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Properties of Halogens

Properties of Halogens

Fluorine, chlorine, bromine, iodine - these are all examples of halogens. But although they're members of the same family, the halogens have very different properties.

  • This article is about the properties of halogens.
  • We'll define halogen before looking at their physical and chemical properties.
  • This will involve considering properties such as atomic radius, melting and boiling points, electronegativity, volatility and reactivity.
  • We'll end by exploring some of the uses of halogens.

Halogen definition

Halogens are a group of elements found in the periodic table. They all contain five electrons in their outer p-subshell and commonly form ions with a charge of -1.

The halogens are also known as group 7 or group 17.

According to the International Union of Pure and Applied Chemistry (IUPAC), group 7 technically refers to the group in the periodic table containing manganese, technetium, rhenium, and bohrium. The group we are talking about is instead systematically known as group 17. To avoid confusion, it’s a lot easier to refer to them as the halogens.

Group 7, group 17, the halogens - whatever you know them as, these elements are shown highlighted in blue. StudySmarter OriginalsFig. 1 - The halogens, shown in the periodic table highlighted in green

Depending on who you ask, there are either five or six members of the halogen group. The first five are fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Some scientists also consider the artificial element tennessine (Ts) to be a halogen. Although tennessine follows many of the trends shown by the other halogens, it also acts strangely by showing some of the properties of metals. For example, it doesn’t form negative ions. Astatine also shows some of the properties of a metal. Because of their unique behaviour, we will largely ignore both tennessine and astatine for the rest of this article.

Tennessine is extremely unstable and has only ever existed for fractions of a second. This, alongside its cost, means that many of its properties haven’t actually been observed. They are only hypothetical. Similarly, astatine is also unstable, with a maximum half-life of just over eight hours. Many of the properties of astatine haven’t been observed either. In fact, a pure sample of astatine has never been collected, because any specimen would immediately vaporise under the heat of its own radioactivity.

Like most of the groups in the periodic table, the halogens have certain shared characteristics. Let’s explore some of them now.

Physical properties of halogens

The halogens are all non-metals. They show many of the physical properties typical of non-metals.

  • They are poor conductors of heat and electricity.

  • When solid, they are dull and brittle.

  • They have low melting and boiling points.

Physical appearance

The halogens have distinct colours. They are also the only group to span all three states of matter at room temperature. Take a look at the table below.

Element

State at room temperature

Colour

Other

F

Gas

Pale yellow

Cl

Gas

Green

Br

Liquid

Dark red

Forms a red-brown vapour

I

Solid

Grey-black

Forms a purple vapour

Here's a diagram to help you visualise these four halogens.

Properties of halogens physical appearance halogens group 7 group 17 studysmarterFig. 2 - The physical appearance of the first four halogens at room temperature

Atomic radius

As you move down the group in the periodic table, the halogens increase in atomic radius. This is because they each have one more electron shell. For example, fluorine has the electron configuration 1s2 2s2 2p5, and chlorine has the electron configuration 1s2 2s2 2p6 3s2 3p5. Fluorine has just two main electron shells, whilst chlorine has three.

properties of halogens, fluorine and chlorine with their electron configurations, StudySmarterFig. 3 - Fluorine and chlorine with their electron configurations. Notice how chlorine is a larger atom than fluorine

Melting and boiling points

As you can tell from their states of matter shown in the table earlier on, melting and boiling points increase as you go down the halogen group. This is because the atoms get larger and have more electrons. Because of this, they experience stronger van der Waals forces between molecules. These require more energy to overcome and so increase the element's melting and boiling points.

Element

Melting point (°C)

Boiling point (°C)

F-220-188
Cl-101-35
Br-759
I114184

Volatility

Volatility is very closely related to melting and boiling points - it is the ease with which a substance evaporates. From the data above, it is easy to see that the volatility of the halogens decreases as you move down the group. Once again, this is all thanks to van der Waals forces. As you move down the group, the atoms get larger and so have more electrons. Because of this, they experience stronger van der Waals forces, decreasing their volatility.

