Which statement best explains why Mendeleev sometimes changed the order based on relative atomic mass?
He used atomic numbers, which were already known at the time
He placed elements with similar properties in the same group
He ordered the elements by their numbers of neutrons instead
He left out any elements that had more than one isotope
80 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.