Problem 98 - Chemistry: The Central Science

Question

The electron affinities, in \(\mathrm{kJ} / \mathrm{mol}\), for the group \(1 \mathrm{~B}\) and group 2B metals are as follows: (a) Why are the electron affinities of the group \(2 B\) 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 affinities of other groups as we proceed down the periodic table.]

Step-by-Step Solution

Answer

(a) The electron affinities of group 2B elements are greater than zero because they have a completely filled s-orbital in their outermost energy level. Adding an electron would require a higher energy level, resulting in a less stable configuration and a positive electron affinity. (b) As we move down the group 1B elements, the atomic size and number of electron shells increase. This leads to a reduced electron-electron repulsion due to increased shielding and greater distance between outermost electrons and the nucleus, allowing the additional electron to release more energy upon addition. Consequently, electron affinity becomes more negative as we move down the group.

Step 1: Part (a) Explanation of the electron affinities of the group 2B elements being greater than zero

To understand why the electron affinities of the group 2B elements are greater than zero, we need to examine their electronic configurations and the overall stability of these elements. Group 2B elements have a completely filled s-orbital in their outermost energy level. Adding an electron to an already filled s-orbital in these elements would require a higher energy level, which is less stable than the current configuration. This results in a positive electron affinity as energy must be supplied for the addition of an electron.

Step 2: Part (b) Explanation of the electron affinities of the group 1B elements becoming more negative as we move down the group

As we move down the group 1B elements in the periodic table, the atomic size and the number of electron shells increases. When an additional electron is added to the outermost shell, the electron-electron repulsion within the atom becomes less significant due to an increased shielding effect provided by the inner electrons and a greater distance between the outermost electrons and the nucleus. This lessening of repulsion allows the additional electron to come closer to the nucleus and release more energy upon addition. Hence, the electron affinity becomes more negative as we move down the group.

Key Concepts

Group 1B Elements

Group 1B elements occupy a special section of the periodic table. This group mainly includes transition metals such as copper (Cu), silver (Ag), and gold (Au). These elements are known for their notable conductivity and lustrous appearance. One interesting property of group 1B elements is their electron affinity, which reveals how these metals react to gaining an extra electron. As we move down this group, the electron affinity becomes more negative. This happens because the atomic size increases, and there are more inner electron shells. This shields the outermost electrons from the nucleus' positive charge. Consequently, when an extra electron is added, it repels less against the nucleus and can more easily release energy, which makes the electron affinity more negative. Here are some takeaway points about group 1B elements:

• Includes copper, silver, and gold.
• Notable for increasing negative electron affinities down the group.
• Conductivity and lustrous appearance are key characteristics.

Group 2B Elements

Group 2B elements also belong to the transition metals and include zinc (Zn), cadmium (Cd), and mercury (Hg). These elements have a completely filled s-orbital in their outermost energy level. Because of their electronic configuration, these elements have a positive electron affinity. This is because adding another electron requires placing it in a higher energy subshell, which is less stable than the arrangements of already filled lower energy levels. Hence, energy must be supplied to the metal to accept an electron, and the electron affinity is positive. Some characteristics of group 2B elements include:

• Includes zinc, cadmium, and mercury.
• Consists of metals with positive electron affinities.
• Filled s-orbitals make them more stable in their current state without extra electrons.

Periodic Table Trends

The periodic table reveals a lot about the chemical properties of the elements, including trends in electron affinity, atomic size, and more. Generally, electron affinity varies across the periodic table. For many groups, electron affinity becomes more negative as you move down a group in the periodic table. This trend is mainly due to the increase in atomic size and the number of electron shells. The further out the outermost shell is from the nucleus, the less energy is required to add an extra electron. However, there are exceptions, such as group 2B elements that have positive electron affinities due to their full s-orbitals. Here are some important periodic table trends:

• Electron affinity usually becomes more negative down a group.
• Except for some elements like those in group 2B with filled orbitals.
• Atomic size increases as you proceed down a group.