The halogens are a group of nonmetallic elements occupying Group 17 of the periodic table. This family includes fluorine (\(\text{F}\)), chlorine (\(\text{Cl}\)), bromine (\(\text{Br}\)), iodine (\(\text{I}\)), and the radioactive elements astatine (\(\text{At}\)) and tennessine (\(\text{Ts}\)). The name “halogen” comes from Greek roots meaning “salt-former,” reflecting their ability to readily form salts when they react with metals.
Defining Atomic Characteristics
The properties of halogens stem from the configuration of their outer electron shell. All halogens possess seven valence electrons, following the general electron configuration of \(ns^2np^5\). This configuration leaves them one electron short of a complete, stable octet. This deficiency creates a strong electronic drive to acquire a single electron.
The attraction for an additional electron is quantified by electronegativity, which is high for halogens. Fluorine is the most electronegative element on the periodic table. Electronegativity generally decreases as one moves down the group, because the increasing atomic radius places the outer electrons farther from the nucleus.
Halogens exist as diatomic molecules (\(\text{X}_2\)) to satisfy their valence electron requirements. In this form, two atoms share a single pair of electrons, forming a non-polar covalent bond. Halogens also exhibit a high ionization energy, which is the energy required to remove an electron, indicating their tendency to gain rather than lose electrons.
Diverse Physical States and Appearance
The halogen group is unique because its members exist in all three standard states of matter—gas, liquid, and solid. Fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids. The physical appearance of the elements also becomes progressively darker as one moves down the group.
Fluorine is a pale yellow gas, while chlorine is a denser, pale yellow-green color. Bromine is the only nonmetal liquid at room temperature, appearing as a volatile, reddish-brown liquid. Iodine is a crystalline solid with a dark, metallic luster that readily changes directly into a purple vapor when heated, a process known as sublimation.
The increase in melting and boiling points down the group is due to a systematic increase in the strength of intermolecular forces. As the size of the atoms increases, the strength of the temporary London dispersion forces between the diatomic molecules also increases. More energy is therefore required to overcome these stronger forces, resulting in higher melting and boiling temperatures for the heavier elements.
Primary Chemical Behavior
The tendency of halogens to acquire an electron makes them powerful oxidizing agents. The halogen atom accepts an electron from another substance, causing that substance to be oxidized while the halogen is reduced to a halide ion with a \(-1\) charge. This oxidizing power decreases from fluorine to astatine, which parallels the decrease in electronegativity down the group.
Fluorine is the most reactive element, reacting explosively with hydrogen even in the dark. The other halogens also react with hydrogen to form hydrogen halides, such as hydrogen chloride (\(\text{HCl}\)) and hydrogen bromide (\(\text{HBr}\)). These hydrogen halides dissolve in water to form strong acids.
Halogens react vigorously with metals to form ionic compounds called metal halides (salts). For instance, reacting sodium with chlorine gas produces sodium chloride (\(\text{NaCl}\)), which is common table salt. This high reactivity enables a more reactive halogen to displace a less reactive halogen from its compound in solution. For example, chlorine can displace bromine from a bromide salt solution because chlorine is more reactive.