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

• 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

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

   

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

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 

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

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

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

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