A halogen is a chemical element that forms a salt when it reacts with metal. ... There are five halogens in the periodic table of chemical elements: fluorine, chlorine, bromine, iodine, and astatine. The halogens are all highly reactive, which means they're quick to form bonds with other elements.

F2(g) = pale yellow

Cl2(g) = green/yellow

Br2(I) = orange/brown

I2(s) = black

The colour gets darker and changes from gas to solid as it goes down the group

Melting point generally increases down the group. This is because of the increase in van der Waal's forces. Thus volatility decreases down the group

Atomic radius increases down Group 17 from top to bottom

Reactivity decreases down the group. This is because the force of attraction between the nucleus and the outer electrons gets further apart

Electronegativity decreases down the group. This is because the force of attraction between the nucleus and the outer electrons gets further apart

The volatility of the Group 17 elements decrease down the group due to the increse the numnber of electrons

Thermal stability decreases down the group because bond length increases and bond energy decreases