2.5.2 The Second Failure: Atomic Spectra
Since electrons are much smaller than atoms, they must fit inside them somehow, and since atoms are electrically neutral, electrons must be joined in that structure by positively charged electrical matter. Just how these electrical entities fitted into an atom was still an open question at the end of the 19th Century. J.J. Thomson thought that electrons were swimming inside a sea of positively charged matter whose total charge equaled that of all the electrons involved. This structure was abandoned as soon as Rutherford’s 1911 experiments, in which he scattered positively charged α particles off thin plates of gold, showed that the positive charge inside an atom must be all concentrated in the center. This suggested a planetary structure for the inside of an atom with electrons rotating around that center.
That inescapable suggestion fundamentally contradicted the then current corpuscular theory of matter which envisioned the inside of things to be made up of molecules, atoms, negatively charged electrons, and positively charged protons, all acting upon each other by means of mechanical and electromagnetic forces. Indeed, if electrons moved around the nucleus in closed circular motions they should, according to Maxwell’s theory, continuously emit electromagnetic energy. As they did so, however, the radius of their orbit would have to continuously decrease until the electrons, having lost all their energy, spiraled into the nucleus, and the atom collapsed upon itself, a result manifestly at odds with the stability of matter attested by all experience. The corpuscular theory of matter could not walk itself out of this dilemma. Moreover, as the electron losses energy, the radiation emitted should have a continuous spectral compositions. Experience shows that things stand quite otherwise: atomic spectra contain only discrete frequencies the values of which follow simple rules, a fact which was totally unaccountable on Maxwell’s theory. Indeed, in 1890, Johannes Rydberg found that the wavelengths of the series of spectral lines emitted by a hydrogen atom can be described by the following rule:
where is Rydberg’s constant equal to 1.0973×107 measured in inverse meters and is an integer larger than the integer . The case when was already described by Johann Jacob Balmer as early as 1885.
While one could still argue that Planck’s inability to explain the large temperature behavior of the black body spectral distribution was a temporary failure of the corpuscular program, a failure that theoretical development would eventually overcome, it was immediately clear that the same could not be said of the atomic model; the inability of the corpuscular construct to explain atomic spectra constituted an irreconcilable contradiction at the heart of that construct. The scheme of explaining the inside of matter in general, and of atoms in particular, in terms of constituents extending Newtonian and Maxwellian forces towards each other was beginning to run into serious trouble.