Option (C) Nuclear mass is the correct answer. The chemistry of an element depends on its electronic configuration and atomic number, not on its relative atomic mass. The valence shell is affected by the valence principal quantum number ($n$), which determines the size and energy of the outermost orbitals. It is also influenced by the nuclear charge ($Z$), which dictates the attraction between the nucleus and the valence electrons, and the number of core electrons, which provides shielding to the valence electrons. Nuclear mass, however, does not influence the valence electrons or their chemistry.