INTRODUCTION TO HALOGEN COMPOUNDS AND THEIR PHYSICAL PROPERTIES

Halogen Compounds

    • Hydrogen halides are binary compounds of halogens with hydrogen. They are strong hydrohalic acids when dissolved in water, with the exception of HF. All of these acids are dangerous; some are widely used in chemical manufacturing plants.
    • Metal halides are compounds of halogens and metals. They include highly ionic compounds, monomeric covalent compounds, and polymeric covalent compounds. Metal halides can be obtained through direct combination or through neutralization of a basic metal salt with a hydrohalic acid.
    • Interhalogen compounds are formed when halogens react with each other. Some resemble the pure halogens in some respects, but mostly their properties and behaviors are intermediates of those of the two parent halogens. Some properties, however, are found in neither parent halogen.
    • Halogenated compounds, or organic halides, are organic compounds that contain halogen atoms. In the human body, some halogens perform multiple regulatory functions, while others are not essential. Organohalogens are synthesized through the nucleophilic abstraction reaction.
    • Compounds substituted with multiple halogens are known as polyhalogenated compounds. Many of them are very toxic and bioaccumulate in humans, but they have many potential applications.

Hydrogen Halides

The halogens all form binary compounds with hydrogen, and these compounds are known as the hydrogen halides: hydrogen fluoride (HF), hydrogen chloride (HCl), hydrogen bromide (HBr), hydrogen iodide (HI), and hydrogen astatide (HAt). All of these except HF are strong chemical acids when dissolved in water. However, hydrofluoric acid does have quite destructive properties towards animal tissue, including that of humans.

When in aqueous solution, the hydrogen halides are known as hydrohalic acids. The names of these acids are as follows:

  • hydrofluoric acid
  • hydrochloric acid
  • hydrobromic acid
  • hydroiodic acid


All of these acids are dangerous and must be handled with great care. Some of these acids are also widely used in chemical manufacturing plants. Hydrogen astatide should also be a strong acid (hydroastatic acid), but it is seldom included in presentations about hydrohalic acids because of the extreme radioactivity of astatine (via alpha decay) and the fact that it readily decomposes into its constituent elements (hydrogen and astatine).

Metal Halides

The halogens form many compounds with metals. These include highly ionic compounds such as sodium chloride, monomeric covalent compounds such as uranium hexafluoride, and polymeric covalent compounds such as palladium chloride. Metal halides are generally obtained through direct combination or, more commonly, through neutralization of a basic metal salt with a hydrohalic acid.

Silver ChlorideSilver chloride is the precipitate formed when silver nitrate solution is added to chloride solution.

Interhalogen Compounds

The halogens react with each other to form interhalogen compounds. Diatomic interhalogen compounds such as BrF, ICl, and ClF bear resemblance to the pure halogens in some respects. The properties and behavior of a diatomic interhalogen compound tend to be intermediates of those of its parent halogens. Some properties, however, are found in neither parent halogen. For example, Cl2 and I2are soluble in CCl4, but ICl is not since it is a polar molecule (due to the electronegativity difference between I and Cl).

Organic Halides

Many synthetic organic compounds, such as plastic polymers, as well as a few natural organic compounds, contain halogen atoms; these are known as halogenated compounds, or organic halides. Chlorine is by far the most abundant of the halogens and is the only one needed (as chloride ions) in relatively large amounts by humans. For example, chloride ions play a key role in brain function by mediating the action of the inhibitory transmitter GABA. They are also used by the body to produce stomach acid. Iodine is needed in trace amounts for the production of thyroid hormones, such as thyroxine. On the other hand, neither fluorine nor bromine is believed to be essential for humans. Organohalogens are also synthesized through the nucleophilic abstraction reaction.

Polyhalogenated Compounds

Polyhalogenated compounds are industrially created compounds substituted with multiple halogens. Many of them are very toxic and bioaccumulate in humans, but they have many possible applications. Polyhalogenated compounds include the much publicized PCBs, PBDEs, and PFCs, as well as numerous other compounds.