Chemical properties of halogens

Halogens also have some characteristic chemical properties. For example:

  • They have high electronegativity values.
  • They form negative anions.
  • They take part in the same types of reaction, including reacting with metals to form salts, and reacting with hydrogen to form hydrogen halides.
  • They are found as diatomic molecules.
  • Chlorine, bromine, and iodine are all sparingly soluble in water. There is no point even considering the solubility of fluorine - it reacts violently the instant it touches water!

Let's look at some trends in chemical properties within the halogen group.

Electronegativity

Knowing what you know about atomic radius, can you predict the trend in Electronegativity as you go down the halogen group? Take a look at Polarity if you need a reminder.

As you move down the group in the periodic table, the halogens decrease in electronegativity. Remember that electronegativity is an atom’s ability to attract a shared pair of electrons. Let’s investigate why this is the case.

Take fluorine and chlorine. Fluorine has nine protons and nine electrons - two of these electrons are in an inner electron shell. They shield the charge of two of fluorine’s protons, so each electron in fluorine’s outer shell only feels a charge of +7. Chlorine has seventeen protons and seventeen electrons. Ten of these electrons are in inner shells, shielding the charge of ten protons. As in fluorine, each of the electrons in chlorine’s outer shell only feels a charge of +7. This is the case for all of the halogens. But as chlorine has a larger atomic radius than fluorine, the outer shell electrons feel the attraction towards the nucleus less strongly. This means that chlorine has a lower electronegativity than fluorine.

In general, as you go down the group, electronegativity decreases. In fact, fluorine is the most electronegative element on the periodic table.

properties of halogens electronegativity StudySmarterFig. 4 - Halogen electronegativity

Electron affinity

Electron affinity is the enthalpy change when one mole of gaseous atoms each gains one electron to form one mole of gaseous anions.

Factors affecting electron affinity include nuclear charge, atomic radius, and shielding from inner electron shells.

Electron affinity values are always negative. For more information, check out Born Haber Cycles.

As we go down the group in the periodic table, the halogen’s nuclear charge increases. However, this increased nuclear charge is offset by extra shielding electrons. This means that in all of the halogens, the incoming electron only feels a charge of +7.

As you go down the group, atomic radius also increases. This means that the incoming electron is further away from the nucleus and so feels the nucleus’s charge less strongly. Less energy is released when the atom gains an electron. Therefore, electron affinity decreases in magnitude as you go down the group.

properties of halogens electron affinity StudySmarterFig. 5 - Halogen electron affinity

There is one exception - fluorine. It has a lower magnitude electron affinity than chlorine. Let’s look at it a little more closely.

Fluorine has the electron configuration 1s2 2s2 2p5. When it gains an electron, the electron goes into the 2p subshell. Fluorine is a small atom and this subshell isn’t very big. That means the electrons already in it are densely clustered together. In fact, their charge is so dense that they partially repel the incoming electron, offsetting the increased attraction from the decreased atomic radius.

Reactivity

To understand the reactivity of halogens, we need to look at two different aspects of their behaviour: their oxidising ability and their reducing ability.

Oxidising ability

Halogens tend to react by gaining an electron. This means that they act as oxidising agents and are reduced themselves.

As you move down the group, oxidising ability decreases. In fact, fluorine is one of the best oxidising agents out there. You can show this by reacting halogens with iron wool.

  • Fluorine reacts vigorously with cold iron wool - well, to tell the truth, fluorine reacts instantly with almost anything!

  • Chlorine reacts quickly with heated iron wool.

  • Gently warmed bromine reacts more slowly with heated iron wool.

  • Strongly heated iodine reacts very slowly with heated iron wool.

Halogens can also react by losing electrons. In this case they act as reducing agents and are oxidised themselves.

The reducing ability of halogens increases as you go down the group. For example, iodine is a much stronger reducing agent than fluorine.

You can look at reducing ability in more detail in Reactions of Halides.

Overall reactivity

Because halogens mostly act as oxidising agents, their overall reactivity follows a similar trend - it decreases as you go down the group. Let's explore this a little further.

