What Are Halogens?
Halogen is a term that was derived from the Greek word ‘halo’ which means salt and ‘gen’ meaning producer. The term halogen therefore means salt producer. This is because they produce a wide range of salts when they react with metals. The name halogen was first proposed by a German chemist by the name Johann Schweigger in 1881 Examples of halogens include substances such as fluorine (F), chlorine (Cl), bromine (Br), iodine (I) astatine (At) and tennessine (Ts). The names of all these elements were derived from Greek words with the exception of tennessine which is named after Tennessee, a state in the US. Fluorine is the most abundant of all the halogens in the earth’s surface.
Properties of Halogens
Physical Properties of Halogens
- The group of halogens is the only periodic table group that contains elements in all three familiar states of matter at standard temperature and pressure
- Fluorine (F) is a pale yellow gas
- Chlorine (Cl) is a greenish gas
- Bromine (Br) is a dark red liquid
- Iodine (I) is a black solid and when heated it forms a purple vapour
- Astatine (At) is a black solid
- The halogens all have a strong and often nasty smell
- The halogen elements are extremely toxic
- Poor conductors of heat and electricity
- Low melting and boiling points
Halogens are generally non-metals belonging to Group VII of the periodic table. The modern IUPAC nomenclature places these chemically related elements in group 17. Halogens are non conductors of heat and electricity because there are no delocalized electrons in their structures
They have seven electrons in the outer energy level. The atomic and ionic radius of the halogens increases down the group because there is an increase in the number of occupied energy levels. However, the atomic radius of a halogen atom is less than the radius of its ion. This is due to the repulsive effect that exists between the existing electrons and the incoming electron resulting in the outer electrons moving further causing an increase in the size of the ion.
When it comes to appearance, fluorine is pale yellow in color, chlorine is greenish-yellow or pale green in color, bromine has a dark reddish-brown appearance while iodine has a shinny purple appearance when in solid form and forms a very dark violet appearance when it’s melted. Very little is known about the artificially produced tennessine element.
Fluorine and chlorine exist as gases at room temperature. Iodine and astatine are solids at room temperature while bromine is a liquid. Astatine is believed to be dark to black in color.
Halogens such as Fluorine are generally insoluble in water. Chlorine, bromine and iodine are slightly soluble in water since water is polar while these halogens are non-polar. Water molecules have stronger intra-molecular forces whereas halogens have weak Vander Waals forces. All halogens are soluble in tetrachloromethane. Solubility of halogens decreases down the group. Chlorine for example, is more soluble in water than bromine and iodine. All halogens are more soluble in non-polar solvents than in water.
When heated, iodine sublimes to form a purple vapor mainly because its particles are held by weak forces which require little energy to break. The melting and boiling point of halogens increases down the group since their molecular size increases and more heat will be required to overcome the strong Vander Waals forces.
The density of halogens increases down the group due to an increase in atomic mass.
Chemical Properties of Halogens
- Molecules of all halogens are homonuclear diatomic. What this means is that their molecules exist with two atoms each.
- Halogens have seven valence electrons because halogens have one electron missing, they form negative ions and are highly reactive.
- They can gain an electron by reacting with atoms of other elements.
- Fluorine is one of the most reactive elements in existence.
- They have relatively weak intermolecular forces.
Halogens react by gaining one electron to attain a stable electron configuration and form negatively charged ions (halides). This is because they require very little amount of energy to gain an electron than to lose electrons. This oxidizing ability decreases down the group as the electron affinity decreases. Fluorine is therefore the stronger oxidizing agent in the group due to its low bond energy.
Halogens are highly reactive due to their high electronegativity. Their electron affinity decreases down the group as the size of the atoms increases. Reactivity of halogens decreases down the group because of this. As such, fluorine is the most reactive of all the halogens. It is commonly handled using Teflon since it is highly resistant to thermal and chemical attacks. Other substances used to handle it include dry glass and metals such as steel and copper.
They react with hydrogen to for hydrogen halides such as hydrogen fluoride, hydrogen chloride, hydrogen bromide, hydrogen iodide and hydrogen astatide. Hydrogen halides are usually colorless gases at room temperature and dissolve in water to form acids. These acids include; hydrofluoric acid, hydrochloric acid, hydrobromic acid, hydroiodic acid and hydroastatic acid.
A reaction between halogens and metals results in the formation of salt or metal halides such as ferric chloride, sodium chloride, zinc bromide and zinc iodide.
A displacement reaction occurs when a more reactive halogen is placed into a solution containing a less reactive halide. For example, fluorine would displace halides belonging to the halogens.
Use of Halogens
- Bromine and chlorine are often used as disinfectants for purifying water, swimming pools, fresh wounds, dishes and sterilizing surfaces.
- Small amounts of a halogen such as iodine or bromine are found in Halogen lamps.
- Fluoride is found in products such as toothpaste, baby formulas and vitamin supplements.
- Chlorine accounts for about 0.15 percent of human body weight and plays several important roles in the body’s functioning. Compounds of both chlorine and bromine are used as disinfectants for sterilization.
- Fluoride anions are found in some quantity in different organisms. It is essential for humans. Iodine is also another compound.
- Halogen atoms are mostly lipophilic and less water-soluble. Thus, it has been used in drug components to provide improved penetration through lipid membranes and tissues. However, halogenated drugs can pile up in adipose tissue.
- Polyhalogenated compounds (PHCs) are used in a wide array of manufactured products and in pest control.
Further References
- Halogens: https://en.m.wikipedia.org/wiki/Halogen
- Group 7 Elements: http://wwwchem.uwimona.edu.jm/courses/CHEM3101/Halogens.html