Hey there! Have you heard of the halogens? They're a group of elements that you can find on the periodic table. But get this - they're not in group 7 like some people might think. Actually, they're in group 17! Group 7 has a bunch of different elements, like manganese and technetium. But when people talk about groups, they usually don't include those transition metals. So when they say "group 7," they're really talking about the halogens, which are all the way on the right side of the periodic table.
This article is an introduction to the halogens. We’ll look at their properties and characteristics before taking a closer look at each member in turn. We’ll then outline some of the reactions they take part in and their uses. Finally, we’ll also explore how you can test for the presence of halide ions in compounds.
Did you know that the halogens are actually non-metals? That means they have a lot of similar properties to other non-metals you might know about. They're not great at conducting heat or electricity, and they produce acidic oxides. Plus, when they're in their solid form, they're dull and brittle. Oh, and here's something interesting - they can sublime easily, meaning they can go straight from a solid to a gas without becoming a liquid first. They also have low melting and boiling points. Another cool thing about halogens is that they have high electronegativity values. In fact, fluorine has the highest electronegativity value of any element on the periodic table. Halogens also form anions, which are negatively charged ions. The first four halogens typically form anions with a charge of -1, meaning they've gained one electron. And finally, halogens form diatomic molecules - that means they're usually found in pairs of two.
We call ions made from halogen atoms halides. Ionic compounds made from halide ions are called halide salts. For example, the salt sodium chloride is made from positive sodium ions and negative chloride ions.
Reactivity and electronegativity decrease going down the group whilst atomic radius and melting and boiling points increase. Oxidising ability decreases going down the group whilst reducing ability increases. You'll learn more about these trends in Properties of Halogens. If you would like to see halogen reactivity in action, visit Reactions of Halogens.
At the start of this article, we said that the halogen group contains six elements. But it depends on who you ask. The first four members are known as the stable halogens. These are fluorine, chlorine, bromine, and iodine. The fifth member is astatine, an extremely radioactive element. The sixth is the artificial element tennessine, and you’ll find out why some people don’t include it in the group later on. Let’s now take a look at the elements individually, starting with fluorine.
Fluorine is the smallest and lightest member of the group, with an atomic number of 9 and a pale yellow gas at room temperature. It is the most electronegative element in the periodic table, making it one of the most reactive elements. Fluorine reacts readily with almost all other elements and can even react with glass. It is stored in special containers using metals such as copper, which form a protective layer of fluoride on their surface. The name fluorine comes from the Latin verb fluo-, meaning 'to flow', which reflects its origins. It was originally used to lower the melting points of metals for smelting. In the 1900s it was used in refrigerators in the form of CFCs, or chlorofluorocarbons, which are now banned due to their harmful effect on the ozone layer. Nowadays fluorine is added to toothpaste and is a part of Teflon™.
For more on CFCs, check out Ozone Depletion.
Teflon™ is a brand name for the compound polytetrafluoroethylene, a polymer made from chains of carbon and fluorine atoms. C-C and C-F bonds are extremely strong, which means that the polymer doesn’t react with much else. It is also extremely slippery, which is why it is often used in non-stick pans. In fact, polytetrafluoroethylene has the third-lowest friction coefficient of any known solid, and is the only material a gecko can’t stick to!
Chlorine, with an atomic number of 17, is the next smallest member of the halogens group. It is a green gas at room temperature, and its name comes from the Greek word chloros, meaning 'green'. Chlorine has a high electronegativity, only behind oxygen and its close cousin, fluorine. It is incredibly reactive and is never found in its elemental state in nature.
While chlorine has higher melting and boiling points than fluorine due to its position in the periodic table, it has a lower electronegativity, reactivity, and first ionisation energy. Chlorine is used for many purposes, such as making plastics and disinfecting swimming pools, and is essential to life for all known species. However, too much chlorine can be toxic, and it was first used as a weapon in World War One.
Take a look at Chlorine Reactions to see how we use chlorine in everyday life.
Bromine is the next element after chlorine, with an atomic number of 35. It is a dark red liquid at room temperature, and like fluorine and chlorine, it does not occur freely in nature but instead forms other compounds. Bromine forms organobromides, which are commonly used as fire retardants. In fact, over half of the bromine produced globally each year is used in this way. While bromine can also be used as a disinfectant like chlorine, the latter is preferred due to bromine’s higher cost. It is worth noting that the only other element that is a liquid at room temperature and pressure is mercury, which we use in thermometers.