Properties of the Halogens

Key Points

    • Halogens are nonmetals in group 17 (or VII) of the periodic table. Down the group, atom size increases. As a diatomic molecule, fluorine has the weakest bond due to repulsion between electrons of the small atoms.
    • Due to increased strength of Van der Waals forces down the group, the boiling points of halogens increase. Therefore, the physical state of the elements down the group changes from gaseous fluorine to solid iodine.
    • Due to their high effective nuclear charge, halogens are highly electronegative. Therefore, they are highly reactive and can gain an electron through reaction with other elements. Halogens can be harmful or lethal to biological organisms in sufficient quantities.

The halogens are a series of non-metal elements from group 17 of the periodic table (formerly VII). The halogens include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). The artificially created element 117 (ununseptium) may also be considered a halogen.

Physical Properties

Atoms get bigger down the group as additional electron shells are filled. When fluorine exists as a diatomic molecule, the F–F bond is unexpectedly weak. This is because fluorine atoms are the smallest of the halogens—the atoms are bonded close together, which leads to repulsion between free electrons in the two fluorine atoms.

The boiling points of halogens increase down the group due to the increasing strength of Van der Waals forces as the size and relative atomic mass of the atoms increase. This change manifests itself in a change in the phase of the elements from gas (F2, Cl2) to liquid (Br2), to solid (I2). The halogens are the only periodic table group containing elements in all three familiar states of matter (solid, liquid, and gas) at standard temperature and pressure.

Physical States of HalogensHalogens represents all of the three familiar states of matter: (left to right) chlorine is a gas, bromine is a liquid, and iodine is a solid. Highly reactive fluorine is not included in the picture.

Chemical Properties

Electronegativity is the ability of an atom to attract electrons or electron density towards itself within a covalent bond. Electronegativity depends upon the attraction between the nucleus and bonding electrons in the outer shell. This, in turn, depends on the balance between the number of protons in the nucleus, the distance between the nucleus and bonding electrons, and the shielding effect of inner electrons. In hydrogen halides (HX, where X is the halogen), the H-X bond gets longer as the halogen atoms get larger. This means the shared electrons are further from the halogen nucleus, which increases the shielding of inner electrons. This means electronegativity decreases down the group.

Halogens are highly reactive, and they can be harmful or lethal to biological organisms in sufficient quantities. This reactivity is due to high electronegativity and high effective nuclear charge. Halogens can gain an electron by reacting with atoms of other elements.

Fluorine is one of the most reactive elements. It reacts with otherwise inert materials such as glass, and it forms compounds with the heavier noble gases. It is a corrosive and highly toxic gas. Fluorine’s reactivity means that once it does react with something, it bonds so strongly that the resulting molecule is inert and non-reactive. Fluorine can react with glass in the presence of small amounts of water to form silicon tetrafluoride (SiF4). Thus fluorine must be handled with substances like the inert organofluorine compound Teflon.

Fluorine reacts vigorously with water to produce oxygen (O2) and hydrogen fluoride:

[latex]2 F_2 (g) + 2 H_2O (l) \rightarrow O_2 (g) + 4 HF (aq)[/latex]

Chlorine has maximum solubility of 7.1 g per kg of water at ambient temperature (21 °C). Dissolved chlorine reacts to form hydrochloric acid (HCl) and hypochlorous acid (HClO), a solution that can be used as a disinfectant or bleach:

[latex]Cl_2 (g) + H_2O (l) \rightarrow HCl (aq) + HClO (aq)[/latex]

Bromine has a solubility of 3.41 g per 100 g of water. It slowly reacts to form hydrogen bromide (HBr) and hypobromous acid (HBrO):

[latex]Br_2 (g) + H_2O (l) \rightarrow HBr (aq) + HBrO (aq)[/latex]

Iodine is minimally soluble in water, with a solubility of 0.03 g per 100 g water. However, iodine will form an aqueous solution in the presence of iodide ion. This occurs with the addition of potassium iodide (KI), forming a triiodide ion.

 

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