Resources | Subject Notes | Chemistry
Group 17, also known as the halogens, is a group of non-metal elements located in Group 17 of the periodic table. The elements in this group are fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). They are highly reactive and exhibit characteristic chemical properties due to their electronic configuration.
The electronic configuration of the halogens is $ns^2 np^5$. This means they have seven valence electrons and require only one more electron to achieve a stable octet. This strong tendency to gain an electron makes them highly electronegative. Electronegativity values increase down the group: F < Cl < Br < I < At. Fluorine is the most electronegative element.
Halogens readily form covalent bonds with other non-metals. They can also form ionic bonds with metals.
The reactivity of the halogens decreases down the group. This is due to the increasing atomic size and weaker effective nuclear charge. The smaller the atom, the stronger the attraction between the nucleus and the valence electrons, making it easier to gain an electron.
| Element | Reactivity with Alkali Metals | Reactivity with Hydrogen |
|---|---|---|
| Fluorine (F) | Very reactive | Very unreactive |
| Chlorine (Cl) | Reactive | Requires a catalyst |
| Bromine (Br) | Reactive | Requires a catalyst |
| Iodine (I) | Less reactive | Requires a catalyst |
| Astatine (At) | Least reactive | Requires a catalyst |
Halogens react directly with most metals to form ionic halides. The reaction is generally exothermic. For example:
$$2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)$$Halogens react with hydrogen to form hydrogen halides (HX). The reactivity of the hydrogen halide decreases down the group. For example:
$$H_2(g) + Cl_2(g) \rightarrow 2HCl(g)$$Fluorine reacts with water, but the reaction is slow and vigorous. Other halogens react with water to form hydrohalic acids (HX). The acidity of these acids increases down the group. For example:
$$HCl(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + Cl^-(aq)$$Alkenes and alkynes react with halogens in an addition reaction. The rate of addition decreases down the group. For example, the addition of chlorine to an alkene is faster than the addition of iodine.
Atomic radius increases down the group due to the addition of electron shells.
Ionization energy increases down the group. This is because the valence electrons are held less tightly due to the larger atomic radius and weaker effective nuclear charge.
Electron affinity increases down the group. This is because the ability to attract an electron decreases down the group.
Rarely used due to its radioactivity. Some research is being conducted into its potential medical applications.
Halogens are corrosive and toxic. They should be handled with care in a well-ventilated area. Appropriate personal protective equipment (PPE) such as gloves and goggles should be worn.