Group 7 – the halogens



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Group 7 – the halogens

  • fluorine
  • chlorine
  • bromine
  • iodine
  • astatine
  • I
  • Br
  • Cl
  • F
  • At

What are the halogens?

Why are they called the ‘halogens’?

  • Halogens are very reactive non metals.
  • They are all toxic or harmful because they are so reactive. Before antiseptics, iodine was used to clean wounds as it is harmful to all things, including bacteria.
  • They are also never found free in nature because of their reactivity – they are found as compounds with metals.
  • These halogen-metal compounds are salts, which give halogens their name – ‘halo-gen’ means ‘salt-former’.

What is the electron structure of the halogens?

  • All halogens have seven electrons in their outer shell.
  • fluorine
  • 2,7
  • chlorine
  • 2,8,7
  • bromine
  • 2,8,8,7
  • They can easily obtain a full outer shell by gaining one electron.
  • They have similar chemical properties.
  • They all gain an electron in reactions to form negative ions with a -1 charge.
  • This means that:

How do halogen molecules exist?

  • All halogen atoms require one more electron to obtain a full outer shell and become stable.
  • Each atom can achieve this by sharing one electron with another atom to form a single covalent bond.
  • This means that all halogens exist as diatomic molecules: F2, Cl2, Br2 and I2.
  • +
  • F
  • F
  • F
  • F

What are the general properties of the halogens?

  • poisonous and smelly.
  • brittle and crumbly when solid
  • All the halogens are:
  • They become darker in colour down the group:
  • non-metals and so do not conduct electricity
  • is green-yellow
  • is blue-black.
  • is red-brown

What is the physical state of the halogens?

  • The melting and boiling points of the halogens increase down the group, as the molecules become bigger.
  • Halogen
  • Relative size
  • Melting point (°C)
  • Boiling point (°C)
  • State
  • -220
  • -118
  • -101
  • -7
  • 114
  • -34
  • 59
  • 184
  • gas
  • gas
  • liquid
  • solid
  • What is the state of each halogen at room temperature?

Melting and boiling points of halogens

Halogen vapours

  • Bromine and iodine are not gaseous, but have low boiling points. This means that they produce vapour at relatively low temperature. They are volatile.
  • When iodine is heated gently, it changes directly from a solid to a gas without first becoming a liquid.
  • This is called sublimation.

True or false?

How do the halogens react with metals?

  • The reactivity of halogens means that they readily react with most metals.
  • Halogens need to gain electrons for a full electron shell and metals need to lose electrons for a full electron shell.
  • This means that halogens and metals react to form ionic compounds.
  • copper (II) chloride
  • These are metal halides, which are a type of salt.

What are halides?

  • When halogens react with another substance, they become negative ions, as they are gaining an extra electron.
  • The name of each of the halogens changes slightly once it has reacted – instead of ending with ‘–ine’, they end with ‘-ide’.
  • Halogen
  • Halide
  • reaction
  • fluoride (F-)
  • chloride (Cl-)
  • bromide (Br-)
  • iodide (I-)
  • (F)
  • (Cl)
  • (Br)
  • (I)
  • When this happens, they are called halides.

Halogens reacting with iron wool

What is the order of reactivity?

What is the reactivity of the halogens?

  • The iron wool experiment shows that the reactivity of halogens decreases as you go down the group.
  • Astatine is the halogen that appears directly below iodine in the periodic table.
  • How do you think astatine would react with iron wool?
  • Iron wool burns and glows brightly.
  • Iron wool has a very slight glow.
  • Iron wool glows but less brightly than with chlorine.
  • Halogen
  • Reaction with iron wool

Equations of halogens and iron

  • When a halogen reacts with iron it forms an iron halide:
  • What would the equation be for the reaction that forms iron (III) bromide?
  • iron
  • iron (III) chloride
  • chlorine
  • +
  • 3Cl2 (g)
  • 2Fe (s)
  • 2FeCl3 (s)
  • +
  • halogen + iron  iron (III) halide
  • The word and chemical equations for the reaction between chlorine and iron are:
  • iron
  • iron (III) bromide
  • bromine
  • +
  • 3Br2 (g)
  • 2Fe (s)
  • 2FeBr3 (s)
  • +

How does electron structure affect reactivity?

  • The reactivity of alkali metals decreases going down the group. What is the reason for this?
  • The atoms of each element get larger going down the group.
  • This means that the outer shell gets further away from the nucleus and is shielded by more electron shells.
  • The further the outer shell is from the positive attraction of the nucleus, the harder it is to attract another electron to complete the outer shell.
  • This is why the reactivity of the halogens decreases going down group 7.
  • decrease in reactivity
  • F
  • Cl
  • Br

How do the halogens react with non-metals?

  • Halogens also react with non-metals.
  • All hydrogen halides are gases. They dissolve easily in water and become strong acids.
  • Unlike their reactions with metals, halogens share electrons with non-metals, and so react to form covalent compounds.
  • hydrogen
  • H
  • chlorine
  • Cl
  • hydrogen chloride
  • H
  • Cl
  • +

Displacement of halogens

  • If a halogen is added to a solution of a compound containing a less reactive halogen, it will react with the compound and form a new one.
  • sodium chloride
  • sodium fluoride
  • chlorine
  • fluorine
  • +
  • +
  • F2 (aq)
  • 2NaCl (aq)
  • 2NaF (aq)
  • Cl2 (aq)
  • +
  • +
  • A more reactive halogen will always displace a less reactive halide from its compounds in solution.
  • This is called displacement.

Displacement of halogens

  • Why will a halogen always displace a less reactive halogen?

Displacement theory

  • Cl
  • -
  • If a metal halide is mixed with a more reactive halogen, the extra electron will be transferred from the less reactive to the more reactive halogen.
  • -
  • sodium
  • chloride
  • fluorine
  • fluoride
  • Na
  • +
  • Cl
  • -
  • F
  • chlorine

Displacement reactions of halogens

Displacement reactions: summary

  • The reactions between solutions of halogens and metal halides (salts) can be summarised in a table:
  • 2KCl + I2
  • 2KBr + I2
  • halogen
  • chlorine
  • bromine
  • iodine
  • salt (aq)
  • potassium chloride
  • potassium iodide
  • potassium bromide
  • 2KCl + Br2
  • no reaction
  • no reaction
  • no reaction

Is there a displacement reaction?

Reactions of halogens: summary

What are the uses of halogens?

  • How many everyday uses of halogens can you see below?

What are the uses of halogens?

What are the uses of halogens?

Glossary

  • diatomic – Molecules that exist as two atoms covalently bonded together.
  • displacement – The reaction when a more reactive halogen reacts with a compound containing a less reactive halogen.
  • halide – The name of a halogen when it has reacted with another substance and gained a full outer electron shell.
  • halogen – An element that belongs to group 7 of the periodic table.
  • hydrogen halide – A compound formed from the reaction between hydrogen and a halogen.
  • metal halide – A compound formed from the reaction between a metal and a halogen.
  • sublime – To change from a solid to a gas without first becoming a liquid.
  • volatile – A substance that evaporates or produces vapour
  • at relatively low temperatures.

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