<Tr> <Td> β, n (17%) </Td> <Td> Al </Td> </Tr> <Tr> <Td> Mg </Td> <Td> 12 </Td> <Td> 22 </Td> <Td> 34.00946 (25) </Td> <Td> 20 (10) ms </Td> <Td> β (> 99.9%) </Td> <Td> Al </Td> <Td> 0 + </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> β, n (<. 1%) </Td> <Td> Al </Td> </Tr> <Tr> <Td> Mg </Td> <Td> 12 </Td> <Td> 23 </Td> <Td> 35.01734 (43) #</Td> <Td> 70 (40) ms </Td> <Td> β, n (52%) </Td> <Td> Al </Td> <Td> (7 / 2 −) #</Td> <Td> </Td> <Td> </Td> </Tr>

Give the reason why the two isotopes of magnesium have different mass number