1) Bohr's model of electrons orbiting the nucleus in circular orbits was ineffective beyond simple atoms like hydrogen and lithium. 2) De Broglie, Heisenberg, and Schrödinger pioneered quantum mechanics, proposing that electrons behave as waves rather than particles in orbits. 3) In Schrödinger's wave mechanical model, electrons are described by wave functions that represent probability distributions of their location and discrete energy levels.