How does atomic radius change down the group in halogens?

The atomic radius increases down the group in halogens due to the addition of electron shells.

In the periodic table, halogens are found in Group 17. As you move down this group, from fluorine (F) at the top to astatine (At) at the bottom, the atomic radius or size of the atoms increases. This is due to the addition of electron shells or energy levels.

Each halogen atom has seven electrons in its outermost shell, but the number of shells increases as you go down the group. Fluorine has two shells, chlorine has three, bromine has four, iodine has five, and astatine has six. Each additional shell means that the outermost electrons are further from the nucleus, which increases the atomic radius.

The increase in atomic radius down the group is also due to the shielding effect. The inner shells of electrons shield the outermost electrons from the positive charge of the nucleus. As more shells are added, the shielding effect becomes stronger, which means the outermost electrons are less attracted to the nucleus and the atomic radius increases.

However, it's important to note that the increase in atomic radius is not proportional to the increase in atomic number. This is because the increase in positive charge in the nucleus also pulls the outermost electrons closer, which somewhat offsets the increase in atomic radius. But overall, the addition of electron shells and the shielding effect are stronger factors, so the atomic radius still increases down the group.

In summary, the atomic radius of halogens increases down the group due to the addition of electron shells and the shielding effect. This is a key trend in the periodic table and is important for understanding the properties and reactivity of halogens.