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2.55: Halogens

Difficulty Level: At Grade Created by: CK-12
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You’ve probably seen halogen lights like the ones pictured here. You may even have halogen lights in your home. If you do, you may have noticed that they get really hot and give off a lot of light for their size. A halogen light differs from a regular incandescent light bulb in having a small amount of halogen gas inside the bulb. The gas combines chemically with the metal in the filament, and this extends the life of the filament. It allows the lamp to get hotter and give off more light than a regular incandescent light without burning out quickly. What is halogen gas, and which elements are halogens? In this article, you’ll find out.  

Meet the Halogens

Halogens are highly reactive nonmetallic elements in group 17 of the periodic table. As you can see in the periodic table below, the halogens include the elements fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). All of them are relatively common on Earth except for astatine. Astatine is radioactive and rapidly decays to other, more stable elements. As a result, it is one of the least common elements on Earth.

Q: Based on their position in the periodic table from the Figure above, how many valence electrons do you think halogens have?

A: The number of valence electrons starts at one for elements in group 1. It then increases by one from left to right across each period (row) of the periodic table for groups 1–2 and 13–18 (numbered 3-0 in the periodic table above.) Therefore, halogens have seven valence electrons.

Chemical Properties of Halogens

The halogens are among the most reactive of all elements, although reactivity declines from the top to the bottom of the halogen group. Because all halogens have seven valence electrons, they are “eager” to gain one more electron. Doing so gives them a full outer energy level, which is the most stable arrangement of electrons. Halogens often combine with alkali metals in group 1 of the periodic table. Alkali metals have just one valence electron, which they are equally “eager” to donate. Reactions involving halogens, especially halogens near the top of the group, may be explosive. You can see some examples in the video at the URL below. (Warning: Don’t try any of these reactions at home!)

http://www.youtube.com/watch?v=mY7o28-l_WU (1:00)

Physical Properties of Halogens

The halogen group is quite diverse. It includes elements that occur in three different states of matter at room temperature. Fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids. Halogens also vary in color, as you can see in the Figure below. Fluorine and chlorine are green, bromine is red, and iodine and astatine are nearly black. Like other nonmetals, halogens cannot conduct electricity or heat. Compared with most other elements, halogens have relatively low melting and boiling points. You can watch a lab demonstration of the melting and boiling points of halogens at this URL: http://www.youtube.com/watch?v=yP0U5rGWqdg.

Uses of Halogens

Most halogens have a variety of important uses. A few are described in the Figure below.

Q: Can you relate some of these uses of halogens to the properties of these elements?

A: The ability of halogens to kill germs and bleach clothes relates to their highly reactive nature.


  • Halogens are highly reactive nonmetal elements in group 17 of the periodic table.
  • Halogens include solids, liquids, and gases at room temperature, and they vary in color.
  • Halogens are among the most reactive of all elements. They have seven valence electrons, so they are very “eager” to gain one electron to have a full outer energy level.
  • Halogens have a variety of important uses, such as preventing tooth decay and killing germs.


  • halogen: Nonmetal in group 17 of the periodic table that has seven valence electrons and is highly reactive.


The Table below gives the melting and boiling points of halogens. Create a graph with the data, and then describe in words the trends that you see in your graph.

Melting and Boiling Points of Halogens
Halogen Melting Point (°C) Boiling Point (°C)
Fluorine -220 -188
Chlorine -101 -35
Bromine -7 58
Iodine 114 184
Astatine 302 337


  1. What are halogens?
  2. Why are halogens very reactive?
  3. Describe the physical properties of halogens.
  4. Why is chlorine added to swimming pool water?

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halogen Nonmetal in group 17 of the periodic table that has seven valence electrons and is highly reactive.

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Difficulty Level:
At Grade

Concept Nodes:

7 , 8
Date Created:
Oct 31, 2012
Last Modified:
Sep 13, 2016
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