Halogens: Properties and Applications

The Group 17 Elements: Halogens Defined

Halogens are a group of elements known for their seven valence electrons in the outermost p-subshell, which predisposes them to gain one electron and form anions with a -1 charge. Located in group 17 of the periodic table, these elements include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Tennessine (Ts), a synthetic element, is sometimes considered part of the group. However, due to the metallic characteristics and instability of tennessine and astatine, this summary will focus on the more commonly encountered halogens: fluorine, chlorine, bromine, and iodine.

Halogens' Physical States and Properties

Halogens are nonmetals with properties such as poor conductivity of heat and electricity, and they are typically brittle when in the solid state. Their states at room temperature vary: fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. These elements also display a range of colors, from the pale yellow of fluorine to the violet of iodine. The atomic radius of halogens increases down the group due to the addition of electron shells, which affects their physical properties.

Trends in Halogens' Melting and Boiling Points

The melting and boiling points of halogens increase from fluorine to iodine. This trend is due to the larger atomic size and greater number of electrons in heavier halogens, which enhance van der Waals forces and require more energy to break. As a result, the volatility of halogens decreases down the group, with fluorine being the most volatile and iodine the least.

Electronegativity and Electron Affinity of Halogens

Electronegativity in halogens decreases from fluorine to iodine. Although all halogens have a nominal effective nuclear charge of +7, the increased atomic radius and electron shielding in heavier halogens reduce the attraction for shared electrons, leading to lower electronegativity. Fluorine is the most electronegative element in the periodic table. Electron affinity generally decreases down the group, but fluorine's electron affinity is lower than chlorine's due to electron repulsion in its small, densely packed 2p orbitals.

Chemical Reactivity and Halogen Bonding

Halogens are highly reactive, especially in forming anions and engaging in reactions such as salt formation with metals and hydrogen halides with hydrogen. Reactivity decreases down the group, with fluorine being the most reactive due to its high electronegativity and strong bond formation. Bond enthalpies for halogen-halogen (X-X) and hydrogen-halogen (H-X) bonds generally decrease down the group. The F-F bond is weaker than the Cl-Cl bond because of the repulsion between fluorine's lone pairs of electrons.

Halogens in Daily Use and Environmental Impact

Halogens have diverse applications in daily life. Chlorine and bromine are used as disinfectants in pools and cleaning products. Halogens are incorporated into lighting to improve bulb efficiency and in pharmaceuticals to increase drug solubility. Fluoride ions help prevent dental cavities by reinforcing tooth enamel, and iodine is vital for thyroid health. However, some halogen compounds, such as chlorofluorocarbons (CFCs), have been regulated due to their ozone-depleting effects.