If one were to base the atomic mass scale on 19F at 1900000

If one were to base the atomic mass scale on 19F at 19.00000 atomic mass units (amu), calculate the mass (grams) of the 19F atom and the mass (grams) of one amu. Why is its mass on the 12C scale different from 19.00000?

Solution

the number of protons in 19F are 9 and neutrons are 10

                       mass of 19 F atom M = 9 * m_p + 10 m_n            (igonoring the mass of electrons)

                                        m_p = 1.672 * 10^-24 gm and m_n = 1.6749 * 10^-24 gm

                                                       = 15.048 +16.749

                                                       M = 31.797 * 10^-24 gm

               defining     1 amu = 31.797 * 10^-24 / 19

                                                      = 1.673526 * 10^-24 gms which is different from 1 amu of 12 C = 1.66054 * 10 ^-24 gms

               the differece is due to the mass defect , or difference in the binding energy between the nucleus in 19F and 12C

usually the nucleus with even number of protons and even number of neutrons will stay with pocesses high binding energy and hence the mass defect is high , but the contrary is true in the case of 19 F.