Explain the trend in atomic radius as you progress up column 17

As we progress up in any column atomic radius decreases.

Explanation:

Atomic radii:

It is the inter nuclear distance between bonded atom.

Trend along group:

When we move from top to bottom in a group atomic size increases with increase of atomic number because of addition of electron in next energy level. Thus nuclear attraction becomes weaker on valence electrons and size gets increased. Moreover shielding effect also protect the valance electrons from nuclear attraction.

when we move from bottom to top the atomic size decreases because of decrease in atomic number and energy level.

For example

In halogens the fluorine has smaller size than rest of halogens elements because it is present in top of this column. while iodine is present at the end so it has larger atomic size.