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Halogens (salt formers) are found in group VII of the periodic table. They are the most reactive nonmetals. They have seven valence electrons, they exist as diatomic molecules, and they are coloured and ionizes to form univalent negative ions. They form electrovalent compounds with metals. In the group are chlorine, fluorine, bromine, iodine and astatine.



The halogens meaning salt makers are found in group VII of the periodic table. The

electronic configuration of the halogens is one electron short of the noble gas structure (i.e. contains seven electrons on their outermost shells), and the chemistry of the group is dominated by the tendency to complete the octet arrangement by receiving an electron. The electronic configurations of the halogens are shown below:

Fluorine = 9: 1s2 2s2 2p5

Chlorine = 17: 1s2 2s2 2p6 3s2 3p5

Bromine = 35: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5

Iodine   = 53: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p5



  1. They are usually univalent, and easily accept one electron from other atoms to form ionic compounds (especially from metals e.g Na & K). They also share electrons with themselves or with non-metals to form covalent compounds.
  2. They exist in their natural states as non-polar diatomic molecules.
  3. Fluorine and chlorine are gases, bromine is a liquid and iodine is a solid.
  4. The halogens are coloured, with typical penetrating odour. The colours deepen down the group. Fluorine is pale-yellow, chlorine is greenish- yellow, bromine is red and iodine is violet.
  5. They are volatile substances. Their volatility decreases down the group.
  6. All the halogens except fluorine, dissolve to some extent in water, fluorine reacts with water to give oxygen and hydrogen fluoride.



The halogens are very reactive elements. Their reactivity decreases down the group. Fluorine is the most reactive halogen. They are also strongly electronegative. Their Electronegativity decreases down the group.

  1. As oxidizing agents. Halogens are strong oxidizing agent. They do so by accepting electrons and forming halide ions especially in the reaction with metals. The oxidizing power decreases down a group.
  2. Reaction with metals: Halogens react with metals to form ionic compounds. 2Na(s) +  F2(g) →  2NaF(s)
  3. Reaction with hydrogen: Fluorine explodes with hydrogen even in the dark, chlorine reacts slowly in the dark but explode in bright sunlight, bromine reacts with hydrogen in the presence of platinum catalyst; while iodine reacts partially with hydrogen on heating. Example

H2(g)  +  Cl2(g)  →  2HCl(g)

Stability of the hydrogen halides decreases down the group. Hydrogen fluoride is a liquid with a boiling point of 19OC. The other hydrogen halides are gases.

  1. Reaction with water: Fluorine reacts vigorously with water to give off oxygen gas. Chlorine reacts very slowly with water to give a mixture of hydrochloric acid and oxochlorate (I) acid which later decomposes to give hydrochloric acid and oxygen gas. The oxygen gas given off by the oxochlorate (I) acid is responsible for the bleaching action of moist chlorine gas and chlorine water.

H2O(g)  +  Cl2(g)  → HCl(aq)  +  HOCl(aq)



  1. Write the electronic configuration of the following atoms/ions: Cl, F, Br.
  2. Give three physical properties of the halogens



Chlorine is the most important element in the halogen family. It does not occur as free element in nature because it is too reactive. It is usually found in combined state as chlorides.



  1. By the oxidation of concentrated HCl with strong oxidizing agent such as MnO2 or KMnO4

MnO2(s)   +  4HCl(aq)  → MnCl2(aq)   +   2H2O(l)  + Cl2(g)


  1. By heating concentrated H2SO4 with a mixture of NaCl and MnO2

                2NaCl(s) + MnO2(s) + 2H2SO4(aq)  →  Na2SO4(aq) + MnSO4(aq) + H2O(l)  +  Cl2(g)



Chlorine is manufactured industrially by the electrolysis of brine and molten NaCl, MgCl2 or CaCl2.


  1. Explain one laboratory preparation of dry chlorine gas.
  2. Name the method of collection of chlorine gas and explain why it can be collected by the method.


  1. Chlorine is a greenish-yellow gas with unpleasant chocking smell.
  2. It is a poisonous gas.
  3. It is about 2.5 times denser than air.
  4. It can be liquefied under a pressure of about 6atm.
  5. It is moderately soluble in water.



