How does electron affinity vary in a group?

Electron affinity generally decreases as you move down a group in the periodic table.

In the periodic table, a 'group' refers to a vertical column. As you move down a group, the atomic radius (the size of the atom) increases. This is due to the addition of energy levels (shells) around the nucleus. As a result, the outermost electrons are further away from the nucleus and are less attracted to it because of the increased distance and the shielding effect of the inner shells.

The term 'electron affinity' refers to the energy change when an electron is added to a neutral atom to form a negative ion. If the energy change is negative, it means that energy is released when an electron is added, indicating that the atom has a high electron affinity. Conversely, if the energy change is positive, it means that energy is absorbed when an electron is added, indicating that the atom has a low electron affinity.

As you move down a group, the electron affinity decreases because the outermost electrons are less attracted to the nucleus. This is due to the increased atomic radius and the shielding effect of the inner shells. Therefore, less energy is released (or more energy is absorbed) when an electron is added to an atom lower down in a group, indicating a lower electron affinity.

In summary, the electron affinity varies in a group due to the changes in atomic radius and the shielding effect. As you move down a group, the atomic radius increases and the shielding effect becomes more significant, leading to a decrease in electron affinity.