How does the electron affinity change across alkali metals?

Electron affinity decreases across alkali metals from lithium to francium.

Alkali metals, which include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr), are located in Group 1 of the periodic table. As we move from lithium to francium, the electron affinity decreases. Electron affinity is the energy change that occurs when an electron is added to a neutral atom to form a negative ion. It is a measure of the attraction of the atom for the added electron.

The decrease in electron affinity across alkali metals can be explained by the increase in atomic size and the shielding effect. As we move down the group, the atomic radius increases due to the addition of energy levels (shells). This means the outermost electron is further away from the nucleus, which contains the positive charge. As a result, the attraction between the nucleus and the outermost electron decreases, making it easier for the electron to be removed.

Additionally, the shielding effect also plays a role. The shielding effect refers to the reduction in effective nuclear charge on the electron cloud, due to a difference in the attraction forces of the electrons for the nucleus. As we move down the group, the number of inner shell electrons (which shield the outermost electron from the nucleus) increases. This further reduces the attraction between the nucleus and the outermost electron.

In summary, the electron affinity decreases across alkali metals from lithium to francium due to the increase in atomic size and the shielding effect. This results in a lower attraction for added electrons, making alkali metals excellent reducing agents.