Assertion: deuterium's proton | /r/HasProperty | known particle
Facets:
Source sentences:
text: However, it was expected that the element hydrogen with a measured average atomic mass very close to 1 u, the known mass of the proton, always has a nucleus composed of a single proton (a known particle), and could not contain a second proton.
text: It was expected that hydrogen, with a measured average atomic mass very close to 1 u, and a nucleus thought to be composed of a single proton (a known particle), could not contain nuclear electrons, and thus could have no heavy isotopes.