[billnotgatez]

I was looking up the atomic weight of Cobalt from 2 different resources.
I noticed that there was a slight disparity between wikipedia.org and webqc.org.
I then looked up the values for Oxygen, Carbon, and Hydrogen.




http://www.webqc.org/periodictable-Cobalt-Co.html 
Co          Atomic weight   58.9331955 amu
http://en.wikipedia.org/wiki/Cobalt               
Co Standard atomic weight   58.933194(4)

http://www.webqc.org/periodictable-Oxygen-O.html   
O           Atomic weight   15.99943 am
http://en.wikipedia.org/wiki/Oxygen               
O  Standard atomic weight   15.999(4)

http://www.webqc.org/periodictable-Carbon-C.html   
C           Atomic weight   12.01078 amu
http://en.wikipedia.org/wiki/Carbon               
C  Standard atomic weight   12.011(1)

http://www.webqc.org/periodictable-Hydrogen-H.html
H           Atomic weight    1.007947 amu
http://en.wikipedia.org/wiki/Hydrogen             
H  Standard atomic weight    1.008(1)

Name, symbol, number    cobalt,   Co,  27
Name, symbol, number    oxygen,   O,    8
Name, symbol, number    carbon,   C,    6
Name, symbol, number    hydrogen, H,    1



I noticed that the differences between the 2 sources appeared to be rounding except for the Cobalt.
I notice the Cobalt difference was slight.
But, I am still puzzled.

By the way, the WIKI has a parenthesis number at the end of each weight but I have forgotten what that means and my search powers fail me.

[Borek]

For me NIST is a definite source:

http://www.nist.gov/pml/data/comp.cfm

So Co is 58.9331950(7).

Number in parentheses means uncertainty of the given number:

http://physics.nist.gov/cgi-bin/cuu/Info/Constants/definitions.html

[billnotgatez]

Here is an extract from NIST so I can compare



The National Institute of Standards and Technology (NIST)
Atomic Weights and Isotopic Compositions

Cobalt
http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=Co&ascii=html&isotype=some
Isotope
27 Co 59
Relative Atomic Mass
58.933 195 0(7)
Isotopic Composition
1.0000
Standard Atomic Weight
58.933 195(5)

Oxygen
http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=o&ascii=html&isotype=some
Isotope
 8 O  16                 17                   18
Relative Atomic Mass
15.994 914 619 56(16)    16.999 131 70(12)    17.999 161 0(7)
Isotopic Composition
 0.997 57(16)             0.000 38(1)          0.002 05(14)
Standard Atomic Weight
15.9994(3)
 
Carbon
http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=c&ascii=html&isotype=some
Isotope
 6 C  12                 13                   14
Relative Atomic Mass
12.000 000 0(0)          13.003 354 8378(10)  14.003 241 989(4)
Isotopic Composition
 0.9893(08)               0.0107(08)
Standard Atomic Weight
12.0107(08)
 
Hydrogen
http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=h&ascii=html&isotype=some
Isotope
 1 H   1                D 2                  T 3
Relative Atomic Mass
 1.007 825 032 07(10)     2.014 101 777 8(4)   3.016 049 2777(25)
Isotopic Composition
 0.999 885(70)            0.000 115(70)   
Standard Atomic Weight
 1.007 94(7)

[billnotgatez]

I know the differences are very very small, but it is interesting to see them

Atomic Weight from 3 sources



Cobalt   27 Co 59
NIST  Standard Atomic Weight   58.933 195(5)
WIKI  Standard atomic weight   58.933194(4)
WebQC          Atomic weight   58.9331955 amu

Oxygen    8 O  16
NIST  Standard Atomic Weight   15.9994(3)
WIKI  Standard atomic weight   15.999(4)
WebQC          Atomic weight   15.99943 amu

Carbon    6 C  12
NIST  Standard Atomic Weight   12.0107(08)
WIKI  Standard atomic weight   12.011(1)
WebQC          Atomic weight   12.01078 amu

Hydrogen  1 H   1
NIST  Standard Atomic Weight    1.007 94(7)
WIKI  Standard atomic weight    1.008(1)
WebQC          Atomic weight    1.007947 amu

[Borek]

Looks like WebQC copies data from NIST not understanding what it means. Shame on them.

Could be data in Wikipedia is based on some other, earlier source, as NIST data seems to be more accurate.

[408]

These things get revised over the years as techniques get more accurate.  Depends what their reference was and when it was from.  This is why for all intents and purposes, rounding to 2 decimals is good enough.