<Ul> <Li> Fricke <Ul> <Li> large cells </Li> </Ul> </Li> <Li> Pyykkö </Li> </Ul> <Li> Fricke <Ul> <Li> large cells </Li> </Ul> </Li> <Ul> <Li> large cells </Li> </Ul> <Tr> <Td> Periodic table history (show) <Ul> <Li> Dmitri Mendeleev <Ul> <Li> predictions </Li> </Ul> </Li> </Ul> <Ul> <Li> Discovery of elements </Li> </Ul> <Ul> <Li> Naming & etymology </Li> </Ul> <Ul> <Li> <Ul> <Li> for places </Li> <Li> for scientists </Li> <Li> controversies </Li> </Ul> </Li> </Ul> <Ul> <Li> (in East Asia) </Li> </Ul> <Ul> <Li> Systematic element names </Li> </Ul> </Td> </Tr>

Why does the fourth period have 18 elements