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Group 7 Elements


Group 7 elements are also called Group 17 elements or Halogens

They are present as Diatomic atoms always

They end with an electronic configuration of P5

They only have weak induced dipole forces between molecules as they are non polar

Melting & Boiling points

There is a clear increase in boiling and melting points down the group. This is because due to the increase in induced dipole forces between molecules. The increase in electrons increases the induced dipole forces!


The properties of the main halogens are given. You need to remember these points

Halogens Formula Color State
Fluorine F2 Pale yellow Gas
Chlroine Cl2 Pale green Gas
Bromine Br2 Brown orange Liquid
Iodine I2 Black / Gray Solid
There are different states that iodine can exist and they all have different colors

Oxidising Power

Halogens are powerful oxidising agents but, their oxidising power decreases down the group. This means that down the group, they get easily oxidised

For example:

2NaBr + Cl2 → 2NaCl + Br2

This shows that chlorine is a more powerful oxidising agent than bromine. In fact, this is a displacement reaction and this is used to show that the reactivity decreases down the group

Solubility of Halogens

Halogens are slightly soluble in water and they are present in the diatomic state. However, halogens are very soluble in cyclohexanes. In fact, when dissolved they have their unique colors

Displacement Reactions

When the more reactivity element displaces the less reactive element from its salt solution

Like the above example, Chlorine gas will displace bromine from its salt solution

This table will summarise whether a reaction will occur. Always remember the more reactive one will displace the less reactive element or else there will be no reaction

Hydrogen Halides

When a halogen reacts with hydrogen, a hydrogen halide is formed. For example:

Br2 + H2 → 2HBr

We will consider how each halide reacts:

Name Process Stablity
HF Reacts explosively under cool and dark conditions Most Stable
HCl Reacts explosively in sunlight
HBr Reacts slowly to form a stable product
HI Reacts very slowly to form an unstable product that decomposes. Forms an equilibrium Least Stable

The stability decreases down the group. This means that the HF molecule is more stable than HI

This is because, the bond enthalpy or strength of HF is more stronger than HI. As HI has the weakest bonds, it is very unstable and easily decomposes

Reactions with Conc Sulfuric acid

Sulfuric acid is a powerful oxidising agent also and it is powerful enough to oxide bromine and iodide ions but, not Chlorine. Instead, it acts as an acid

  • Sodium Chloride
  • When Concentrated sulfuric acids is added to Sodium Chloride crystals, HCl gas is made.

    2NaCl + H2SO4 → 2HCl + Na2SO4

    This is not a redox reaction but an acid base reaction as sulfuric acid only acts as an acid. This is important for the production of hydrogen chloride gas.

  • Sodium Bromide
  • The products are further oxidised. The first equation is the same which means in this reaction sulfuric acid acts as both an acid and an oxidising agent

    2NaBr + H2SO4 → 2HBr + Na2SO4

    NaBr + H2SO4 → Br2 + SO2 + H2O

    This means that the HBr undergoes further oxidation and the Sulfur undergoes futher reduction

    This will give a mixture of products so it is a bad way to make HBr

  • Sodium Iodide
  • The above 2 equation are the same but the products oxidise further

    2NaI + H2SO4 → 2HI + Na2SO4

    NaI + H2SO4 → I2 + SO2 + H2O

    NaI + H2SO4 → I2 + S + H2O

    NaI + H2SO4 → I2 + S + H2S

    The understanding is very simple but, it is good to remember these equations and the products

    This reaction will release white fumes ( HI ), a yellow solid (S) & rotten eggs smell and Purple fumes

    In fact all of these above reactions will release white fumes as a hydrogen halide gas is formed

    Test For Halides

    We add aqueous silver nitrate and nitric acid to test the ion. Then we add aqueous ammonia to distinguish them

    Halide Silver Nitrate Then add Ammonia
    Chloride White ppt Dissolves in Dilute Ammonia
    Bromide Cream ppt Dissolves in excess concentrated ammonia
    Iodide Yellow ppt No visible change

    The reason why the precipitate dissolves in ammonia is because a complex ligand structure is made. More will be covered later

    Another important point to remember is that iodide compounds undergo a faster chemical reaction than chloride compounds. This is also again due to the weak bonds in the iodide compound. So it has a lower activation energy and so it reacts faster. This means for the above table, the yellow ppt is formed first then cream then white

    Fastest Iodocompounds
    Slowest Flurocompounds


    A process in which the atoms of an element both oxidises and reduces simultaneously

    This means that the products are an oxidised and reduced form of the reactant. We use NaCl + Cl2 as the example. At different conditions, different products are made

  • Dilute NaOH 15°C
  • NaOH + Cl2 → NaCl + NaClO + H2O

    As you can see one chlorine has an oxidation number of -1 and the other one has an oxidation number +1 in NaClO. NaClO is used to make bleach to kill bacteria

  • Conc NaOH 70°C
  • NaOH + Cl2 → NaCl + NaClO3 + H2O

    Now we see that one chlorine atom has an oxidation number of -1 and the other one is +5

    These equation must be memorised and the products and oxidation numbers must be known as they exams do ask you

    Uses of Group 7 Elements

    There are many uses of group 7 elements but we will only note down the important ones:

  • Chlorination
  • Chlorine is added to water to bleach and sterlize the water by killing harmful bacteria

    H2O + Cl2 → HClO

    HClO is a chloric acid which we saw before

    HClO → HCL + [O]

    This decomposes to HCL and activated oxygen that kills bacteria

  • Bromine and Iodine
  • They are simply used in many situations such as disinfectors, retarder, textile and to make hydrogen halides and gases such as HFC or CFC that are inert and used in aerosol sprays and refrigants

  • Polymers
  • They are used to make polymers such as PVC or Teflon ( polytetrafluroethene ) that are strong. More will be dicussed in the polymerisation section

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