Why did Mendeleev leave gaps in his periodic table?
He could not measure any relative atomic masses
He placed elements strictly in order of relative atomic mass
He wanted every period to have the same length
He expected undiscovered elements to fit those positions
50 exam-style questions on Edexcel GCSE Chemistry 2.2 The periodic table, covering 2.2.1 Mendeleev's periodic table, 2.2.2 Periods, groups, metals and non-metals, and 2.2.3 Electronic configuration and position in the periodic table. Each one has a worked solution and a mark scheme showing where the marks go.