edited bv
RALPH K. BIRDWHISTEL; University of West Florida Pensacola, FL 32504
textbook forum On Neutron Numbers and Atomic Masses
R. Heyrovske The J. HeyrovskG Institute of Physical Chemistry and Electrochemistry, DolejHkova 3,18223 Prague 8, Czechoslovakia Assigning Neutron Numbers Standard textbooks (13)give the neutron number N of
Correct Neutron Numbers From the available (4-6) atomic mass data for the most
a nuclide
abundant isotope (or the one with the longest half-life) of all the elements, it was found that the following values of N satisfy eq 4.
zxM as the difference N=M-Z
where Z is the atomic number and M is the mass number. The mass number M, defined as the integer nearest to the atomic mass A, is also the nucleon number Z + N. This is based on two approximations. The masses, mp and m,, of a proton and a neutron, respectively, are each 1 amu. The mass me of the electron is negligible. The atomic mass unit (amu) is based on the standard &lZ = 12 arnu in which 1 arnu is 931 megaelectronvolts (MeV). The exact values of the rest masses ( 3 , 4 )available now mp = 1.0072765 amu m,
= 1.0086649 amu me = 0.00054858 amu
This is the conventional approximation that is valid only for nuclides whose mass numbers M are less than 108, that is, up to 47Ag'07.For example, for .&" A = 12.000000 arnu N=6
AZ, N = 12.098940 amu
MZ, = 0.098940 arnu < m, N = M - Z - 1 When 108251
This is valid for nuclides whose mass numbers M is greater than 251. For example, for lozNo2" A = 259.100931 amu Az,N = 259.141215 arnu
is considered as the nuclear binding energy (13), where c is the velocity of light. As per eq 3 , l arnu amounts to 931 megaelectron volts (MeV). Because the mass defect depends on the value o f A & ~it, is essential to use the correct neutron number N in eq 1. The minimum integral number N of neutrons used in eq 1 must be such that A,,,>A
where OSA4z,N
