<Table> Carbon - 13 <Tr> <Th_colspan="2"> General </Th> </Tr> <Tr> <Th> Name, symbol </Th> <Td> Carbon - 13, C </Td> </Tr> <Tr> <Th> Neutrons </Th> <Td> 7 </Td> </Tr> <Tr> <Th> Protons </Th> <Td> 6 </Td> </Tr> <Tr> <Th_colspan="2"> Nuclide data </Th> </Tr> <Tr> <Th> Natural abundance </Th> <Td> 1.109% </Td> </Tr> <Tr> <Th> Isotope mass </Th> <Td> 13.003355 u </Td> </Tr> <Tr> <Th> Spin </Th> <Td> − ​ ⁄ </Td> </Tr> <Tr> <Td_colspan="2"> Complete table of nuclides </Td> </Tr> </Table> <Tr> <Th_colspan="2"> General </Th> </Tr>

What is the mass number of a carbon-13 (13c) atom