Hund's rule is based on the Aufbau principle that electrons are added to the lowest available energy level (shell) of an atom. Developed by the German scientist, Friedrich Hund (1896–1997), Hund's rule allows scientists to predict the order in which electrons fill an atom's suborbital shells. Lastly, Hund's rule states that even though each orbital can hold two electrons, the electrons will occupy the orbitals such that there are a maximum number of orbitals with only one electron. No two electrons in an atom will have the same set. The Pauli exclusion principle states that each electron in an atom has its own distinct set of four quantum numbers. The first, the Aufbau principle, states that an electron will occupy the lowest possible energy orbital available. The placement of electrons in a particular configuration is based on three principles. The ground state electron configuration of an atom represents the lowest energy arrangement of the electrons in an atom. The atoms for each element have their own distinct electron configuration. This arrangement is known as the electron configuration. The quantum model of the atom uses four quantum numbers to describe the arrangement of electrons in an atom, much like an address describes the locations of houses on a street. The probable location of the electrons in an orbital is described by a series of numbers called quantum numbers. An orbital indicates a probable location of the electrons in an atom instead of a definite path that they follow. Therefore, the electrons are not located in discrete orbits, as hypothesized in the Bohr model, but instead occupy a hazier region, called an orbital. This meant that the solutions to the Schrödinger wave equation, known as wave functions, could describe only the probability of finding an electron in a given orbit. This hypothesis states that the position and velocity of an electron, or any moving particle, cannot both be known at the same time. In 1927, German physicist Werner Heisenberg (1901–1976) developed what is now known as the Heisenberg uncertainty principle. Further experiments demonstrated that Broglie was correct in his assertion that matter could behave as waves, as electrons were diffracted and exhibited interference. This demonstrated that only certain energies are possible for the electrons in an atom. Furthermore, only waves of specific frequencies could be solved using his equation. It could also be used for atoms with more than one electron. Schrödinger's equation gave a much more precise description of an electron's location and energy than Bohr's model could. His wave equation could be used to find the frequency of the electrons and then Planck's equation could be used to find the corresponding energy. Schrödinger proposed that electrons also behaved like waves. Broglie's proposal was not supported by experimental or mathematical evidence until 1926 when the Austrian physicist Erwin Schrödinger (1887–1961) developed his mathematical wave equation.
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