The Periodic Table of Elements - Elements in Group 17

The Periodic Table of Elements

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elements in group 17

Group 17 Elements

The elements in Group 17 are fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts).  

Group 17 elements:

  • are also known as halogens
  • have seven valence electrons (electrons in the outermost shell)
  • exists as diatomic molecules (each molecule contains two atoms)

Group 17 is located on the right side of the periodic table. (refer to the periodic table above)

We will not be discussing the properties of tennessine in this post.

Diatomic Molecules

  • Group 17 elements form diatomic molecules – F2, Cl2, Br2, I2 and At2.
  • This is because they have seven valence electrons.
  • When two atoms of group 17 elements share one electron each, they achieve the stable octet electron arrangement (more on this in Chemical Bonds).

 

chlorine as a diatomic molecule
Diatomic molecule of chlorine
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Reactivity of Group 17 Elements

Atomic radius increases down the group.

The reactivity of Group 17 elements decreases down the group. This is because:

  • Group 17 elements react by receiving 1 electron into the outermost shell.

X2 + e– → 2X–

  • This makes alkali metals electronegative (they have a tendency to receive electrons).
  • The electron is pulled into the outermost shell by the electrostatic force of attraction with the positively charged nucleus.
  • The atomic radius of elements increases down the group (refer to diagram).
  • The distance between the nucleus and the outermost shell increases.
  • This causes the electrostatic force of attraction between the nucleus and the electron to be received to become weaker.
  • Therefore, it gets harder to receive the valence electron going down the group. (The electronegativity of halogens decreases down the group)
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Physical Properties of Group 17 Elements

  • Atomic radius increases down the group. This is due to an increase in the number of electron shells down the group.
  • Density increases down the group. As the atomic mass increases, the density increases (density = mass/volume). 
  • Low melting and boiling points which increase down the group.
    • Molecules are held together by weak Van der Waal forces of attraction. (refer to diagram).
    • As the number of electrons increases down the group, the size of the molecule increases.
    • The larger the molecule, the stronger the Van der Waal forces of attraction.
  • Since alkali metals are non-metals, they do not conduct electricity and heat.
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Chemical Properties of Group 17 Elements

metals water and alkali

Halogens react with:

  • Metals
  • Water
  • Alkalis

Reaction of Group 17 Elements with Metals

Reaction of halogens with iron wool
Reaction of halogens with iron wool

Halogens react with metals to form salts (metal halides).

Halogen + Metal → Metal halide

3Cl2 (g) + 2Fe (s) → 2FeCl3 (s)
Chlorine + Iron → Iron(III) chloride

3Br2 (g) + 2Fe (s) → 2FeBr3 (s)
Bromine + Iron → Iron(III) bromide

3I2 (g) + 2Fe (s) → 2FeI3 (s)
Iodine + Iron → Iron(III) iodide

Observations (refer to diagram)

  • Iron wool glows brightly when chlorine gas passes over it.
  • Iron wool glows when bromine gas passes over it.
  • Iron wool glows dimly when iodine vapour passes over it.

All three reactions produce a brown solid.

TIME TO THINK

What is the purpose of the soda lime at the end of the combustion tube?

  • Halogens are toxic when inhaled.
  • Soda lime will absorb any excess halogen gas that comes out of the combustion tube.
  • The experiment should also be carried out in a fume chamber as an added measure of precaution.

Reaction of Group 17 Elements with Water

reaction of chlorine with water
Reaction of chlorine with water
Reaction of bromine and iodine with water
Reaction of bromine and iodine with water

Halogens react with water to form two acids.

Halogen + Water → Acid + Acid

Cl2 (g) + H2O (l) → HCl (aq) + HOCl (aq)
Chlorine + Water → Hydrochloric acid + Hypochlorous(I) acid

Br2 (l) + H2O (l) → HBr (aq) + HOBr (aq)
Bromine + Water → Hydrobromic acid + Hypobromous(I) acid

I2 (g) + H2O (l) → HI (aq) + HOI (aq)
Iodine + Water → Hydroiodic acid + Hypoiodous(I) acid

Blue litmus paper turns red and then is bleached
Blue litmus paper turns red and then is bleached

Observations (refer to diagram)

  • Chlorine dissolves rapidly in water.
  • A pale yellow solution is formed.
  • The solution turns blue litmus paper red and then decolourises (bleaches) it.
  • Bromine dissolves slowly in water.
  • A brown solution is formed.
  • The solution turns blue litmus paper red and then decolourises (bleaches) it slowly.
  • Iodine dissolves slightly in water.
  • A brown solution is formed.
  • The solution turns blue litmus paper red 
  • Litmus paper is NOT decolourised (bleached).

Reaction of Group 17 Elements with Alkali

Halogens react with alkaline solutions to form metal halides, metal halates and water.

Halogen + Alkaline solution → Metal halide + Metal halate(I) + Water

Cl2 (g) + 2NaOH (aq) → NaCl (aq) + NaOCl (aq) + H2O (l)
Chlorine + Sodium hydroxide → Sodium chloride + Sodium chlorate(I) + Water

Br2 (g) + 2NaOH (aq) → NaBr (aq) + NaOBr (aq) + H2O (l)
Bromine + Sodium hydroxide → Sodium bromide + Sodium bromate(I) + Water

I2 (g) + 2NaOH (aq) → NaI (aq) + NaOI (aq) + H2O (l)
Iodine + Sodium hydroxide → Sodium iodide + Sodium iodate(I) + Water

Halate(I) Ions

  • Halate(I) ions can be written in two ways: XO– or OX–
  • Example: ClO– or OCl–

Both ways are correct and refer to the same ion.

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