Lithium reacts less vigorously than potassium mainly because lithium's outer electron
is closer to the nucleus, so it is held more strongly
is attracted less strongly, so it is lost more slowly
is further from the nucleus, so it is held more weakly
is in a full outer shell, so it is not easily lost
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.