Halogens in the Periodic Table

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The Halogens in the periodic table correspond to the elements of Group 17 or Family VIIA. In Greek, the word halogen means “salt former”. Elements in Group 17 have the ability to form inorganic salts. Let’s learn more about the elements that make up this family.

What are Halogens?

The elements of family 7A of the periodic table are all non-metals, with 7 valence electrons and found naturally in the diatomic form (X 2 ). This group includes the elements Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), Astatine (As), and Tennessine (Ts).

The electronic distribution of all these elements is similar, according to their placement in family 7A of the table. In all cases, the distribution ends with the p sublevel, always with 5 electrons.

In general, they are not very abundant in nature in their diatomic elemental form. On the other hand, when they are associated with alkali or alkaline earth metals, forming inorganic salts, they are much more popular. This is the case of table salt, made up of an atom of chlorine and sodium (NaCl).

Characteristics of Halogens

Halogens are very important elements in the periodic table and have unique characteristics. Let’s see some of them:

  • They have high electronegativity. The group of halogens is considered the most electronegative in the periodic table, explained by its atomic structure. Among them, fluorine is the most electronegative of all existing elements;
  • They are strong oxidizers and react with other elements, mainly receiving electrons from family 1A elements but also with noble gases and other metals;
  • In the diatomic form, they form gases (fluorine and chlorine), liquids (bromine), and solids (iodine, astatine, and Tennessine);
  • They can form ionic bonds with other metals or covalent bonds when bonded to carbon, for example;
  • It has a tendency to receive an electron, that is, it results in a monatomic anion (X 1- ) called a halide;

There are still specific characteristics of these elements regarding ionization energy, atomic radius and boiling/melting points, but they vary from one element to another. Also, with the exception of iodine, these elements are toxic and can cause skin burns.

Properties of Halogens

Now, let’s discuss the elements of this group and what some of their applications are in everyday life.

Fluoride (F)

Fluorine is the first halogen most abundant in the Earth’s crust, and is the most electronegative of all the elements in the periodic table. In diatomic form, it is a corrosive and toxic gas. Atomic number (Z) = 9 and atomic mass (A) = 19 u. 

It forms hydrofluoric acid (HF), one of the strongest in existence. Fluoride is used in the pharmaceutical industry as a tranquilizer and to strengthen teeth, in addition to being present in drinking water treatment.

Chlorine (Cl)

Chlorine, like fluorine, is found in nature in its gaseous form.

Its atomic number is 17, 17 protons, and 17 electrons. Its atomic mass is 35.45 u.

It has the ability to extinguish harmful microorganisms (bacteria, fungi) present in water, as they have a disinfecting action.

Thus, they are used in water and sewage treatment, cleaning swimming pools, and industrial activities (paper manufacturing).

In addition, they are used in food. For example, sodium chloride (NaCl) is popular table salt. Its deficiency can cause problems in the thyroid gland.

Note that the chloride ion (Cl – ) is a necessary component for forming hydrochloric acid (HCl), an essential component of our gastric juice.

Bromine (Br)

Bromine is a volatile and not very stable element. In its diatomic form, it forms a reddish brown liquid with extremely toxic vapors to our health. It is highly oxidizing and used to manufacture medicines, insecticides, dyes, and disinfectants.

It has the atomic number 35 (35 protons and 35 electrons) and an atomic mass of 80 u.

This element, in aqueous presence, is considered a strong oxidant. It is found in a liquid state, and its toxicity can cause allergic reactions to the eyes, tissues, and throat.

It is widely used in industry, agriculture, in the manufacture of medicines, dyes, disinfectants, and insecticides. It is also used in some organic reactions, photographic developments, and gasoline additives.

Iodine (I)

Iodine is a halogen with the atomic number 53; that is, it has 53 protons and 53 electrons and an atomic mass of 126.9 u.

Iodine is sodium at room temperature that has the characteristic of sublimating. In solid form, it has a silvery metallic appearance, but in gaseous form, it is a purple-colored gas.

It has the lowest electronegativity among the most common halogens found in nature. Iodine deficiency in the body can cause problems in the thyroid gland. It is used as an antiseptic in an alcoholic iodine solution, capable of eliminating microorganisms.

In addition, the lack of this element for the human being can generate a disease called goiter. For this reason, iodine is added to sodium chloride (iodized table salt).

Astatine (At)

Astatine is a rare element (found in small amounts in nature) and is highly radioactive. It has atomic number 85 (85 protons and 85 electrons) and atomic mass of 210 u.

Astatine is no longer common, like its four predecessors, being an element found in trace amounts in the Earth’s crust; it is rare. It is solid and is considered the heaviest and most oxidizing of the halogens, with 5 oxidation states.

This rare element is used in the development of scientific research and little is known about its real effects.

Tennessee (Ts)

Z = 117 and A = 291 u (predicted mass). Tennessine is the most recently discovered halogen, first demonstrated in 2010 and confirmed by IUPAC in 2015. It is highly unstable and radioactive. It does not exist under normal conditions. Therefore, it needs to be synthesized in particle accelerators and only exists for short periods.

Finally, halogens are elements that have several applications and, at the same time, can be extremely dangerous to our health, especially when they are in their natural and diatomic form.

Reactivity of Halogens

The reactivity of halogens is an essential characteristic of these elements. Each of them behaves differently in similar situations, and, with this, it is possible to distinguish one element from another based on qualitative tests, as shown in the video. Watch the video and try to find out what the mysterious element is shown in the experiment is.

In summary, we have seen the elements that make up the halogen family and their applications and main characteristics. Don’t stop your studies here. See more about alkali metals, compounds that form inorganic salts with halogens.


Bachelor in Industrial Chemistry from Covenant University with experience in academic research in the area of ​​environmental ecology. M.Sc. (Petrochemical and Hydrocarbon Chemistry),(Manchester), Ph.D. (Chemistry) - Industrial/Applied( Manchester).