Approximate atomic masses are Na 23.0, Mg 24.3, C 12.0, N 14.0, O 16.0, F 19.0, Ar 39.9 and K 39.1. If elements were arranged strictly by increasing atomic mass, which pair would be placed in the wrong order, a problem that the modern table resolves by using atomic number?

ANa and Mg
BC and N
CO and F
DAr and K

Answer: D

By mass K (39.1) would come before Ar (39.9), putting an alkali metal in the noble gas position. Atomic numbers (Ar 18, K 19) give the chemically correct order. The other pairs already have mass and atomic number in the same order.

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Approximate atomic masses are Na 23.0, Mg 24.3, C 12.0, N 14.0, O 16.0, F 19.0, Ar 39.9 and K 39.1. If elements were arranged strictly by increasing atomic mass, which pair would be placed in the wrong order, a problem that the modern table resolves by using atomic number?

Answer: D

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