Potassium atomic weight

A similar activity is found in Mendeleevs first attempt at a periodic system as presented in a hand-written table. If one examines the calculations that he is carrying out one finds again an attempt to compute differences between the atomic weights of elements in the columns of his table. For example Mendeleev writes the number 27 in smaller writing below the symbols for potassium (Zn - K = 65 - 39 = 27) and again below rubidium (Cd-Rb = 112-85 = 27). 

The atomic weight of an element is a weighted average of the mass of the naturally occurring isotopes of that element. Therefore, the atoms in a naturally occurring sample of argon must be heavier than the atoms in a naturally occurring sample of potassium. 

Atoms of argon have 18 protons, whereas atoms of potassium have 19 protons. In order for the atomic weight of argon to be greater than that of potassium, argon atoms must have more neutrons. 

From these data, we can see that argon would have a higher atomic weight than potassium. 

Thus Moseley s series is almost the same as Mendeleev s series of increasing atomic weights. When, however, the elements are arranged, not according to their atomic weights, but according to their atomic numbers (Moseley numbers), the discrepancies between argon and potassium and between iodine and tellurium disappear (10). 

Gamer, E. L., Murphy, T. J., Bramlich, J. W., Paulsen, P. J. and Barnes, I. L. (1975) Absolute isotopic abundance ratios and the atomic weight of a reference sample of potassium. Journal of Research of the National Bureau of Standards A. Physics and Chemistry, 79A, 713—725. 

The atomic weight of bromine.—The at wt. of bromine has been determined by methods which follow in principle those employed for chlorine. A. J. Balard (1826),14 the discoverer of bromine, transformed a known weight of potassium bromide into the sulphate, and also reduced silver bromide to metallic silver by means of zinc the numbers 74 7 and 75 3 were respectively obtained. J. von Liebig (1826)... 

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