, the electron affinities of Group 1B elements become more negative as we move down the group because the increase in atomic size and electron shielding prevails over the electron-electron repulsion, resulting in a stronger attraction between the nucleus and the added electron.

Key Concepts

Periodic trends

When exploring the periodic table, you might notice patterns or periodic trends in various properties of elements. Electron affinity is one such property. Electron affinity refers to the amount of energy released when an electron is added to a neutral atom. In the periodic table, we observe that electron affinities generally become more negative across a period from left to right. This is because atoms tend to have a stronger nuclear charge and smaller atomic size, pulling added electrons closer and releasing more energy.

In contrast, as we move down a group, electron affinities usually become less negative. This happens due to two main reasons:

• Increased atomic size, meaning the added electron is further from the nucleus.
• Increased electron shielding, which reduces the effective nuclear charge felt by the outer electrons.

However, it is important to notice that there are exceptions, such as the group 1B elements, which show more negative electron affinities as you move down the group. Understanding these exceptions deepens our grasp of electron interactions within atoms.

Group 1B elements

Group 1B elements include copper (Cu), silver (Ag), and gold (Au), and they display unique electron affinity trends. These elements have a filled s subshell and a partially filled d subshell with a single unpaired electron. When an extra electron is added to these atoms, it joins the same d subshell but occupies a different d orbital. This setup minimizes the repulsion between existing electrons and the new electron added, making it easier for atoms to gain them.

When moving down this group in the periodic table, their electron affinities become more negative. This is because:

• The increase in atomic size and electron shielding effects still leave a significant positive impact on the overall addition of electrons.
• Nucleus attraction to the outer electron increases, making the affinity more negative as we move from Cu to Ag to Au.

This reversal in trend gives rise to a stronger attraction between the added electron and the nucleus, dominating over any repulsion forces present. Hence, less energy is required to affiliate an electron to Group 1B elements further down the period.

Group 2B elements

Introducing you to Group 2B elements such as zinc (Zn), cadmium (Cd), and mercury (Hg). Unlike their Group 1B neighbors, Group 2B elements have completely filled s subshells and partially filled d subshells. The addition of an electron here often results in a scenario where the energy becomes positive, meaning energy must be added for the electron to stay, as shown with their electron affinity values greater than zero.

Why do these elements exhibit positive electron affinities? Here's why:

• When adding an electron to a Group 2B element, it must go into the same d subshell.
• This increases electron-electron repulsion as the electron-electron distance is shorter within the d subshell.

As a result, energy is required to overcome the repulsion, resulting in a positive electron affinity. This understanding underscores how subtle changes in electron configuration can impact the energy dynamics within an atom.

Electron-electron repulsion

Understanding electron-electron repulsion is crucial to knowing why electron affinities can vary between elements. When electrons are added to an atom, existing electrons exert repulsive forces on the new addition due to their like charges. The distribution and configuration of electrons determine how significant this repulsion might be.

Let's look at two different scenarios across the periodic table:

• In Group 1B elements, when electrons are added, there is a lesser degree of repulsion because the new electron enters a different orbital in the same d subshell.
• Conversely, in Group 2B elements, adding another electron leads to high electron-electron repulsion within the same d subshell due to tightly packed electrons in the same orbital region.

The positioning of electrons, therefore, plays a vital role in dictating how easily an atom might accept an electron, indicating the negative or positive nature of its electron affinity. Managing these repulsive forces determines the stability and energy changes associated with electron addition.