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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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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- 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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