Question

The electron affinities, in \(\mathrm{kJ} / \mathrm{mol}\), for the group 11 and group 12 metals are as follows: (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? [Hint: Examine the trends in the electron affinities of other groups as we proceed down the periodic table.]

Short answer

(a) Group 12 elements have positive electron affinities because the incoming electron occupies a higher energy level orbital that is further away from the nucleus, resulting in a poor shielding effect and less attraction between the electron and nucleus. (b) The electron affinities of group 11 elements become more negative as we move down the group due to an increase in atomic radius, which leads to weaker attraction between valence electrons and the nucleus, allowing the atom to more easily gain an electron.

Step-by-step solution

Define Electron Affinity

Electron affinity is the energy change that occurs when an electron is added to a gaseous atom, forming an anion. It's a measure of the attraction between the nucleus and the incoming electron. A negative electron affinity value means that the atom releases energy when it gains an electron, while a positive value means that energy must be supplied to the atom for it to gain an electron.

Group 12 Elements Electron Affinities

Group 12 elements have electron affinities greater than zero, meaning they require energy to gain an electron. This is because when an electron is added to the atom, it occupies the (n+1) s orbital. The (n+1) s orbital is further away from the nucleus and has a higher energy level, leading to a poorer shielding effect. This results in less attraction between the incoming electron and the nucleus, and thus a positive electron affinity.

Group 11 Elements Electron Affinities

As we move down the group 11 elements in the periodic table, the electron affinities become more negative. This indicates that the atom releases more energy as it gains an electron. The atomic size increases down the group due to the addition of more electron shells and increased shielding effect of the inner electrons. This increase in atomic radius leads to an increased distance between the nucleus and the electrons in the outermost energy level (valence electrons). Consequently, valence electrons experience weaker attraction to the nucleus, which allows them to be more easily gained or removed.

Summarizing the Reasons

(a) Group 12 elements have electron affinities greater than zero due to poor shielding effect of the incoming electron, which occupies a higher energy level orbital further away from the nucleus. (b) Group 11 elements have more negative electron affinities down the group due to an increase in atomic radius, resulting in weaker attraction between valence electrons and the nucleus, making it easier for the atom to gain an electron.

Key concepts

Group 11 Elements

Group 11 elements consist of copper (Cu), silver (Ag), and gold (Au). These elements are known as the coinage metals because they have historically been used to make coins. They are positioned in the d-block of the periodic table and have one electron in their outermost s-orbital.
Their electron affinities are unique and typically become more negative as you move down the group. This means these elements release more energy when they gain an electron. Why does this happen? Well, as you go down the group, the size of the atom increases because new electron shells are added.
This causes valence electrons, which are farthest from the nucleus, to experience a weaker pull. As a result, it's easier for these atoms to attract an extra electron, leading to more negative electron affinities. This trend is common in many groups where elements become more prone to gaining electrons as atomic size increases.

Group 12 Elements

Group 12 elements include zinc (Zn), cadmium (Cd), and mercury (Hg). They are adjacent to group 11 and share some physical properties, but their chemistry sets them apart. One notable feature is that these elements have electron affinities that are greater than zero.
This is unusual because it means that energy must be supplied for these atoms to gain an electron. The key reason lies in their electron configuration. When an electron is added to a group 12 element, it goes into the next higher energy level orbital, specifically the (n+1) s orbital.
This orbital is farther from the nucleus and experiences less attraction from it, partly due to poor shielding by inner electrons. Poor shielding means inner electrons don't effectively block the charge of the positive nucleus from the outermost electrons, leading to a less stable electron gain process. Hence, the electron affinity is positive, indicating a lower tendency to accept additional electrons.

Periodic Table Trends

Studying the periodic table reveals many intriguing trends, especially in electron affinity. Understanding these trends makes it easier to predict chemical behaviors and reactions. In general, electron affinity tends to become more negative across a period from left to right. This is due to increasing nuclear charge that attracts additional electrons more strongly. However, a different trend emerges down a group.
In groups like group 11, electron affinities become more negative moving down. Conversely, in group 12, electron affinities are positive and less encouraging for electron gain. This difference highlights the influence of atomic size and electron shielding, both crucial for electron affinity values.
As you explore these trends, notice how they interconnect with atomic size, proton number, and electron configuration, providing valuable insights into elemental behavior. Recognizing these patterns can enhance your grasp of chemistry and predict how atoms will interact in various scenarios.