A Level Chemistry Revision: Inorganic Chemistry – Group 7(17), The Halogens

The halogens are the six nonmetallic, highly reactive elements under Group VIIa (column 17) of the periodic table. They’re very strong oxidising agents, which means they take electrons from other substances. They’re also very reactive and usually form salts with Group 1a, i.e. the alkali metals. The name halogen literally means salt-producing. Halogens in elemental form do not exist in nature because they’re so highly reactive.

Many compounds of halogens are also very reactive and have oxidising properties, making them ideal as disinfectants. Many laundry bleaching products, for example, have halogen-based compounds, such as sodium hypochlorite. Halogen compounds have a wide range of applications, including water sanitation, paper production, plastic polymer synthesis, and petroleum refining.

What Are the General Properties of Halogens?

Just like other groups in the periodic table of elements, the six elements in column 17 have similar chemical and physical properties. This is mainly because of their atomic structures and electron configuration.

Chemical Properties

Halogens have seven valence electrons in their outermost energy level, which can be denoted by ns2np5. Additionally, they all have an ‘s’ (spherical shape) and a ‘p’ (dumbbell shape) orbital in the outermost shell. All halogens are highly electronegative due to the high effective nuclear charge of the elements.

Electronegativity measures the ability or tendency of an atom to attract electrons in either shared covalent bonds or in ionic bonds. Therefore, higher electronegativity means stronger reactivity. As elements, halogens exist as diatomic molecules (molecules made up of two atoms). However, as the atomic radius increases, the stability of the diatomic bonds decreases.

Fluorine is the most reactive element of all halogens. In fact, it can be argued that it’s the most reactive naturally-occurring element that we know of. It can even react with materials that are considered inert, like glass. With the presence of a small amount of water, fluorine can react with its glass container, forming silicon tetrafluoride, or SiF4.

Various chemicals can be formed by the reaction of halogens with other elements and compounds. Their products can be categorised in the following ways:

  • Hydrogen halides: All compounds formed with the reaction between hydrogen and a halogen. Examples of these are hydrogen fluoride, hydrogen chloride, and hydrogen bromide. These are essentially salts but, when dissolved in water, they become acids, namely hydrofluoric acid, hydrochloric acid, and hydrobromic acid.
  • Metal halides: These are compounds formed with many types of metals, including alkali metals and radioactive metals such as uranium (uranium hexafluoride). The bonds can be highly ionic, such as those formed with alkali metals. e.g. sodium chloride. Halogens can also form polymeric covalent compounds with metals like palladium chloride. Generally, halogens can react directly with metals, but they can also react via their acidic compounds.
  • Interhalogen compounds: Halogens can also react with other halogens to form compounds. Some examples include bromine fluoride (BrF), iodine monochloride (ICl), and chlorine monofluoride (ClF).
  • Organic halides: Many of these compounds are synthetic, like plastic polymers, but a few are also naturally occurring. Some also have essential roles in biological functions. In humans, for instance, chloride ions are necessary for brain functionality. Similarly, iodine is needed in very small amounts to synthesise thyroid hormones.
  • Polyhalogenated compounds: These are synthetic compounds with multiple halogen constituents. They’re industrially formed as waste products that are very toxic to humans and other animals. However, some have potential practical applications.

Refer to this pattern in the periodic table to compare the trends in the ionisation energy, electron affinity, atomic radius, and the metallic/non-metallic characteristics of the elements: 

As you can see, the electron affinity and ionisation energy of the halogens tend to decrease as you move down the column, which is highlighted in pale blue on the right hand side of the table:

Periodic table colour coordinated into groups

Physical Properties of Halogens

At room temperature, halogens vary in state of matter. For example, fluorine is a gas, bromine is liquid, and iodine is a crystalline solid. Astatine has never been assembled at macroscopic scale but is probably semi metallic or metallic. Meanwhile, tennessine is a highly transitory synthetic element with just a few milliseconds of half-life.

All halogens are highly reactive because of their electron configuration. They’re also corrosive and poisonous, making them ideal disinfectants…but also potential chemical weapons. 

In elemental form, halogens must be kept in inert containers like sealed glass flasks. The cover must also be glass. Typical cork or rubber stoppers will not work because halogens can corrode the cellulose of cork and polymers of rubber.

As you go down the column of the group, the boiling points of the elements increase because the Van der Waals forces between the molecules also increase, along with the size of the atoms and their respective mass. There’s also a correlation between the atomic weight and the boiling point of an element.

In terms of the first ionisation energy in kilojoules per mole, the required energy tends to decrease as you go down the column. For example, the first ionisation energy of chlorine is 1,251.20 kJ/mol while the first ionisation energy of iodine is 1,008.40 kJ/mol. Again, there is a clear correlation here between the atomic weight and the first ionisation energy. But why?

The first ionisation energy refers to the amount of energy required to remove the loosely held electrons of one mole of neutral gaseous atoms in order to yield one more of gaseous ions of the lament with a particle charge of positive one (1+).

As the atomic size and weight of the element increase, the diatomic bonding between the atoms in a molecule of halogen decreases. This means that the bonds become easier to break and the single atoms also become easier to ionise.

Illustration showing the chemical properties of the halogens

What Are the Elements in the Halogen Group?

Group VIIa(17) has five naturally-occurring elements and one synthetic element. While the halogens have similar chemical and physical properties, the synthetic member of the group, tennessine, has some properties that are dissimilar to the halogens. For example, it’s solid and semi metallic at room temperature.

Here’s some key information about the elements in the halogen group:

  • Fluorine (F): Fluorine is the lightest element among the halogens, with the atomic number 9 and an atomic weight of 18.998403163. It’s also the most reactive element in the group, and the entire periodic table. It’s a diatomic molecule in elemental form.
  • Chlorine (Cl): Like fluorine, chlorine also exists as a diatomic molecule in elemental form. Its atomic number is 17 and its atomic weight is 35.453. It’s the twentieth most abundant element on earth and has several practical uses, but its main application is as a disinfectant. It’s a yellow-green gas at room temperature.
  • Bromine (Br): Bromine’s atomic number is 35, and its atomic weight is 79.904. It’s a dark, brownish liquid at room temperature. When heated above its boiling point to become a gas, its molecules become diatomic. It’s used in the synthesis of many organic compounds.
  • Iodine (I): At room temperature, iodine forms dark purple crystals. It’s also diatomic in gaseous form. It has the atomic number 53 and an atomic weight of 126.90447. It has an important biological role as part of a hormone. Tincture of iodine is used as antiseptic for wounds and surgical procedures.
  • Astatine (At): Astatine a radioactive element that occurs naturally as a transitory elemental product of nuclear decay of heavier elements. It has the atomic number 85 and an atomic weight of 210. Its most stable isotope has a half-life of only 8.1 hours. Its physical properties, like its state of matter at room temperature, can only be interpolated because the samples are microscopic and unstable. Any macroscopic sample would be immediately vaporised by its own radioactive heat. Some studies suggest that it might be useful in cancer treatment.
  • Tennessine (Ts): Tennessine is the heaviest element in the group and the second heaviest element in the periodic table. Its atomic number is 117 and its atomic weight is 294. This element doesn’t occur naturally and can only be synthesised in particle accelerators – but it lasts momentarily and only in trace, microscopic amounts. Its most stable isotope has a half-life of 80 milli-seconds. Therefore, like astatine, its physical properties can only be interpolated.

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