Ask A Scientist Chemistry Archive

Question:
Why does cobalt have a higher atomic mass (58.93) than
nickle (58.63) even though it has a lower atomic number
(27 vs. 28 for nickle)?
 al gawt

Answer 1:
The atomic masses you see on a periodic table are not the exact masses
of the element.  They're actually a weighted average of all the isotopes
of the element.  An example I know off the top of my head is for carbon:
It has two isotopes...C12 and C13.  The natural abundance of C12 is
98.89% and that of C13 is 1.11%.  So, (0.9889x12.00000) + (0.0111x13.00335)
is equal to 12.0 (3 sig figs).  So cobalt probably has a higher percentage
of higher mass isotopes than does nickel thereby giving it a larger
average atomic mass.
-joe
 
Answer 2:
Joe is, of course, absolutely correct. Just a few numbers
from Kenneth Krane's "Modern Physics" (1983);
Only 5 isotopes of Co are known to exist at all, and 4 of them
have only been created in the laboratory. Naturally
occuring samples of cobalt are 100% Co(59), which weighs
58.933198 amu.  Naturally occuring Ni, however, exists in
5 isotopes...
N
Isotope    atomic mass (amu)   abundance
 
Ni58       57.935347           68.3 %
Ni60       59.930789           26.1 %
Ni61       60.931059            1.13%
Ni62       61.928346            3.59%
Ni64       63.927968            0.91%
 
There are, in addition, 5 radioactive isotopes of Ni listed.
Two of them last a LONG time, as measured by their half-lives...
 
Isotope    atomic mass (amu)   halflife
 
Ni59       58.934350           0.075 x 10^6 years
Ni63       62.929670           100 years
 
(actually that last half-life is pretty short). -topper