A halogen’s reactivity depends a lot on how well it attracts electrons. This is all to do with its electronegativity. As we’ve already discovered, fluorine is the most electronegative element. This makes fluorine extremely reactive.

We can also use bond enthalpies to show the trend in reactivity. Take the bond enthalpy of carbon, for example. Bond enthalpy is the energy required to break a covalent bond in gaseous state, and decreases as you move down the group. Fluorine forms much stronger bonds to carbon than chlorine does - it is more reactive. This is because the bonded pair of electrons is further from the nucleus, so the attraction between the positive nucleus and the negative bonded pair is weaker.

When halogens react, they generally gain an electron to form a negative anion. This is what happens in the process of electron affinity, right? You might therefore be wondering why fluorine is more reactive than chlorine when it has a lower value for its electron affinity.

Well, reactivity isn’t just to do with electron affinity. It involves other enthalpy changes as well. For example, when a halogen reacts to form halide ions, it is first atomised into individual halogen atoms. Each atom then gains an electron to form an ion. The ions may then dissolve in solution. Reactivity is a combination of all of these enthalpies. Although fluorine has a lower electron affinity than chlorine, this is more than made up for by the size of the other enthalpy changes in the reaction, making fluorine more reactive.

Bond strength

The final chemical property of halogens that we'll look at today is their bond strength. We'll consider both the strength of the halogen-halogen bond (X-X), and the hydrogen-halogen bond (H-X).

Halogen-halogen bond strength

Halogens forms diatomic X-X molecules. The strength of this halogen-halogen bond, also known as its bond enthalpy, generally decreases as you move down the group. However, fluorine is an exception - the F-F bond is much weaker than the Cl-Cl bond. Take a look at the graph below.

Properties of halogens halogen-halogen X-X bond strength enthalpy StudySmarterFig. 6 - Halogen-halogen (X-X) bond enthalpy

Bond enthalpy depends on the electrostatic attraction between the positive nucleus and the bonding pair of electrons. This in turn depends on the atom's number of unshielded protons, and the distance from the nucleus to the bonding electron pair. All halogens have the same number of electrons in their outer subshell and so have the same number of unshielded protons. However, as you move down the group in the periodic table, atomic radius increases, and so the distance from the nucleus to the bonding electron pair increases. This decreases the bond strength.

Fluorine breaks this trend. Fluorine atoms have seven electrons in their outer shell. When they form diatomic F-F molecules, each atom features one bonding pair of electrons and three lone pairs of electrons. Fluorine atoms are so small that when two come together to form a F-F molecule, the lone pairs of electrons in one atom repel those in the other atom quite strongly - so much so that they decrease the F-F bond enthalpy.

Hydrogen-halogen bond strength

Halogens can also form diatomic H-X molecules. The strength of the hydrogen-halogen bond decreases as you move down the group, as you can see from the graph below.

Properties of halogens hydrogen-halogen H-X bond strength enthalpy StudySmarterFig. 7 - Hydrogen-halogen (H-X) bond enthalpy

Once again, this is due to the increasing atomic radius of the halogen atom. As atomic radius increases, the distance between the nucleus and the bonding pair of electrons increases, and so bond strength decreases. But note that in this instance, fluorine follows the trend. Hydrogen atoms don't have any lone pairs of electrons, and so there isn't any additional repulsion between the hydrogen atom and the fluorine atom. Therefore, the H-F bond has the highest strength out of all of the hydrogen-halogen bonds.

Thermal stability of hydrogen halides

Let's take a moment to consider the relative thermal stabilities of hydrogen halides. As you move down the group in the periodic table, the hydrogen halides become less thermally stable. This is because the H-X bond decreases in strength and so is easier to break. Here's a table comparing the thermal stability and bond enthalpy of hydrogen halides:

properties of halogens thermal stability hydrogen halide bond strength StudysmarterFig. 8 - Thermal stability and bond strength of hydrogen halides

Uses of halogens

To finish, we'll consider some of the uses of halogens. In fact, they have a number of applications.

