How does the atomic radius of alkali metals change down the group?

The atomic radius of alkali metals increases as you move down the group.

In the periodic table, alkali metals are found in Group 1. This group includes elements such as lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). As you move down this group, the atomic radius, which is essentially the size of the atom, increases.

This increase in atomic radius is due to the addition of energy levels (also known as shells) as you move down the group. Each subsequent element in the group has an extra electron shell compared to the one before it. For example, lithium has two shells, sodium has three, potassium has four, and so on. These additional shells increase the distance between the nucleus and the outermost electrons, thereby increasing the atomic radius.

Another factor contributing to the increase in atomic radius is the shielding effect. The shielding effect refers to the phenomenon where inner shell electrons shield outer shell electrons from the attractive force of the protons in the nucleus. As more shells are added, the outermost electrons are less attracted to the nucleus due to the increased shielding effect. This results in the electrons being held less tightly and thus the atomic radius increases.

However, it's important to note that while the atomic radius increases, the ionisation energy decreases down the group. Ionisation energy is the energy required to remove an electron from an atom. As the atomic radius increases and the outermost electrons are further from the nucleus and less attracted to it, less energy is required to remove these electrons. This is why alkali metals are highly reactive, especially those lower down the group with larger atomic radii and lower ionisation energies.