<Tr> <Td> 37 </Td> <Td> rubidium </Td> <Td> </Td> <Td> </Td> <Td> Rb </Td> <Td> Rb </Td> <Td> </Td> </Tr> <Tr> <Td> 55 </Td> <Td> caesium </Td> <Td> </Td> <Td>--</Td> <Td> Cs </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> 87 </Td> <Td> francium </Td> <Td>--</Td> <Td>--</Td> <Td_colspan="3"> No primordial isotopes (Fr is a radiogenic nuclide) </Td> </Tr> <Tr> <Td_colspan="7"> Radioactive: K, t 1.25 × 10 years; Rb, t 4.9 × 10 years; Fr, t 22.0 min . </Td> </Tr>

How do alkali metals illustrate the periodic law