Group VII Properties (Cambridge International IGCSE Chemistry)

Group VII properties & trends

Spec point — 8.2.2 · Describe the Group VII halogens, chlorine, bromine and iodine, as diatomic non-metals with general trends down the group, limited to: increasing density decreasing reactivity State the appearance of the halogens at r.t.p. as: chlorine, a pale yellow-green gas bromine, a red-brown liquid iodine, a grey-black solid

Group VII properties & trends

• These are the Group VII non-metals that are poisonous and include fluorine, chlorine, bromine, iodine and astatine

• Halogens are diatomic

  • This means that they form molecules containing two atoms

• The formulae of the halogens are:

  • Fluorine = F₂
  • Chlorine = Cl₂
  • Bromine = Br₂
  • Iodine = I₂
  • Astatine = At₂

• All halogens have seven electrons in their outer shell

  • They form halide ions by gaining one more electron to complete their outer shells

• Fluorine is not allowed in schools so observations and experiments tend to only involve chlorine, bromine and iodine

Properties of the halogens

Physical state, colour and density

• At room temperature (20 °C), the physical state of the halogens changes as you go down the group

  • Chlorine is a pale yellow-green gas
  • Bromine is a red-brown liquid
  • Iodine is a grey-black solid

• This demonstrates that the density of the halogens increases as you go down the group:

The physical state of the halogens at room temperature

Exam Hint Solid iodine, iodine in solution and iodine vapour are different colours.

• Solid iodine is dark grey-black
• Iodine vapour is purple
• Aqueous iodine is brown.

Reactivity

• The reactivity of Group 7 elements decreases as you go down the group

  • This is the opposite trend to Group 1

• When a halogen reacts, it needs to gain one outer electron to achieve a full outer shell of electrons

• As you go down Group 7, the number of electron shells increases:

• The increasing number of electron shells has two main effects:

  • The atomic radius increases, so the outer shell is further from the nucleus
  • There is more electron shielding from the inner shells

• These factors reduce the force of attraction between the nucleus and an incoming electron

• Therefore, it becomes harder to attract an electron

• This means that the reactivity of the halogens decreases down the group

Predicting group VII properties

Spec point — 8.2.2 · Predict the properties of other elements in Group VII, given information about the elements

• You may be given information about some elements and asked to predict the properties of other elements in the group
• The information you might be given could be in relation to melting/boiling point or physical state/density so it is useful to know the trends in properties going down the group

Predicting melting and boiling point

• The melting and boiling point of the halogens increases as you go down the group

• Fluorine is at the top of Group 7

  • It has the lowest melting and boiling point

• Astatine is at the bottom of Group 7

  • It has the highest melting and boiling point

Predicting physical states and density

• The halogens become denser as you go down the group

• Fluorine is at the top of Group 7

  • It is a gas

• Astatine is at the bottom of Group 7

  • It is a solid

Predicting colour

• The colour of the halogens becomes darker as you go down the group

• Fluorine is at the top of Group 7 so the colour will be lighter

  • Fluorine is yellow

• Astatine is at the bottom of Group 7 so the colour will be darker

  • Astatine is black

Exam Hint If you are doing the extended course you can be asked to identify trends in chemical or physical properties of the Group 7 elements, given appropriate data.

Firstly, make sure that you have placed the elements and associated data in either ascending or descending order according to their position in Group 7. Then look for any general patterns in the data.