  1. It is very reactive and tends to attain stability by forming electrovalent compound with metals and a single covalent bond compounds with non-metals.

2Na(s)         +          Cl2(g)  →  2NaCl(s)

Cl2(g)           +          H2(g) → 2HCl(g)

  1. It displaces other halogens from solution of their acids and salts

Cl2(g)           +          NaI(aq)  →  2NaCl(aq)     +          I2(g)

  1. It combines directly with other elements except oxygen, nitrogen carbon and the noble gases; to form chlorides

Ca(s)           +          Cl2(g)     → CaCl2(s)

  1. It has a very strong affinity for hydrogen; it removes hydrogen from its compounds.

C10H12(l)                 +          8Cl2(g)         →  10C(s)    +          16HCl(g)

  1. It is a powerful oxidizing agent: it oxidizes green Fe2+ to yellow Fe3+

2FeCl2(aq)               +          Cl2          →2FeCl3(aq)

  1. It has a bleaching action: in the presence of water, chlorine bleaches most dyes and inks except printer’s ink. The bleaching action of chlorine is due to its ability to react with water to form oxochlorate (I) acid which decomposes to release oxygen which oxidizes the dye to form a colourless compound.

H2O(l)         +      Cl2(g)         → HCl(aq)          +          HOCl(aq)

HOCl(aq)     →   HCl(aq)        +          [O]

Dye            +    [O] →        [Dye + O]

Coloured                                                         Colourless            

  1. It reacts with hot concentrated NaOH solution to give a mixture of trioxochlorate (V) and chloride of the metal.

6NaOH    +     3Cl2(g) →      NaClO3(aq)        +   NaCl(aq)       +          H2O(l)

     Sodium trioxochlorate (V)

With cold dilute solution of NaOH, a pale yellowish mixture of oxochlorate (I) and chloride of the metal is formed.

2NaOH(aq)  +  Cl2(g)  →  NaOCl(aq)  +  NaCl(aq)  +  H2O(l)          

  1. It reacts with slaked lime solutions to produce bleaching powder

Ca(OH)2(aq)  +  Cl2(g) →  CaOCl2.H2O(s)

Bleaching powder


  1. It turns damped blue litmus paper pink and then bleaches it. It is acidic gas.
  2. It turns damped starch-iodide dark blue. Chlorine turns starch-iodide paper blue because it displaces iodine from the iodide. The iodine liberated then turns the starch blue.



  1. It is a powerful germicide [due to its oxidizing nature].
  2. It is used as a bleaching agent for cotton, wool, pulp etc.
  3. It is used in the manufacture of polyvinyl chloride (PVC) and synthetic rubber.
  4. It is used in the manufacture of organic compound e.g CHCl3, CCl
  5. It is used in producing KClO3, for making matches and fireworks.
  6. It is used for making NaClO3, a weed killer.
  7. It is used for making domestic antiseptics e.g acidified NaClO solution.


  1. Mention FOUR physical properties of chlorine.
  2. Using balanced equations, state FIVE chemical properties of chlorine.




Hydrogen chloride (marine-acid gas) exists as a gas at room temperature. It dissolves in water to form hydrochloric acid. It occurs in traces in the air as industrial by-product and is considered as air pollutant; but it can be easily washed down as acid rain since it is very

soluble in water.



The gas is prepared by the action of hot concentrated H2SO4 on any soluble chloride. Example    2NaCl(s)            +          H2SO4(aq)          →        Na2SO4(aq)        +          2HCl(g)

Note: NaHSO4 is first formed at a lower temperature and later at higher temperature HCl gas is formed. The gas is dried by passing it through concentrated H2SO4 in another flask and collected.



Pure HCl gas can be produced in large scale by direct combination of hydrogen and chloride gas obtained from the electrolysis of brine.

