If multi-electron atoms did have all of their electrons placed in the lowest energy levels, then very different periodic patterns would have resulted from what was actually observed. The German physicist Wolfgang Pauli made a quantum leap by realizing that in order for there to be differences in ionization energies and atomic volumes among atoms with many electrons, there had to be a way that the electrons were not all placed in the lowest energy levels. (This is discussed in more detail in our module The Periodic Table of Elements.) By the early 1920s, other periodic trends, such as atomic volume and ionization energy, were also well established. In the late 19 th century, the father of the periodic table, Russian chemist Dmitri Mendeleev, had already determined that the elements could be grouped together in a manner that showed gradual changes in their observed properties. Periodic trends lead to the distribution of electrons As such, at the beginning of the second decade of the 20 th century, another rich vein of scientific work was about to be mined. As is consistent with all new scientific discoveries, a fresh way of thinking about the universe at the atomic level would only lead to more questions, the need for additional experimentation and collection of evidence, and the development of expanded theories. Bohr’s new idea that electrons could only be found in specified, quantized orbits was revolutionary (Bohr, 1913). The Pauli exclusion principle states that no two electrons with the same spin can occupy the same orbital.Īs discussed in our Atomic Theory II module, at the end of 1913 Niels Bohr facilitated the leap to a new paradigm of atomic theory – quantum mechanics. The discovery of electron spin defines a fourth quantum number independent of the electron orbital but unique to an electron. The Heisenberg Uncertainty Principle establishes that an electron’s position and momentum cannot be precisely known together, instead we can only calculate statistical likelihood of an electron’s location. These ‘maps’ have come to be known as the s, p, d, and f orbitals. Max Born’s interpretation of the Schrödinger equation allows for the construction of three-dimensional probability maps of where electrons may be found around an atom. Using quantum numbers, one can write the wave function, and find a solution to the equation that helps to define the most likely position of an electron within an atom. The Schrödinger equation describes how the wave function of a wave-particle changes with time in a similar fashion to the way Newton’s second law describes the motion of a classic particle.

Electrons, like light, have been shown to be wave-particles, exhibiting the behavior of both waves and particles.

Erwin schrodinger atomic theory