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The non-metal elements found in group 7 of the periodic table are collectively known as the halogens. They include fluorine, chlorine, bromine, iodine and astatine, as highlighted in blue on the diagram below:
Physical properties
The halogens all have low melting and boiling points which means that they tend to be gases or liquids at room temperature. You must know the colours and physical states (at room temperature) of the halogens, focusing on chlorine, bromine and iodine.
- Chlorine – yellow-green gas
- Bromine – red-brown liquid which turns into a red-brown vapour when heated
- Iodine – shiny dark purple/black solid crystals which turn into a purple vapour when heated
You must also know the trends in physical properties of the halogens, focusing on chlorine, bromine and iodine.
As you descend the group, the melting points and boiling points increase. This is evidenced by the change in state at room temperature as you descend the group. You can use this trend to predict the state of fluorine and astatine at room temperature. Fluorine is above chlorine so would have the lowest melting and boiling points and must be a gas at room temperature. Astatine is below iodine so has the highest melting and boiling points and must be a solid at room temperature.
Each of the halogens exist naturally as a pair of atoms covalently bonded together as diatomic molecules. When a halogen is heated up, it is the diatomic molecules which are separated from each other. This trend occurs because the melting and boiling points are determined by the strength of intermolecular forces of attraction acting between the halogen molecules.
As you descend the group, the atoms become larger. This means that the diatomic molecules made up of two of the atoms will also become larger. The larger the molecule, the stronger the intermolecular forces become. The stronger the intermolecular forces of attraction between the halogen molecules, the harder it is to separate them. More energy is required to overcome the forces of attraction and the higher the melting and boiling points will be.
As you descend group 7 the colour of the halogens becomes darker. We can use this trend to predict the colours of fluorine and astatine. Fluorine is above chlorine so would be paler in colour and astatine is below iodine so would be darker in colour. Fluorine is a pale yellow gas and astatine is a dark black solid.
Uses of halogens
Halogens are used as bleaching agents, for example chlorine is used to bleach wood to make white paper. Halogens are toxic and can be used to kill bacteria. For example, very low concentrations of chlorine compounds can be added to swimming pools to eliminate bacteria, and iodine can be used to sterilise wounds.
Chemical properties
One of the typical types of reactions which halogens are involved in are displacement reactions. A displacement reaction is one in which a more reactive metal displaces a less reactive metal from its compound. The observations made during displacement reactions between solutions of halogens and halide ions can be used as evidence for the trend in reactivity down group 7. The expected observations from adding halogen solutions to solutions of halide salts are summarised below
- Adding chlorine solution (Cl2(aq)) to colourless sodium bromide solution (NaBr(aq)) causes the solution to become brown in colour, indicating that bromine is present in the solution
- Adding chlorine solution to colourless sodium iodide solution (NaI(aq)) causes the solution to become brown, indicating that iodine is present in the solution
- When bromine solution is added to colourless sodium chloride solution (NaCl(aq)), no colour change is observed
- When bromine solution (Br2(aq)) is added to colourless sodium iodide solution, a brown colour is produced, indicating that iodine is present
- When iodine solution is added to either colourless sodium chloride solution or colourless sodium bromide solution, no colour change is observed
The addition of chlorine solution results in a colour change for both sodium bromide and sodium iodide. The addition of bromine solution only causes a colour change in sodium iodide and the addition of iodine solution does not result in a colour change for either sodium chloride or sodium bromide. When chlorine is added to the sodium bromide solution, sodium chloride (NaCl(aq)) and bromine (Br2(aq)) are produced in solution, as shown by the equation:
![\[Cl_{2(aq)} + 2NaBr \rightarrow 2NaCl + Br_{2(aq)} \]](https://online-learning-college.com/wp-content/ql-cache/quicklatex.com-cd410fdbdbfd7a6427e1ac601f485ff5_l3.png "Rendered by QuickLaTeX.com")
Chlorine is able to displace bromine as it is further up in group 7 and therefore more reactive than bromine. In this reaction the chlorine acts as an oxidising agent and is reduced to chloride ions, and the bromide ion acts as a reducing agent and is oxidised to bromine. This is known as a redox reaction as both reduction and oxidation have occurred in the same reaction.
When chlorine is added to the sodium iodide solution, a brown colour is seen, as sodium chloride (NaCl(aq)) and brown iodine (I2(aq)) are produced in solution:
![\[Cl_{2(aq)} + 2NaI \rightarrow 2NaCl + I_{2(aq)} \]](https://online-learning-college.com/wp-content/ql-cache/quicklatex.com-15489a9d1ca7e10bbdcde9b1cce38834_l3.png "Rendered by QuickLaTeX.com")
The chlorine displaces the iodine from sodium iodide as it is further up in group 7 and therefore more reactive than iodine. When bromine is added to the sodium chloride solution, no change occurs in the colour of the solution because bromine is less reactive than chlorine and is therefore not able to displace it from the sodium chloride solution.
When bromine is added to the sodium iodide solution, a brown colour is seen, as sodium bromide (NaBr(aq)) and brown iodine (I2(aq)) are produced in solution:
![\[Br_{2(aq)} +2NaI \rightarrow 2NaBr + I_{2(aq)} \]](https://online-learning-college.com/wp-content/ql-cache/quicklatex.com-c708c43a02cacd57a750cf347a902e9b_l3.png "Rendered by QuickLaTeX.com")
The bromine displaces the iodine from sodium iodide as it is further up in group 7 and therefore more reactive than iodine. Iodine is lower down in group 7 and therefore less reactive than chlorine and bromine. Iodine is not able to displace either of these halogens, so no colour change is seen as no displacement reaction happens. These observations show a trend in the reactivity of the halogens. As you go down through the halogens, the reactivity decreases.
Explaining the trend in reactivity of Group 7
All of the halogens in group 7 have very similar chemical properties because they all have seven electrons in their outer shell. Fluorine has an electronic configuration of 2.7, chlorine has an electronic configuration of 2.8.7, and bromine has an electronic configuration of 2.8.8.17.
This means that to achieve stability, they need to gain one electron by reacting with other atoms. For example, halogens will try and achieve stability by gaining the electron from a metal ion to form a non-metal ion which is negatively charged. This gaining of electrons is known as reduction. For example, a fluorine atom gains an electron to become a fluoride ion:
![\[F + e^- \rightarrow F^- \]](https://online-learning-college.com/wp-content/ql-cache/quicklatex.com-634b7d3c08d1e5fd19c8524859bb9959_l3.png "Rendered by QuickLaTeX.com")
This is because the alkali metals, which we’ve just learnt about, have one electron in their outer shell and a halogen is missing one electron to form a full outer shell. When they react with metals, halogens produce salts such as calcium fluoride, sodium chloride etc. These are called metal halides.
Halogens in their natural forms exist as covalently bonded pairs of atoms known as diatomic molecules. Diatomic molecules are stable as both atoms achieve the stability of a full outer electron shell by sharing one pair of electrons. The force of attraction between the positively charged nucleus and the negatively charged outer electrons makes it easier for electrons to be gained. We know that the reactivity of the halogens decreases as we move down the group.
As we move down group 7, the atoms get larger and the number of electron shells between the nucleus and outer shell increases. The force of attraction between the nucleus and the outer shell electrons gets weaker and it becomes more difficult to gain an electron into the outer shell.
It is easier for the halogens at the top of the group to gain electrons and they are therefore more reactive than those further down. From these observations and trends we can predict that astatine would not displace any other halogens from their compounds since it is at the bottom of the group and therefore the least reactive. We can also predict that fluorine would be the most reactive and would therefore displace all of the other halogens from their compounds.
