Outer shell of electrons is closest to the nucleus so it is difficult to overcome the force of attraction between the protons and electrons
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Sodium
Reaction with water:
Melts to form a ball
Moves around on the surface
Fizzes rapidly
May burn with an orange flame before disappearing
Forms sodium hydroxide and hydrogen
More reactive than sodium, less reactive than potassium
Outer shell is getting further away from the nucleus so it is easier to lose those electrons
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Potassium
Reaction with water:
Melts
Floats and moves quickly
Metal burns with a lilac flame
Sometimes a small explosion at the end
Forms potassium hydroxide and hydrogen
Most reactive of the three elements
Outer shell is furthest from the nucleus so are easier to lose
(Rubidium, caesium and francium are more reactive still, but these are not covered in IGCSE Chemistry)
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Summary
All Group 1 elements react vigorously with water to produce a metal hydroxide and hydrogen
All Group 1 elements form ionic compounds-white solids which form colourless alkaline solutions in water
Reactivity increases down the group because of the distance of the outer shell from the nucleus- electrons are easier to lose when they are further from the positive nucleus
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