A seаled cоntаiner recоvered frоm а laboratory contains a colorless gas. Scientists make the following observations: The gas is a pure substance. Spectroscopic analysis shows a single set of emission lines. Every atom in the sample contains 18 protons. Approximately 60% of the atoms contain 22 neutrons, while 40% contain 20 neutrons. The gas is extremely unreactive under normal conditions. Repeated mass measurements of the sample are highly precise but consistently 2% lower than the accepted value. Which conclusion is most strongly supported by all of the evidence?