# 3 development of quantum theory accomplishments of

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6.3: Development of Quantum Theory Accomplishments of the Bohr Model: Explains the structure of the hydrogen atom very well. Indicated that the location of an electron in an atom is restricted to fixed locations! But when applied to atoms with two or more electrons, the theory deviates significantly from that observed by experiment.

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Development of Quantum Theory Classical physics sufficiently explains what happens in our macroscopic world. But by the 1920’s it became clear that very small pieces of matter, such at atoms and electrons follow a different set of rules. Early 20 th century: Louis de Broglie – If light can be treated as a particle, then a particle, such as an electron, can be treated as a wave. This thinking ushered in Quantum Theory.
Development of Quantum Theory 2 Heisenberg Uncertainty Principle: It is fundamentally impossible to determine simultaneously and exactly both the momentum and the position of a particle. Erwin Schrodinger (1926): Schrodinger equation We can only specify the probability of finding an electron in a particular region of space known as an orbital.

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If electrons have wave-like properties and particle-like, then we can’t know both its position and velocity (momentum) at the same time In order to determine the position of an electron, we hit it with a photon of light, but this will change its position and velocity. Heisenberg Uncertainty Principle
Quantum Mechanics was developed (by Schrödinger in the 1920’s) to describe the motion of subatomic particles Did not attempt to describe exact position of particles; used mathematical equations to describe the probability of finding the particles The probability density (map of likely locations) is the “electron cloud” Quantum Mechanical Model

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The region of highest probability for finding an electron is an “electron cloud”. This region of high probability is also called an atomic orbital . Each orbital holds at most 2 electrons . Atomic Orbitals (NOT ORBITS) 38
Probability graph

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Quantum Numbers Every electron in an atom has its own unique set of four quantum numbers. Three of these quantum numbers are used to described the region of space (orbital) that the electron occupies. n m The fourth quantum number, m s , is associated with the electron’s spin.
Principle Quantum Number, n Principle quantum number (n) – Defines the location of the orbital. Essentially the same concept as n in the Bohr Model. n is also called the shell number. n = 1, 2, 3, 4, … n = 1 is the first shell n = 2 is the second shell, and so on … As the value of n increases, the distance the electron is from the nucleus increases, and the energy of the electron increases.

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Principle Quantum Number, n 2 Different shells are numbered by principle quantum numbers.
Angular Momentum Quantum Number, ℓ Angular momentum quantum number (ℓ) – Defines the shape of the orbital.

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