Halogens are a group of elements present in the periodic table. These elements can remain active at room temperature in three to four different states, i.e., liquid, solids and gases. The name halogen is derived from the Greek roots hal- (“salt”) and -gen (“to make”).

They belong to the Group 17 of the periodic table and consist of six nonmetallic elements (Group VII a). In addition to  Fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and Tennessee (Ts), there are other halogens. They all produce salts of sodium, halite or sodium chloride, commonly known as table salt.

In nature, free elements of halogens do not exist because of their reactivity. Earth’s crust contains the highest amount of fluorine,  a pale yellow gas, in combination with the other halogens, that is, 0.061 Chlorine (Cl), a greenish gas, 0.00003 iodine and 0.00016 bromine in the igneous rocks of Earth’s crust. Histamine and tennessine are radioactive isotopes that do not exist in nature.

Physical Properties

1. Halogens have remarkably high electronegativity.
2. A stable octet consists of eight electrons (seven valence electrons).
3. Alkali metals and alkaline Earth are highly reactive with them. The non-metals with the highest halogen content are halogens.
4. Since elemental halogens are reactive, they can be toxic and lethal. Until astatine, which is radioactive, the toxicity decreases with heavier halogens.
5. As you descend the group, the state of matter changes. Iodine and astatine are solids, while fluorine (F) is a pale yellow gas, and chlorine is green gas. Bromine is a dark red liquid. Element 117 is expected to be solid under normal conditions as well. Since the Van der Waals force increases with increased size and atomic mass, the boiling point increases further down the group. 

Chemical Properties

1. Powerful oxidising properties:- Halogens are potent oxidising agents. All halide particles in a solution can be oxidised to halogen by  fluorine (F), a pale yellow gas. Depending on the group, oxidising power decreases.
2. When hydrogen halides react with halides, acidic hydrogen halides are formed as we move down group 17, and halogen’s reactivity and acidity decreases.
3. Halogen oxides are formed when halogen combines with oxygen, but they do not remain stable. X2O to X2O7 are the general formula for oxides.
4. Reaction with metals:-  Halogens are highly reactive, so they react instantly with metals to form metal halides. For example, sodium reacts with chlorine (Cl), a greenish gas, to form sodium chloride, emitting a lot of heat energy and yellow light.

Halogen Compounds

The fact that Halogens are highly reactive is one of their most defining characteristics. The nature of these compounds allows them to form different combinations, such as halides, polyhalogenated compounds, and inter-halogenated compounds. We will briefly discuss these below.

Hydrogen Halides

Halogens and hydrogen react to form binary compounds. Generally, if we take fluorine, chlorine, and Bromine (Br), the reaction appears as follows:

X2 + H2 → 2HX

Whenever hydrogen halides are dissolved in water, they form hydrohalic solid acids. This can cause severe damage.

Metal halides

Halogens react with metals to form these compounds. There are two types of metal halides: highly ionic combinations and polymeric covalent compounds. An essential metal salt can be directly combined with hydrohalic acid or neutralised.

Interhalogen Compounds

Interhalogen compounds are formed when halogens react with one another. They exhibit properties and behaviours intermediate between those of the two-parent halogens. They may, however, exhibit some differences. The direct combination of pure halogens into interhalogens, except for IF7, can result in all interhalogens except IF7.

Halogenated/Organohalogen Compounds

Halides are organic compounds containing oxygen. Inorganic compounds containing halogen atoms can be categorised as halides. The nucleophilic abstraction reaction is usually used to manufacture these compounds.

Polyhalogenated Compounds

Polyhalogenated compounds are those that contain more than one halogen. Industrially created compounds, many of which are toxic and bioaccumulative in humans, such as PFCs, PCBs, and PBDEs, are examples of polyhalogenated compounds.

Uses of Halogens

• Bromine and chlorine are often used while disinfecting water, swimming pools, wounds, dishes, and surfaces.
• Halogen lamps contain small amounts of halogens such as iodine or bromine.
• Products such as toothpaste, baby formulas, and vitamin supplements may contain fluoride.
• In the human body, chlorine makes up about 0.15 percent of body weight and plays a pivotal role. 
• A halogen atom is primarily lipophilic and water-soluble. Thus, it has been used to improve the penetration of drug components through lipid membranes and tissues. Halogenated drugs, however, can accumulate in the adipose tissue.
• Various manufactured products and pest control products contain polyhalogenated compounds (PHCs).

Conclusion

A halogen is a nonmetal belonging to group 17 (or VII) of the periodic table. As atom sizes decrease in the periodic table, halogens become smaller. Due to the repulsion between electrons of the small atoms in fluorine (F), a pale yellow gas, it has the weakest bond of all diatomic molecules. The boiling points of halogens increase due to increased Van der Waals forces down the group. Therefore, fluorine changes from a gaseous state to solid iodine as we move down the group. Halogens are highly electronegative because their nuclear charges are high.