If we aim a radio telescope at a distant spiral arm of the Milky Way Galaxy, we will probably observe a 21-cm line. If we point a large optical telescope at this same region, we will probably not be able to detect the neutral hydrogen that gives rise to the 21-cm radio signal. Why not

According to rutherford's nuclear theory, most of the volume of an atom is , so the volume of a hydrogen atom cannot be mostly due to the proton. according to rutherford's nuclear theory, the core of an atom (nucleus) contains most of of an atom and is , so the majority of the mass of a fluorine atom cannot be due to its nine electrons. according to rutherford's nuclear theory, the number of negatively charged particles outside the nucleus the number of positively charged particles within the nucleus, so a nitrogen atom has 7 protons and 7 electrons, while a phosphorous atom cannot have 15 protons and 150 electrons. word bank: dense, empty space, more than, mass, particles, the same as, less than, positively charged, negatively charged