Group 2 and Group 7

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Periodicity

Trends Across a Period:

  • Ionisation energy increases across a period, because the number of protons increases so there is more attraction on the electrons, and electrons are added to the same shell, drawing it in slightly.
  • The atomic radius decreases as the increased nuclear charge pulls the electron shells inwards.

Trends Down a Group;

  • Ionisation energy decreases down a group, because the number of shells increases, increasing the distance of the outer electrons from the nucleus, resulting in a weaker force of attraction. Also the effect of shielding is much greater as more electron shells are added.
  • The atomic radius increases as there is less attraction pulling the electron shells inwards.
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Group 2

Reactivity increases down the group

Below, "M" indicates a group 2 element

  • 2M + O2 ----> 2MO
  • M + 2H2O -----> M(OH)2 + H2O
  • Oxides react with water to produce a solution of metal hydroxide.
  • Hydroxides react with water to produce an alkaline solution.
  • Carbonates undergo thermal decomposition to form a metal oxide and carbon dioxide.
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Group 7

Displacement reactions:

  • A halogen solution is mixed with aqueous solutions of different halides, the more reactive halogen will oxidise and displace a halide of a less reactive halogen. The most reactive is chlorine, followed by bromine, followed by iodine.
  • Colours in cyclohexane: Cl2- pale green, Br2- orange, I2- violet
  • Disproportionation is when the same element is both oxidised and reduced in the same reaction.

Testing for Halide Ions:

  • An unknown halide is first dissolved in water, then aqueous silver nitrate (AgNO3) is added.
  • Silver ions react with any halide ions present forming a precipitate.
  • AgCl = white, AgBr = cream, AgI = yellow
  • If the colour can't be determined, aqueous ammonia can be added, chloride ions are soluble in dilute NH3, bromide ions are soluble in concentrated ammonia, and iodide is insoluble in ammonia.
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