Question

I understand that electron affinity is generally exothermic(groups 16 and 17). But, alkali metals are reluctant to gain an electron because losing electrons is beneficial for them. So, the electron gain enthalpy is termed endothermic (source: Chem libretexts) for alkali metals because it is hard to add an electron to their atoms and you need to invest energy. If it is endothermic, the electron affinity values must be positive, but they are still negative. Why? Why do the values still represent the release of energy?

In the image, the EA values of alkali metals are still negative, not positive. But in the description, it says that the process of adding an electron to alkali metals associates with the absorption of energy (endothermic- positive)

Answer 1

Electron affinity is the energy change when a gaseous atom gains an electron; it is typically negative for groups 16 and 17, meaning energy is released. For alkali metals, gaining an electron is usually endothermic and would expect positive values; if negative values occur, there may be a data error.

Step-by-step explanation:

The electron affinity (EA) is defined as the energy change associated with the addition of an electron to a gaseous atom. It can be either exothermic or endothermic. For example, when an alkali metal atom in the gaseous state accepts an electron, it is typically an endothermic process, meaning energy must be absorbed for the reaction to occur.

In the periodic table, the elements in groups 16 and 17 often release energy when gaining an electron, hence their electron affinities are negative, indicating an exothermic process.

However, alkali metals have a different behavior due to their desire to lose an electron and achieve a noble gas configuration, thus requiring energy to gain an electron making their EA positive. If there's a discrepancy in the data you're looking at, it might be a mistake or misinterpretation.

Generally, when considering periodic trends of electron affinity, the EA increases in magnitude as you move from left to right across the periodic table. This tendency is because atoms progressively hold their electrons more tightly and attract additional electrons more effectively.

Nevertheless, there can be exceptions to the expected values of EA due to atomic structure intricacies. If you're observing negative values for alkali metals' electron affinities where they should be positive, it's possible that the data you have encountered may be incorrect or mislabeled.