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

Lithium has a negative electron affinity because adding an electron to its 1s² 2s¹ configuration results in a stable, complete s-subshell (1s² 2s²). In contrast, beryllium has a positive electron affinity because adding an electron to its already-stable 1s² 2s² configuration would result in an unstable 1s² 2s² 2p¹ configuration, requiring an input of energy. This difference in electron configurations accounts for the different signs of their electron affinities.

Step-by-step solution

Determine the electron configurations of lithium and beryllium

In order to determine the electron configurations of lithium and beryllium, we need to follow the periodic table. Lithium (Li) has an atomic number of 3, meaning it has 3 electrons. Its electron configuration is 1s² 2s¹. Beryllium (Be) has an atomic number of 4, meaning it has 4 electrons. Its electron configuration is 1s² 2s². Now that we have the electron configurations, let's take a look at their electron affinities.

Explain the negative electron affinity of lithium

Electron affinity is the energy change that occurs when an electron is added to a gaseous atom. Lithium has a negative electron affinity, implying that energy is released when an electron is added to it. In the case of lithium, when an electron is added, the electron configuration becomes 1s² 2s², which corresponds to a stable and complete s-subshell. This results in a release of energy, leading to a negative electron affinity.

Explain the positive electron affinity of beryllium

Beryllium has a positive electron affinity, which means that energy is required to add an electron to it. Beryllium already has a complete and stable 2s² subshell. Adding an electron to beryllium would create a 2p¹ electron configuration. This results in an unstable and higher energy configuration, requiring an input of energy. Thus, beryllium's electron affinity is positive.

Conclude

Lithium's electron affinity is negative because adding an electron to it results in a stable, lower-energy electron configuration (1s² 2s²). On the other hand, beryllium's electron affinity is positive because adding an electron to it leads to an unstable, higher-energy electron configuration (1s² 2s² 2p¹). This difference in electron configurations is responsible for the observed difference in the electron affinities of lithium and beryllium.