# General Chemistry/Chemistries of Various Elements/Group 17

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## Halogens

The halogens are found in Group 17(VIIA). The halogens are fluorine, chlorine, bromine, iodine, and astatine.

Like the alkali metals, the halogens are extremely reactive. They have seven valence electrons, meaning they require only one more electron for a noble configuration. This gives them very large electron affinities and extreme reactivity to form ions with a -1 charge. They are so reactive that in their homogeneous state, UV light will catalyze a radical reaction.

The halogens exist in diatomic form. Under normal conditions, they will always occur in pairs, covalently bonded. The covalent bond allows them to share an electron and possess a complete octet. F2 is a pale yellowish-brown gas. It is highly reactive, causing organic compounds and hydrogen gas to explode, even without a spark. Cl2 is a pale yellow-green gas. It reacts with water to form disinfectants and bleaches. Br2 is a reddish-brown liquid, but, being volatile, it readily evaporates into a reddish vapor. I2 is a gray solid that forms a violet gas if heated.

Fluorine is the most electronegative of all elements, and it is so reactive that it attacks almost any other element (noble gases, oxygen, nitrogen, and gold are the exceptions) to form fluorides. Chlorine is somewhat less reactive, bromine somewhat less reactive than chlorine, and iodine even less, but even iodine is a formidable ionizer. Extreme radioactivity masks the chemical properties of astatine. With increasing atomic weight for these elements, the elements have higher boiling and melting points. At normal temperatures, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid.

## Reactions

 $2 \hbox{F}_{2(g)} + 2 \hbox{H}_2\hbox{O}_{(l)} \to \hbox{O}_{2(g)} + 4 \hbox{HF}_{(aq)}$ $\hbox{Cl}_{2(g)} + \hbox{H}_2\hbox{O}_{(l)} \to \hbox{HCl}_{(aq)} + \hbox{HClO}_{(aq)}$ $\hbox{Br}_{2(g)} + \hbox{H}_2\hbox{O}_{(l)} \to \hbox{HBr}_{(aq)} + \hbox{HBrO}_{(aq)}$ The halogens in diatomic form react with water to produce acids. Iodine does not react with water and is only slightly soluble. Chlorine also has low solubility, but it will react in water to form hypochlorous acid and hydrochloric acid. $\hbox{X}_2 + \hbox{H}_{2(g)} \to 2 \hbox{HX}_{(g)}$ In chemical reactions, an X is used to symbolize any halogen element. All halogens form gaseous compounds with hydrogen: hydrogen fluoride HF, hydrogen chloride HCl, hydrogen bromide HBr, and hydrogen iodide HI. These are acidic, strongly reactive substances called hydrogen halides. $\hbox{HX}_{(g)} + 2\hbox{H}_2\hbox{O}_{(g)} \to \hbox{H}_3\hbox{O}^+_{(aq)} + \hbox{X}^-_{(aq)} + \hbox{H}_2\hbox{O}_{(l)}$ When hydrogen halides are dissolved in water, they are known as hydrohalic acids. Except for hydrogen fluoride, they are among the strongest known acids. $2 \hbox{M}_{(s)} + \hbox{X}_{2(g)} \to 2 \hbox{MX}_{(s)}$ $\hbox{HX}_{(g)} + \hbox{NH}_{3(g)} \to \hbox{NH}_4\hbox{X}_{(g)}$ These reactions show reactions with metals and ammonia gas to form salts and ammonium halides, respectively. $\hbox{SiCl}_4 + 2\hbox{H}_2\hbox{O} \to \hbox{SiO}_2 + 4 \hbox{HCl}$ Many non-metallic halides react with water to release hydrogen halides. For example, silicon chloride and water react.

## Other Compounds

Halides of metals are known as salts. Sodium chloride, better known as "table salt", is the crystalline substance often used to enhance the flavor of food. Note, however, that not all salts are halides (for example, sodium sulfate Na2SO4), and not all halides are salts (carbon tetrachloride, CCl4).

Interhalogens are molecules composed of two or more different halogen atoms. They are similar to the diatomic halogens. Some examples are chlorine monofluoride ClF and bromine monochloride BrCl. There are many others, and they are all very reactive and somewhat unstable. Interhalogens take the form XYn, where n is 1, 3, 5, or 7. X and Y are both halogens, X being the less electronegative.

Noble gas compounds have been formed using fluorine. Although noble gases are supposedly inert, the larger ones like xenon will form covalent bonds with a very electronegative element like fluorine. Xenon difluoride XeF2, xenon tetrafluoride XeF4, and xenon hexafluoride XeF6 are among the noble gas compounds that have been created.