Thanks for the responses.
Follow on questions:
Is there a difference in weight between polymorphs of the same element? I.e., would a cubic centemeter of diamond weigh the same as a cubic centimeter of graphite?
Relating to another thread, this question;
An atom of Nickel has 28 protons, 28 electrons, and 31 neutrons.
An atom of Barium has 56 protons, 56 electrons, and 81 neutrons.
Would 87 atoms of Barium weigh the same as 193 atoms of Nickel?
(Each would have a total of 16,791 protons, electrons and neutrons.)