Which statement best explains why rubidium (RbRbRb) reacts more vigorously with water than potassium (KKK)?
The outer 5s5s5s electron of rubidium is more shielded and further from the nucleus than the outer 4s4s4s electron of potassium, resulting in a weaker electrostatic attraction and lower first ionisation energy.
The rubidium atom has a larger nuclear charge (Z=37Z = 37Z=37 vs Z=19Z = 19Z=19), which increases the electrostatic attraction on the outermost electron, making it easier to lose.
The metallic bonding in rubidium is stronger than in potassium, reducing the activation energy required to break the metallic lattice during the reaction.
The hydration enthalpy of the Rb+Rb^+Rb+ ion is more exothermic than that of the K+K^+K+ ion, providing a greater thermodynamic driving force for the reaction.
75 exam-style questions on Edexcel GCSE Chemistry 7.1 Group 1, covering 7.1.1 Classifying elements by group in the periodic table and 7.1.2 Properties and reactions of the alkali metals. Each one has a worked solution and a mark scheme showing where the marks go.