What are the Halogens
The halogens, aka halogen family, are a group of reactive elements in group 17 of the periodic table, to the right of the chalcogens, and to the left of the noble gases. Fluorine and chlorine are the “poster children” of the halogens. They are non-metals that consist of diatomic molecules.
Halogen means “salt-producing”. They are salt-producing, because when they react with metals (often violently), they produce ionic compounds known as salts. In 1811 John Schweigger wanted to name the element chlorine “halogen”, but he failed. In 1826, Swedish chemist Jons Berzelius coined the term halogen for the entire group of elements. The Greek word “hal”, meaning salt, also appears in the name of the mineral halite, aka sodium chloride.
Properties of the Halogens
The halogens are highly reactive and highly toxic. Breathing in vapors of any of the halogens is very dangerous, and they have distinctive, unpleasant odors (although the author finds a faint smell of bromine oddly interesting).
Fluorine is the most reactive of all elements, and can only be stored in metals that form passivation layers of the fluoride salt. These unique properties are because of the 7 electrons in their valence shell. Because of their reactivity, there are almost never found in nature in their elemental form.
Moving down the column, halogens increase in atomic radius as they add additional electron shells. In the elemental state, they form diatomic molecules, joined by nonpolar covalent bonds. The fluorine-to-fluorine bond is the weakest
The boiling points also increase due to increased Van Der Waals forces. So fluorine is a gas, and iodine is a solid. It is the only group on the periodic table that has elements in the solid, liquid and gas phase at room temperature. All of the halogens have a high electronegativity, but it decreases as you move down the column. In fact, fluorine has the highest electronegativity of any element.
The ionization energy also decreases as you move down, making it easier to form higher oxidation states. The first ionization energy of fluorine is so high, that it does not exhibit any positive oxidation states.
The halogens all have an interesting color, which can change depending on the medium or what it is dissolved in, see a detailed explanation here.
List of all Halogens
The halogen family includes fluorine, chlorine, bromine, iodine, astatine, and tennessine, element 117 – the most recent element to be discovered. In this article, we focus just on the first four halogens.
The halogens react violently with alkali metals, alkaline earth metals, and aluminum, especially when there is a large surface area of metal. They also form anions like the hypochlorite, chlorate, perchlorate, and bromate ions that are very strong oxidizing agents. Fluorine commonly occurs in minerals like fluorite, CaF2. Halogens also form many different interhalogen compounds, for example IF7, iodine (VII) fluoride.
Why are they so reactive?
Halogens are so reactive, because they have 7 valence electrons, and are very close to having a complete shell of 8 electrons. The halogens will rip an electron from another atom, in order to achieve a very stable state of 8 electrons in their outer shell.
Fun fact about the Halogens
- Many rare-earth fluorides are insoluble in water
- Students learn that silver (I) chloride is insoluble, but most people don’t know that copper (I) chloride is also insoluble in water
- Bromide and bromate ions are key ions in a famous and beautiful oscillating reaction known as the Belousov–Zhabotinsky reaction
- Iodate ions play a key role in many “clock” reactions used for chemical demonstrations
- Astatine is the rarest element that occurs naturally in the earth’s crust. A sample of pure astatine has never been assembled. All isotopes are radioactive and short-lived.
- Although hydrofluoric acid, HF, is a weak acid, it is one of the most dangerous acids to be around. It not only damages the body on the outside but it can be absorbed through the skin, penetrating deep into tissues causing severe systemic injury
- Chlorine gas killed thousands of soldiers during world war I in 1915, when used as a chemical weapon. Ok that is not a very fun fact.
- Iodine will sublime directly from a solid into a gas. It will also enter the liquid state at 114C if heated carefully in a vessel that is not too “open”
- The hypochlorite ion, ClO–, is the active ingredient in bleach.