  • Chlorine and bromine are used as disinfectants in a range of situations, from sterilising swimming pools and wounds to cleaning dishes and surfaces. In some countries, chicken meat is washed in chlorine to rid it of any harmful pathogens, such as salmonella and E. coli.

  • Halogens can be used in lights. They improve the lifespan of the bulb.

  • We can add halogens to drugs to make them dissolve in lipids more easily. This helps them cross through the phospholipid bilayer into our cells.

  • Fluoride ions are used in toothpaste, where they form a protective layer around tooth enamel and prevent it from acid attack.

  • Sodium chloride is also known as common table salt and is essential to human life. Similarly, we also need iodine in our body - it helps maintain optimum thyroid function.

Chlorofluorocarbons, also known as CFCs, are a type of molecule that were previously used in aerosols and refrigerators. However, they are now banned due to their negative effect on the ozone layer. You’ll find out more about CFCs in Ozone Depletion.

Properties of Halogens - Key takeaways

  • The halogens are a group of elements in the periodic table, all with five electrons in their outer p-subshell. They commonly form ions with a charge of -1 and are also known as group 7 or group 17.

  • The halogens are non-metals and form diatomic molecules.

  • As you move down the halogen group in the periodic table:

    • Atomic radius increases.

    • Melting and boiling points increase.

    • Volatility decreases.

    • Electronegativity generally decreases.

    • Reactivity decreases.

    • The X-X and H-X bond strength generally decreases.

  • Halogens aren’t very soluble in water, but are soluble in organic solvents such as alkanes.

  • We use halogens for a variety of purposes, including sterilisation, lighting, medicines, and toothpaste.

Frequently Asked Questions about Properties of Halogens

In general, halogens have low melting and boiling points, high electronegativities, and are sparingly soluble in water. Their properties show trends as you move down the group. For example, atomic radius and melting and boiling points increase down the group whilst reactivity and electronegativity decrease.

In general, halogens have high electronegativities - fluorine is the most electronegative element in the periodic table. Their electronegativity decreases as you go down the group. Their reactivity also decreases as you go down the group. Halogens all take part in similar reactions. For example, they react with metals to form salts and with hydrogen to form hydrogen halides. Halogens are sparingly soluble in water, tend to form negative anions, and are found as diatomic molecules.

Halogens have low melting and boiling points. As solids they are dull and brittle, and they are poor conductors.

Halogens are commonly used to sterilise things such as drinking water, hospital equipment, and work surfaces. They are also used in lightbulbs. Fluorine is an important ingredient in toothpaste as it helps protect our teeth from cavities whilst iodine is essential for supporting thyroid function.

Final Properties of Halogens Quiz

Question

Name the five halogens.

Show answer

Answer

Fluorine, chlorine, bromine, iodine, astatine.

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Question

Halogens form _______ molecules.


Show answer

Answer

Diatomic

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Question

Which of the following trends increase as you go down group 17 in the periodic table?


Show answer

Answer

Atomic radius.

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Question

Which of the following halogens has the largest atomic radius?


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Answer

Fluorine

Show question

Question

Which of the following halogens is the most electronegative?


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Answer

Fluorine

Show question

Question

Fluorine’s first electron affinity enthalpy is lower than chlorine’s. Explain why.

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Answer

In general, electron affinity enthalpies decrease as you go down the group. However, fluorine has a lower electron affinity than chlorine because it is such a small, dense atom, and its other electrons in the 2p subshell repel the incoming electron.

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Question

Which of the following halogens has the highest boiling point?


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Answer

Fluorine

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Question

 Halogens are _____ in water and ______ in organic solvents.


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Answer

Sparingly soluble, soluble.

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Question

Fluorine is relatively unreactive. True or false? 

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Answer

False

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Question

How do we use halogens?


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Answer

(E.g.) As a disinfectant, in lightbulbs, in drugs, in toothpaste.

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Question

 Predict the colour and physical state of astatine at room temperature.


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Answer

Dark brown-black, solid.

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60%

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