The size of atoms in Group 7 (also known as the halogens) generally increases as you move down the periodic table. This is because the number of electron shells or energy levels increases as you go down a group. Each additional electron shell increases the overall size of the atom due to the increase in the number of electrons and the space they occupy. As the atomic number increases within Group 7 from fluorine to iodine, new electron shells are added, resulting in a gradual increase in atomic radii.

Here's a summary of the trend:

1. Fluorine (F): The smallest atom in Group 7. It has only one electron shell and a small atomic radius.

2. Chlorine (Cl): Larger than fluorine. It has two electron shells, which increases the atomic radius.

3. Bromine (Br): Larger than chlorine. It has three electron shells, resulting in a further increase in atomic radius.

4. Iodine (I): The largest atom in Group 7. It has five electron shells, contributing to its significantly larger atomic radius compared to the other elements in the group.

So, as you move down Group 7 from fluorine to iodine, the size of the atoms increases due to the addition of more electron shells, which corresponds to the increasing atomic number.