Complex sentence: The Heisenberg uncertainty principle of quantum mechanics presented a serious objection to the proton--electron model for nuclei, since the principle implied that a particle as light as the electron confined in a region as small as the nucleus would have enormous energy.
Split into simpler sentences: The Heisenberg uncertainty principle of quantum mechanics presented a serious objection to the proton--electron model for nuclei. According to this principle, if a particle as light as the electron were confined in a region as small as the nucleus, it would have enormous energy.