Group VII Properties (Cambridge International IGCSE Science)

Group VII properties & trends

Spec point — 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

Diagram showing the electronic configuration of the first three elements in Group 7

• 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 — Predict the properties of other elements in Group VII, given information about the elements

Extended only

• 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 You can be asked to identify trends in chemical or physical properties of the Group 7 elements, given appropriate data.

1. Place the elements and associated data in either ascending or descending order according to their position in Group 7
2. Look for any general patterns in the data