H2(g)     +          Cl2(g)     →  2HCl(g)


  1. Pure HCl gas is colourless and has sharp irritating smell
  2. It turns damp blue litmus paper red
  3. It is about 1.25times denser than air
  4. It is very soluble in water, forming aqueous HCl acid
  5. It is readily dissolved in non-polar solvent like chloroform and toluene; but the solution does not conduct electricity and has no acidic properties because hydrogen chloride which is a covalent molecule does not ionize when it dissolve in non-polar solvents. But it dissolves in water and ionizes. The ions formed in aqueous solution are responsible for the acidic property and conductivity of its aqueous solution.
  6. It forms misty fumes in moist air because it dissolves in the moisture to form tiny droplets of HCl acid.
  7. It does not support combustion.



  1. It combines directly with NH3 and produces a white fumes of ammonium chloride

HCl(g)        +       NH3(g)     → NH4Cl(s)

  1. It reacts with various heated metals to form their respective chloride and hydrogen

Zn(s)           +       2HCl(g)  →        ZnCl2(s) +          H2(g)



  1. A gas rod that has been dipped in ammonia solution is brought near the gas jar containing the unknown gas, if there are dense white fumes on the glass rod, then the gas is hydrogen chloride gas.
  2. Few drops of silver trioxonitrate (V) is added to the gas jar containing the unknown gas and shaken. If white precipitate of silver chloride is observed, then the gas is hydrogen chloride gas.



  1. State TWO physical and TWO chemical properties of hydrogen chloride gas
  2. An unknown gas is colourless, has an irritating smell, fumes in moist air and turns blue litmus paper red; describe how you will confirm the gas to be hydrogen chloride gas.



Chlorides are normal salts formed when metallic ion replace the hydrogen ion in hydrochloric acid. Chlorides are prepared by neutralization reaction.  Chlorides are soluble in water with exception of few.

Soluble chlorides                                                       Insoluble chlorides

NaCl, NH4Cl, KCl                                                          CuCl2, AgCl, PbCl2

CaCl2 etc



  1. Chlorides are not decomposed by heat. They can only be recovered from solution by evaporation to dryness or sometimes by crystallization.
  2. They react with hot concentrated tetraoxosulphate (VI) acid to produce hydrogen chloride gas.

2NaCl(s)  +  H2SO4(aq)  →  Na2SO4(aq)  +  2HCl(g)

On heating a chloride with concentrated tetraoxosulphate (VI) in the presence of a strong oxidizing agent, chlorine is produced.

ZnCl2(s)  +  KMnO4(s) +  2H2SO4(aq)  → ZnSO4(aq)  +  K2SO4(aq)  + 2MnO2(aq)  +   2H2O(l)  +  Cl2(g)



The test solution is acidified with dilute trioxonitrate (V) acid to prevent precipitation of other salts. Few drops of AgNO3(aq) is then added to the acidified solution in a test tube, a white precipitate of AgCl which readily dissolves in excess NH3(aq) solution indicates the presence of a chloride.


  1. Explain why hydrogen chloride in toluene does not conduct electricity but its aqueous solution does conduct electricity.
  2. Describe a test for a soluble chloride.
  3. Give FOUR uses of chlorine gas.
  4. Define Ionization energy and Electronegativity.
  5. State the second law of thermodynamics.


READING ASSIGNMENT: New School Chemistry for Senior Secondary Schools by O.Y. Ababio (6th edition) pages 356-361


SECTION A: Write the correct option ONLY

  1. The gas released when chlorine reacts with water in the presence of sunlight is a. Cl2 b. H2 c. O2 d. N2
  2. A misty white fume produced when HCl gas react with ammonia is a. NH4OH b. NH4Cl c. NaCl d.  ZnCl2
  3. The reaction between common salt and concentrated tetraoxosulphate (VI) acid liberates a. SO2 b. O2 and Cl2 c. HCl gas d. H2
  4. Apart from hydrogen chloride gas, which other common gas is used in the demonstration of fountain experiment? a. H2S b. SO2 c. NH3 d. CH4
  5. What property makes hydrogen chloride suitable for the fountain experiment? a. It is very soluble in water b. It is slightly soluble in water c. It dissolves in water to give an acidic solution d. It forms acid rain


  1. Explain why hydrogen chloride in toluene does not conduct electricity but its aqueous solution does conduct electricity.
  2. Describe a test for a soluble chloride.

 See also




Test for Anions



See also

  • chlorine and other halogens
  • do halogens react with other halogens
  • why chlorine is called halogen

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