Uranium-238 is 238 times more massive than hydrogen. Why, then, isn't the diameter of the uranium atom 238 times that of the hydrogen atom?
The mass of an atom primarily depends on the number of protons and neutrons it possesses. In the case of Uranium-238, it has 92 protons and 146 neutrons in its nucleus, giving it a total mass of 238 atomic mass units. Hydrogen, on the other hand, has just one proton and no neutrons, resulting in a mass of 1 atomic mass unit.
So, while Uranium-238 is indeed 238 times more massive than Hydrogen, the size or diameter of an atom is mostly determined by the electrons surrounding the nucleus. The protons and neutrons in the nucleus of an atom are highly concentrated and have a relatively small size compared to the overall atom.
The diameter of an atom is mainly influenced by the electron cloud, which determines the outer boundary of the atom. Electrons have negligible mass compared to protons and neutrons, so their presence does not significantly contribute to the overall mass of the atom. Instead, the electrons spread out in a region surrounding the nucleus, occupying an "electron cloud" that extends far beyond the size of the nucleus itself.
Therefore, the difference in mass between Uranium-238 and Hydrogen does not result in a proportional increase in the diameter of their respective atoms. The size of an atom is primarily governed by the arrangement and distribution of the electrons, rather than the mass of the atom itself.