<Tr> <Th_colspan="2"> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> Periodic table </Th> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <Tr> <Th> Periodic table forms </Th> <Td> <Table> <Tr> <Th> Standard </Th> <Td> <Ul> <Li> 18 - column </Li> <Li> 18 - column, detailed cells </Li> <Li> 32 - column, detailed cells </Li> </Ul> </Td> </Tr> <Tr> <Th> Alternative </Th> <Td> <Ul> <Li> Alternatives </Li> <Li> Janet's left step table </Li> </Ul> </Td> </Tr> <Tr> <Th> Extended </Th> <Td> <Ul> <Li> Extension beyond the 7th period <Ul> <Li> Aufbau </Li> <Li> Fricke </Li> <Li> Pyykkö </Li> </Ul> </Li> </Ul> </Td> </Tr> </Table> </Td> </Tr> <Table> <Tr> <Th> Standard </Th> <Td> <Ul> <Li> 18 - column </Li> <Li> 18 - column, detailed cells </Li> <Li> 32 - column, detailed cells </Li> </Ul> </Td> </Tr> <Tr> <Th> Alternative </Th> <Td> <Ul> <Li> Alternatives </Li> <Li> Janet's left step table </Li> </Ul> </Td> </Tr> <Tr> <Th> Extended </Th> <Td> <Ul> <Li> Extension beyond the 7th period <Ul> <Li> Aufbau </Li> <Li> Fricke </Li> <Li> Pyykkö </Li> </Ul> </Li> </Ul> </Td> </Tr> </Table>

What are the seven periods of the periodic table