4.5 - Elements in Group 17

Halogens and Their Uses

• Group 17 contains fluorine, F; chlorine, Cl; bromine, Br; iodine, I; astatine, At; and tennessine, Ts.
• These halogens exist as diatomic molecules.
• Chlorine is used in bleach, bromine is a component in fire extinguishers and iodine is used as a disinfectant.

Physical Properties and Trends

• Down the group at room temperature, physical state changes from gas to liquid to solid.
• Molecular size and attraction between molecules increase, so melting and boiling points rise because more heat is needed to overcome the intermolecular forces.
• Density increases with increasing mass down the group.

Halogen	State and colour	Melting / boiling point (${}^{\circ }\mathrm{C}$)
Chlorine, $\mathrm{C}{\mathrm{l}}_2$	Greenish-yellow gas	$-101/-34$
Bromine, $\mathrm{B}{\mathrm{r}}_2$	Reddish-brown liquid	$-7/59$
Iodine, ${\mathrm{I}}_2$	Purplish-black solid	$114/184$

Halogen	Density ($\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$)
Chlorine	0.00300
Bromine	3.11900
Iodine	4.95000

Chemical Properties

• Each atom has seven valence electrons, accepts one electron during a reaction and forms a $-1$ ion:

• Chlorine, bromine and iodine have similar chemical properties but different reactivities.
• With water, halogens form acidic solutions. Chlorine produces hydrochloric acid and hypochlorous acid:

• With metals, halogens form metal halides; bromine and iron produce iron(III) bromide:

• With alkalis, halogens form a metal halide, metal halate and water; iodine and sodium hydroxide produce sodium iodide and sodium iodate(I):

Reactivity Trend

• Reactivity decreases down Group 17.
• More occupied shells increase atomic size and place the valence shell farther from the nucleus.
• Weaker nuclear attraction makes it more difficult for an atom to attract an electron to complete its valence shell.

Other Halogens and Safety

• Halogens dissolve in organic solvents and do not conduct heat or electricity.
• Fluorine is a poisonous, light-yellow gas. It is very reactive and corrosive and explodes strongly when combined with hydrogen.
• Astatine is rare, radioactive and chemically unstable.
• Handle halogens carefully because they are dangerous.