Edexcel A Level Chemistry:复习笔记2.3.1 Group 7 Trends

Physical Properties

 

  • The Group 7 elements are called halogens
  • The halogens have uses in water purification and as bleaching agents (chlorine), as flame-retardants and fire extinguishers (bromine) and as antiseptic and disinfectant agents (iodine)

Colours

  • All halogens have distinct colours which get darker going down the group

2.3-Group-17-Halogen-Colours 

The colours of the Group 7 elements get darker going down the group

 

Volatility

  • Volatility refers to how easily a substance can evaporate
    • A volatile substance will have a low boiling point

     

Melting-boiling-points-of-the-Halogens

The melting and boiling points of the Group 7 elements increase going down the group which indicates that the elements become less volatile

 

  • Going down the group, the boiling point of the elements increases which means that the volatility of the halogens decreases
    • This means that fluorine is the most volatile and iodine the least volatile

Trend in melting and boiling points 

  • Halogens are non-metals and are diatomic molecules at room temperature
    • This means that they exist as molecules which are made up of two similar atoms, such as F2

     

  • The halogens are simple molecular structures with weak London dispersion forces between the diatomic molecules caused by instantaneous dipole-induced dipole forces

2.3-Group-17-Instantaneous-Dipole-Induced-Dipole_1

The diagram shows that a sudden imbalance of electrons in a nonpolar molecule can cause an instantaneous dipole. When this molecule gets close to another non-polar molecule it can induce a dipole as the cloud of electrons repel the electrons in the neighbouring molecule to the other side

 

  • The more electrons there are in a molecule, the greater the instantaneous dipole-induced dipole forces
  • Therefore, the larger the molecule the stronger the London dispersion forces between molecules
  • This is why as you go down the group, it gets more difficult to separate the molecules and the melting and boiling points increase
  • As it gets more difficult to separate the molecules, the volatility of the halogens decreases going down the group

Trend in electronegativity 

  • The electronegativity of of the halogens decreases down the group 2.3-Group-17-Electronegativity-Halogens

The electronegativity of the halogens decreases going down the group

 

  • The electronegativity of an atom refers to how strongly it attracts electrons towards itself in a covalent bond
  • The decrease in electronegativity is linked to the size of the halogens
  • Going down the group, the atomic radii of the elements increase which means that the outer shells get further away from the nucleus
  • An ‘incoming’ electron will therefore experience more shielding from the attraction of the positive nuclear charge
  • The halogens’ ability to accept an electron (their oxidising power) therefore decreases going down the group

2.3-Group-17-Trend-Oxidising-Power

With increasing atomic size of the halogens (going down the group) their electronegativity, and therefore oxidising power, decreases

Reactivity

Reactivity

  • When a halogen atom reacts it will usually gain an electron, to form a 1- ion (X + e- → X-)
  • The oxidation number has decreased from 0 to -1, therefore reduction has occurred
  • Therefore halogens will act as oxidising agents
  • Down Group 7 we have seen that the atoms become larger so the outer electrons are further away and are therefore more shielded from the positive nucleus
  • Larger halogen actoms such as iodine will find it more difficult to attract incoming electrons needed to form the 1- ion
  • Therefore the reactivity decreases down Group 7

Reaction with hydrogen

  • To demonstrate the decrease in the reactivity we can look a the reaction with hydrogen gas
  • The table outlines the trend in the reactivity of the halogens with hydrogen gas
  • As we can see the reaction becomes less vigorous down the group

Reaction between Halogen & Hydrogen Gas

2.3-Group-17-Table-1_Reaction-with-Hydrogen

Making Predictions

Making Predictions 

  • Boiling and melting point
    • Increases down the group
  • Colour
    • Appear darker in colour down the group

The Appearance, Characteristics and Colour in Solution of the Halogens

1.2.6-Properties-of-the-Halogens-Table

 

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