The definition of a kilogram has recently been changed. What effect, if any, will this change have on data on the periodic table?

  • 1
    $\begingroup$ chemistry.stackexchange.com/questions/110189/… $\endgroup$
    – Mithoron
    May 21, 2019 at 18:21
  • 3
    $\begingroup$ Possible duplicate of Is the kilogram still consistent with the old definition? $\endgroup$
    – Mithoron
    May 21, 2019 at 18:24
  • 1
    $\begingroup$ @Mithoron I think the question and answer linked don't work as duplicates as the question refers to an earlier change of the definition of the kilogram (mass of a cubic centimeter of water changing to a reference cylinder of platinum), while the answer discusses a proposal for a new definition that wasn't ultimately used (the number of atoms in a sphere of silicon-28). $\endgroup$
    – Tyberius
    May 21, 2019 at 19:06
  • $\begingroup$ Check out this video by the chanel veritasium which explains about the various implications of redefining the kg in detail $\endgroup$ May 21, 2019 at 19:16
  • $\begingroup$ The number of significant figures of carbon-12’s molar mass changed on May 20th, but for a different reason. Otherwise, no changes. $\endgroup$
    – Karsten
    May 21, 2019 at 20:14

2 Answers 2


Molar masses

The molar mass of the carbon-12 isotope used to be 12 g/mol exactly (by definition). Now, it is experimentally determined, with the current value $\pu{11.9999999958(36) g mol-1}$ (CODATA 2018). So the molar mass of carbon-12 changed. The standard atomic weights (relative atomic masses) shown in the periodic table are not affected by this. However, atomic weights (dimension one) are no longer exactly equal to molar mass divided by 1 g/mol. Instead, they differ by a tiny bit (you would need 9 or 10 significant digits to see the difference).

No visible changes

Here is an animated gif of a portion of the periodic table at ptable.com over the past four years (Jan 23rd 2016, Jan 10th 2017,Jan 6th 2018, May 2nd 2019). Nothing much has changed near carbon:

enter image description here

The standard atomic weights change a bit each year, as better estimates of the isotope compositions of elements with multiple stable isotopes are made. Also, the publishers of this specific periodic table decided to truncate atomic weights to 5 significant figures in 2018. The biggest change occurred when elements in period 7 were discovered and named.

The last image is from May 2nd 2019, before the change of the kilogram (and mole) definition. On May 29th 2019, the data shown at ptable.com is identical.


I have not checked for every element, but I don't expect any difference to be significant as the goal of any international convention on that subject was to keep "backward compatibility" (as we would say for computers) with existing measures.

Well, of course, the 7th or 8th decimal can change, but it won't change the face of the world. At least, we won't suddenly grow up 4cm :-)


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.