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# 8: Atomic Structure

In this chapter, we use quantum mechanics to study the structure and properties of atoms. This study introduces ideas and concepts that are necessary to understand more complex systems, such as molecules, crystals, and metals. As we deepen our understanding of atoms, we build on things we already know, such as Rutherford’s nuclear model of the atom, Bohr’s model of the hydrogen atom, and de Broglie’s wave hypothesis.

• 8.1: Prelude to Atomic Structure
NGC1763 is an emission nebula in the small galaxy known as the Large Magellanic Cloud, which is a satellite of the Milky Way Galaxy. Ultraviolet light from hot stars ionizes the hydrogen atoms in the nebula. As protons and electrons recombine, radiation of different frequencies is emitted. The details of this process can be correctly predicted by quantum mechanics and are examined in this chapter.
• 8.2: The Hydrogen Atom
In contrast to the Bohr model of the hydrogen atom, the electron does not move around the proton nucleus in a well-defined path. Indeed, the uncertainty principle makes it impossible to know how the electron gets from one place to another. A hydrogen atom can be described in terms of its wave function, probability density, total energy, and orbital angular momentum. The quantum numbers of a hydrogen atom can be used to calculate important information about the atom.
• 8.3: Orbital Magnetic Dipole Moment of the Electron
A hydrogen atom has magnetic properties because the motion of the electron acts as a current loop. The energy levels of a hydrogen atom associated with orbital angular momentum are split by an external magnetic field because the orbital angular magnetic moment interacts with the field. The potential energy of the hydrogen atom associated with this magnetic interaction is given by $$U = -\vec{\mu} \cdot \vec{B}$$.
• 8.4: Electron Spin
The spin angular momentum quantum of an electron is = +½. The spin angular momentum projection quantum number is ms =+½or−½ (spin up or spin down). The energy of the electron-proton system is different depending on whether or not the moments are aligned. Transitions between these states (spin-flip transitions) result in the emission of a photon.
• 8.5: The Exclusion Principle and the Periodic Table
The structure and chemical properties of atoms are explained in part by Pauli’s exclusion principle: No two electrons in an atom can have the same values for all four quantum numbers (n,l,m,ms). This principle is related to two properties of electrons: All electrons are identical and they have half-integral spin (s=1/2).
• 8.6: Atomic Spectra and X-rays
Radiation is absorbed and emitted by atomic energy-level transitions. Quantum numbers can be used to estimate the energy, frequency, and wavelength of photons produced by atomic transitions.  X-ray photons are produced when a vacancy in an inner shell of an atom is filled by an electron from the outer shell of the atom. The frequency of X-ray radiation is related to the atomic number Z of an atom.
• 8.7: Lasers
A laser is device that emits coherent and monochromatic light. Laser light is produced by population inversion and subsequent de-excitation of electrons in a material (solid, liquid, or gas). When a photon of energy triggers an electron in a metastable state to drop in energy emitting an additional photon, it provokes stimulated emission.
Thumbnail: Spin-orbit coupling is the interaction of an electron’s spin magnetic moment $$\vec{\mu}_s$$ with its orbital magnetic moment $$\vec{\mu}_l$$.