The halogens are a periodic table group of elements. They are found on the righthand side of the periodic table, just to the left of the noble gas group. The halogens are group VII or 7 in older nomenclature and group 17 in modern IUPAC nomenclature. Here is a list of the halogens and a look at their properties, uses, and biological role.
List of Halogen Elements
There are six halogen elements:
- Fluorine (F) – Atomic number 9
- Chlorine (Cl) – Atomic number 17
- Bromine (Br) – Atomic number 35
- Iodine (I) – Atomic number 53
- Astatine (At) – Atomic number 85
- Tennessine (Ts) – Atomic number 117
Halogen Properties
The halogens share several common properties:
- All of the halogens are nonmetals. They are poor conductors of heat and electricity and form brittle solids.
- Atoms of halogen elements have seven valence electrons in their outer shell. This is one less electron than needed for a full valence shell, so their usual oxidation state is -1.
- Because of their electron configuration, the halogens are highly reactive. They readily bond with metals, particularly the alkali metals. The group name “halogen” means “salt-producing” because the halogens react with metals to form salts.
- The halogens are highly electronegative. Fluorine is the most electronegative element. Electronegativity decreases moving down the group on the periodic table.
- Similarly, the halogens display high electron affinities.
- The melting and boiling points of the halogens increase as you move down the periodic table. Fluorine and chlorine are gases at room temperature. Bromine is a liquid. Iodine and astatine are solids. Scientists predict tennessine is a solid. This makes the halogen group the only element group that contains all three normal states of matter at room temperature and pressure.
Halogen Uses
The halogens have many uses. Chlorine and bromine are disinfectants used to treat wounds, clean surfaces, and protect pools and spas. These two elements are widely used as flame retardants. Chlorine is also used in bleach. Iodine and bromine are used in halogen lamps, which glow with a whiter color than other incandescent lights. Halogens are used in drugs. Astatine and iodine isotopes find use in nuclear medicine.
Biological Role
Fluorine, chlorine, bromine, and iodine all occur in the human body. Astatine is rare and not found in living organisms. Tennessine is not found in nature.
Chlorine is an essential element for plants and animals. It is mainly used as an ion. The average 70-kilogram person contains about 95 grams of chlorine.
Fluorine is found in bones, teeth, hair, blood, urine, and eggs. It’s possible trace amounts of this element are essential for human nutrition. A typical 70-kg human contains between 3 and 6 grams of fluorine.
Bromine occurs in all organisms. No known biological role in humans is known, but a person consumes 1 to 20 milligrams of the element each day. A 70-kg human contains about 260 mg of bromine.
Iodine is essential for human and animal nutrition, but serves no biological role in plants. The are 10 to 20 milligrams of iodine in an average 70-kg person.
Humans and animals exposed to astatine accumulate it in the thyroid (like iodine), lungs, spleen, and liver. Its radioactivity damages cells. Exposure to tennessine would be dangerous because of its radioactivity.
While several halogens are essential for life, the body uses them as ions. They can be toxic in pure form as diatomic elements. Toxicity decreases moving down the element group, with fluorine being the most toxic halogen and bromine the least toxic. The main issue with the heavier elements is their radioactivity.
References
- Emsley, John (2011). Nature’s Building Blocks. ISBN 978-0199605637.
- Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
- Lide, D. R., ed. (2003). CRC Handbook of Chemistry and Physics (84th ed.). Boca Raton, FL: CRC Press.