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2.55: Halogens

Difficulty Level: At Grade Created by: CK-12
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How do you study a gas that does not exist as such in nature?  It’s not as easy as you think.  Fluorine is so reactive that we cannot find it free.  None of the halogens exist free in nature (unlike some of the metals suchas gold and silver) because they are very reactive.  The video below shows how violently elemental fluorine reacts with other materials.



Some elements are much more reactive than others. The Group I (red)  and Group II (tan) elements can easily lose electrons during a reaction.  Elements of other groups are much more likely to accept electrons as they react.

The elements of Group VII (new Group 17 - fluorine, chlorine, bromine, iodine, and astatine) are called the halogens (tan column).  The term “halogen” means “salt-former” because these elements will readily react with alkali metal and alkaline earth metals to form halide salts. The halogens all have the general electron configuration \begin{align*}ns^2np^5\end{align*}, giving them seven valence electrons. They are one electron short of having the full outer \begin{align*}s\end{align*} and \begin{align*}p\end{align*} sublevel, which makes them very reactive.

Physical Properties of Halogens

As elements, chlorine and fluorine are gases at room temperature, bromine is a dark orange liquid, and iodine is a dark purple-gray solid. Astatine is so rare that its properties are mostly unknown. In the picture below we see chlorine gas on the left (green), bromine solid and vapor in the middle (orange), and solid iodine (grey) on the right. Fluorine is not shown in the picture below because it is too corrosive and will destroy the glass container.

None of these elements are found free in nature because of their reactivity. Fluorine is found in combination with cations in several minerals.  Chlorine is found in table salt, in the oceans (which are about 2% chloride ion by weight) and in lakes such as the Great Salt Lake in Utah. Small amounts of bromide and iodide salts can be found in the oceans and in brine wells in several states.

Watch video experiments of \begin{align*}p\end{align*} block elements:

http://youtu.be/zZAiYc3WVXM - video of bromine reaction with aluminum

http://youtu.be/cbFCWFksYoM - halogen reactions


  • The halogens all have seven electrons in their outer shells.
  • The electron configuration in the outer shell is \begin{align*}ns^2np^5\end{align*}.
  • As the atomic number increases, the reactivity of the halogens decreases
  • Fluorine and chlorine exist as gases at room temperature, while bromine is a liquid, and iodine is a solid.


Use the link below to answer the following questions:


  1. Are the halogen elements monatomic or diatomic molecules?
  2. Why do we not know very much about astatine?
  3. Describe the reactivity of elemental fluorine.
  4. List one use for fluorine, chlorine, bromine, and iodine compounds.


  1. List the halogens by name, chemical symbol, and atomic weight.
  2. What does the term “halogen” mean?
  3. What is the outer shell electron configuration for the halogens?
  4. What is the physical state of each halogen at room temperature?
  5. Where are the halogens found?

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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

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