How do the properties of halogens change down group 17?

As you move down Group 17, halogens become less reactive, have higher atomic radii, lower electronegativities, and higher melting and boiling points.

Group 17 of the periodic table consists of the halogens: fluorine, chlorine, bromine, iodine, and astatine. As you move down this group, there are several noticeable trends in the properties of these elements.

Firstly, the reactivity of the halogens decreases as you move down the group. This is due to the increasing atomic radius, which results in the outer electrons being further from the nucleus and thus less strongly attracted to it. This makes it harder for the atom to gain an extra electron to achieve a stable electron configuration, hence the decrease in reactivity. Fluorine, at the top of the group, is the most reactive of all the halogens, while astatine, at the bottom, is the least reactive.

Secondly, the atomic radius of the halogens increases down the group. This is because each successive element has an additional electron shell compared to the one above it. This increase in the number of electron shells causes the atomic radius to increase, as the electrons are located further from the nucleus.

Thirdly, the electronegativity of the halogens decreases as you move down the group. Electronegativity is a measure of the ability of an atom to attract a bonding pair of electrons. As the atomic radius increases, the outer electrons are less strongly attracted to the nucleus, resulting in a decrease in electronegativity.

Lastly, the melting and boiling points of the halogens increase down the group. This is due to the increase in the number of electrons per atom, which leads to stronger London dispersion forces (a type of van der Waals force) between the atoms. These stronger forces require more energy to overcome, resulting in higher melting and boiling points.

In summary, as you move down Group 17, the halogens exhibit a decrease in reactivity and electronegativity, and an increase in atomic radius, melting point, and boiling point. These trends are a result of the increasing number of electron shells and the corresponding increase in atomic radius.