Question

The electron affinity of lithium is a negative value, whereas the electron affinity of beryllium is a positive value. Use electron configurations to account for this observation.

Short answer

The electron configurations of lithium (Li) and beryllium (Be) are \(1s^2 2s^1\) and \(1s^2 2s^2\), respectively. Lithium has a negative electron affinity because adding an electron to its 2s orbital results in a stable configuration and energy is released in the process. In contrast, beryllium has a positive electron affinity because adding an electron requires energy, as the new electron would have to go into the higher energy 2p orbital.

Step-by-step solution

Write the electron configurations of lithium and beryllium

Determine the electron configurations of lithium (Li) and beryllium (Be) atoms. Lithium (atomic number 3): \(1s^2 2s^1\) Beryllium (atomic number 4): \(1s^2 2s^2\)

Analyze the electron configuration of lithium

Lithium has one electron in its 2s orbital (2s^1), making it one electron short of achieving the stable electron configuration of the nearest noble gas, helium (1s^2 2s^2). The energy released during the process of adding this electron is greater than the energy required to overcome the electrostatic repulsion between the new electron and the existing electrons. Therefore, lithium has a negative electron affinity, as energy is released when it gains an electron to complete its 2s orbital.

Analyze the electron configuration of beryllium

Beryllium has a completely filled 2s orbital (2s^2), which is a very stable electron configuration. If a new electron were to be added to the beryllium atom, it would have to go into the next available orbital, which is the 2p orbital. The energy required to add an electron to the 2p orbital is greater than the energy released due to electrostatic interactions between the new electron and the existing electrons. As a result, beryllium has a positive electron affinity, meaning it needs energy to gain an extra electron.

Conclusion

The different electron configurations of lithium and beryllium account for their contrasting electron affinities. Lithium has a negative electron affinity because adding an electron to its 2s orbital results in a stable configuration and energy is released in the process. On the other hand, beryllium has a positive electron affinity because adding an electron to its electron configuration requires energy, as the new electron would have to go into the higher energy 2p orbital.

Key concepts

Electron Configurations

Understanding electron configurations is pivotal in explaining various chemical behaviors of elements, including trends in electron affinity. Essentially, electron configurations depict the distribution of electrons in an atom's orbitals. These configurations follow a prescribed order dictated by increasing energy levels, starting with the 1s orbital, moving up through 2s, 2p, and so forth.

For instance, lithium, with an atomic number of 3, has the electron configuration of \(1s^2 2s^1\), while beryllium, with an atomic number of 4, has the configuration of \(1s^2 2s^2\). These configurations inform us how each atom would interact when gaining an electron. In lithium's case, the addition of an electron completes the 2s subshell, resulting in the release of energy, thus a negative electron affinity. Conversely, any added electron to beryllium must enter the next higher energy subshell (2p), requiring extra energy, leading to a positive electron affinity.

To improve the exercise's comprehensibility, it is essential to consider the concept of stability which is inherently tied to atomic structures. Atoms are more stable and have a higher electron affinity when they can achieve a noble gas configuration or fill subshells, which is the driving force behind the gain or loss of electrons.

Noble Gas Configuration

The noble gas configuration refers to an atom's electron structure that resembles that of the nearest noble gas—atoms in Group 18 of the periodic table characterized by their chemical inertness. This configuration signifies a full valence shell, which is associated with maximum stability. The noble gas electron configuration is achieved by most elements through the gain, loss, or sharing of electrons until they possess a similar electron structure to a noble gas.

For example, lithium desires to reach the configuration of the nearest noble gas, helium, resulting in a negative electron affinity. On the other hand, once beryllium has filled its 2s orbital to mirror helium, it attains a stable noble gas configuration. The introduction of another electron would disrupt this stability and hence, beryllium's electron affinity is positive, reflecting the additional energy required to force an additional electron into the already stable arrangement. Proper comprehension of noble gas configuration can significantly enhance students' understanding of why elements behave differently regarding electron affinity.

Periodic Table Trends

Periodic table trends are patterns in properties that occur among elements as one moves across a period (row) or down a group (column) of the periodic table. One of these trends is the behavior of electron affinity, which generally becomes more negative across a period from left to right and varies less predictably down a group.

Looking at the periodic trend, lithium and beryllium are adjacent to each other in the same period, but they exhibit differing electron affinities. This variation is a quintessential example of the irregularities that can occur due to electronic configurations and the drive for stability. Elements towards the right of a period tend to want to gain electrons to reach noble gas configurations, thus generally exhibiting more negative values of electron affinity. Meanwhile, as one moves down a group, the added electron shells put the valence electrons further from the nucleus, making the electron affinity less negative.

By highlighting such trends and their exceptions in educational content, students can better predict and rationalize the chemical properties of elements and their likelihood of gaining or losing electrons in chemical reactions.