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Properties of Halogens

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Chemistry

Fluorine, chlorine, bromine, iodine - these are all examples of halogens.

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 commonly known as group 7 or group 17.

According to the International Union of Pure and Applied Chemistry (IUPAC), group 7 actually 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 OriginalsGroup 7, group 17, the halogens - whatever you know them as, these elements are shown highlighted in blue. StudySmarter Originals

Depending on who you ask, there are either five or six members of the halogen group. The first five are fluorine, chlorine, bromine, iodine, and astatine. Some scientists also consider the artificial element tennessine to be a halogen. Although tennessine follows some of the trends shown by the other halogens, it also shows some of the properties of metals. For example, it doesn’t form negative ions. Astatine also shows some of the properties of a metal. In this article, we will largely ignore both tennessine and astatine.

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.

Name

State at room temperature

Colour

Other

Fluorine

Gas

Pale yellow

Chlorine

Gas

Green

Bromine

Liquid

Dark red

Forms a red-brown vapour

Iodine

Solid

Grey-black

Forms a purple vapour

Atomic radius

As you move down the group in the periodic table, the halogens increase in atomic radius. This is because they have more electron shells. For example, fluorine has the electron configuration , and chlorine has the electron configuration .

Properties of Halogens chlorine fluorine electron configuration electron shellFluorine and chlorine, shown with their electron configurations. Notice how chlorine is a larger atom than fluorine. commons.wikimedia.org

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.

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.

Name

Electronegativity

Fluorine

3.98

Chlorine

3.16

Bromine

2.96

Iodine

2.66

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.

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 . When it gains an electron, the electron goes into the 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.

Properties of Halogens electron affinity graph StudySmarterThe electron affinity of the first four halogens. Anna Brewer, StudySmarter Originals

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.

To help you pull together your understanding, we’ve included a table summarising the properties of halogens:

Properties of Halogens halogen properties StudySmarterA table comparing the properties of halogens. Anna Brewer, StudySmarter Originals

Uses of halogens

Halogens have a number of uses.

  • 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 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 group in the periodic table, the halogen’s atomic radius increases in size.

  • Halogens have low melting and boiling points. As you move down the group in the periodic table, their melting and boiling points increase.

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

  • As you move down the group in the periodic table, the halogen’s reactivity decreases.

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

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.

Show question

Question

Halogens commonly form ions with a charge of _____.


Show answer

Answer

-1

Show question

Question

Halogens form _______ molecules.


Show answer

Answer

Diatomic

Show question

Question

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


Show answer

Answer

Atomic radius.

Show question

Question

Which of the following halogens has the largest atomic radius?


Show answer

Answer

Fluorine

Show question

Question

Which of the following halogens is the most electronegative?


Show answer

Answer

Fluorine

Show question

Question

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

Show answer

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.

Show question

Question

Which of the following halogens has the highest boiling point?


Show answer

Answer

Fluorine

Show question

Question

 Halogens are _____ in water and ______ in organic solvents.


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Answer

Sparingly soluble, soluble.

Show question

Question

Fluorine is relatively unreactive. True or false? 

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Answer

False

Show question

Question

Reactivity decreases as you go down the halogen group in the periodic table. True or false?


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Answer

True

Show question

Question

How do we use halogens?


Show answer

Answer

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

Show question

Question

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


Show answer

Answer

Dark brown-black, solid.

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