That quantum mechanics prevents electrons from collapsing into the atomic nucleus is a foundational, textbook principle of quantum physics rooted in the wave-like nature of electrons and the Heisenberg uncertainty principle. As a fundamental physical truism taught in basic coursework, it requires no external citation.
The claim that quantum mechanics prevents electrons from falling into the atomic nucleus is a classic textbook fact of quantum mechanics (explaining atomic stability via the uncertainty principle or wave mechanics). Following Step 0, trivial facts and fundamental textbook principles that represent direct everyday scientific knowledge do not require citations and are classified as COMMON_KNOWLEDGE.