Iodine is the heaviest of the stable halogens, with an atomic number of 53. It is a grey-black solid at room temperature that melts to produce a violet liquid. Its name comes from the Greek word iodes, meaning 'violet'. As you move down the periodic table to iodine, the trends outlined earlier in the article continue. Iodine has a higher boiling point than fluorine, chlorine, and bromine, but a lower electronegativity, reactivity, and first ionisation energy. However, it is a better reducing agent.
Look at Reactions of Halides to see halides at work as reducing agents.
Astatine is a fascinating element with an atomic number of 85. It is the rarest naturally occurring element in the Earth’s crust and is mostly found leftover as other elements decay. Astatine is highly radioactive, and its most stable isotope has a half-life of just over eight hours. Due to its radioactivity, a sample of pure astatine has never been successfully isolated, and scientists have had to make predictions about its properties. They suggest that astatine follows the trends shown in the rest of the group, with a lower electronegativity and reactivity than iodine but higher melting and boiling points. However, astatine also displays some unique properties, lying on the line between metals and nonmetals, which has led to some debate about its characteristics. Some chemists predict that astatine is a dark grey-black, while others consider it more of a metal and predict that it is shiny, lustrous, and a semiconductor. As a result, astatine is often excluded from discussions about the halogens. Despite the challenges in studying astatine, scientists continue to make progress in understanding this enigmatic element.
Tennessine is the final member of the halogens, but some don’t consider it a proper member at all. Tennessine has the atomic number 117 and is an artificial element, meaning that it is only created by colliding two smaller nuclei together. This forms a heavier nucleus that only lasts for a few milliseconds. Once again, this makes it just a little bit tricky to figure out! Chemists predict that tennessine has a higher boiling point than the rest of the halogens, following the trend seen in the rest of the group, but that it doesn’t form negative anions. Most consider it to be a sort of post-transition metal instead of a true nonmetal. For this reason, we often exclude tennessine from group 7.
The halogens are highly reactive and take part in multiple different types of reactions. Fluorine, in particular, is one of the most reactive elements in the periodic table. As you go down the group, reactivity falls.
Halogens can displace other halogens, with a more reactive halogen displacing a less reactive halogen from an aqueous solution. They can also react with hydrogen, metals, sodium hydroxide, alkanes, benzene, and other organic molecules. Halide ions can react with sulfuric acid to form a range of products, with silver nitrate solution to form insoluble silver salts, and can also dissolve in solution to form acids.
For example, chlorine can displace iodide ions to form chloride ions and a grey solid, iodine. Halide ions can be used to test for the presence of halogens in a substance. Hydrogen halides dissolve in solution to form acids, with hydrogen fluoride forming a weak acid and hydrogen chloride, bromide, and iodide forming strong acids. To learn more about the reactions of halogens and halides, check out Reactions of Halogens and Reactions of Halides.
To test for halides, we can carry out a simple test-tube reaction.
Dissolve a halide compound in solution. Add a few drops of nitric acid. This reacts with any impurities that might give a false-positive result. Add a few drops of silver nitrate solution and note down any observations. To further test your compound, add ammonia solution. Once again, note down any observations.
With any luck you should get results a little like the following:
The test works because adding silver nitrate to an aqueous solution of halide ions forms a silver halide. Silver chloride, bromide, and iodide are insoluble in water, and partially soluble if you add different concentrations of ammonia. This enables us to tell them apart.
The halogens have diverse uses in our everyday life. Fluoride is essential for animal health and is added to drinking water and toothpaste to help strengthen teeth and bones. Fluorine is also used in the nuclear power industry to fluorinate uranium tetrafluoride. Chlorine is primarily used to make further compounds, such as 1,2-dichloroethane, which is used to make the plastic PVC. Chlorine also plays an important role in disinfection and sanitation. Bromine is used as a flame retardant and in some plastics, while iodine compounds are used as catalysts, dyes, and feed supplements.
Overall, the halogens have numerous beneficial applications in medicine, industry, and everyday life, making them an essential group of elements in the periodic table.
Key takeaways about halogens include:
What are halogens?
Halogens are a group of elements found in group 17 in the periodic table. This group is sometimes known as group 7. They are nonmetals that tend to form anions with a charge of -1. They show many of the properties typical of nonmetals - they have low melting and boiling points, are poor conductors, and are dull and brittle.
What are four properties of halogens?
Halogens have low melting and boiling points, are hard and brittle, are poor conductors, and have high electronegativities.
Which halogen is the most reactive?
Fluorine is the most reactive halogen.
What group are the halogens in?
The halogens are in group 17 in the periodic table, but some people call this group 7.
What are halogens used for?
Halogens are used as disinfectant, in toothpaste, as fire retardants, to make plastics, and as commercial dyes and feed supplements.
