# 8: Multielectron Atoms

Electrons with more than one atom, such as Helium (He), and Nitrogen (N), are referred to as multi-electron atoms. Hydrogen is the only atom in the periodic table that has one electron in the orbitals under ground state. We will learn how additional electrons behave and affect a certain atom.

- 8.1: Atomic and Molecular Calculations are Expressed in Atomic Units
- Atomic units form a system of natural units which is especially convenient for atomic physics calculations. There are two different kinds of atomic units, Hartree atomic units and Rydberg atomic units, which differ in the choice of the unit of mass and charge. This article deals with Hartree atomic units, where the numerical values of the following four fundamental physical constants are all unity by definition.

- 8.2: Perturbation Theory and the Variational Method for Helium
- Both perturbation theory and variation method (especially the linear variational method) provide good results in approximating the energy and wavefunctions of multi-electron atoms. We address both approximations with respect to the helium atom.

- 8.3: Hartree-Fock Equations are Solved by the Self-Consistent Field Method
- The Hartree method is used to determined the wavefunction and the energy of a quantum multi-electron system in a stationary state. The Hartree method often assumes that the exact, N-body wave function of the system can be approximated by a product of single-electron wavefucntions. By invoking the variational method, one can derive a set of N-coupled equations for the N spin orbitals.

- 8.4: An Electron Has an Intrinsic Spin Angular Momentum
- Spin is one of two types of angular momentum in quantum mechanics, the other being orbital angular momentum. The orbital angular momentum operator is the quantum-mechanical counterpart to the classical angular momentum of orbital revolution. The existence of spin angular momentum is inferred from the Stern–Gerlach experiment, in which particles are observed to possess angular momentum that cannot be accounted for by orbital angular momentum alone.

- 8.5: Wavefunctions must be Antisymmetric to Interchange of any Two Electrons
- The probability |Ψ(r1, r2)|² should be identical to the probability |Ψ(r2, r1)|² because the electrons have no label and they cannot be told apart because of Heisenberg principle. You can naively think that Ψ(r1, r2)=±Ψ(r2, r1) but it turns out that the sign must always be minus for the electrons. This is an additional postulate of quantum mechanics.

- 8.6: Antisymmetric Wavefunctions can be Represented by Slater Determinants
- John Slater introduced an idea of a Slater determinant that is a relatively simple scheme for constructing antisymmetric wavefunctions of multi-electron systems from a product of one-electron functions in the form of a determinant.

- 8.7: Hartree-Fock Calculations Give Good Agreement with Experimental Data
- The Hartree–Fock method is a method of approximation for the determination of the wave function and the energy of quantum many-body systems. The Hartree–Fock method often assumes that the exact, N-body wave function of the system can be approximated by a single Slater determinant of N spin-orbitals. By invoking the variational method, one can derive a set of N-coupled equations for the N spin orbitals. A solution of these equations yields the Hartree–Fock wave function and energy of the system.

- 8.8: A Term Symbol Gives a Detailed Description of an Electron Configuration
- The term symbol is an abbreviated description of the (total) angular momentum quantum numbers in a multi-electron atom (however, even a single electron can be described by a term symbol). Each energy level of an atom with a given electron configuration is described by not only the electron configuration but also its own term symbol, as the energy level also depends on the total angular momentum including spin. The usual atomic term symbols assume LS coupling (Russell-Saunders coupling).

- 8.9: The Allowed Values of J - the Total Angular Momentum Quantum Number
- The total angular momentum quantum number parameterizes the total angular momentum of a given particle, by combining its orbital angular momentum and its intrinsic angular momentum (i.e., its spin).

- 8.10: Hund's Rules Determine the Term Symbols of the Ground Electronic States
- The allocation electrons among degenerate orbitals can be formalized by Hund’s rule: For an atom in its ground state, the term with the highest multiplicity has the lowest energy. Hund's first rule states that the lowest energy atomic state is the one that maximizes the total spin quantum number for the electrons in the open subshell. The orbitals of the subshell are each occupied singly with electrons of parallel spin before double occupation occurs.

- 8.11: Using Atomic Term Symbols to Describe Atomic Spectra
- The electronic states that result from these excited orbital configurations are characterized by term symbols and are essential in understanding the spectra and energy level structure of atoms, and the orbital electron configurations. The orbital configurations help us understand many of the general or coarse features of spectra and are necessary to produce a physical picture of how the electron density changes because of a spectroscopic transition.

- 8.E: Multielectron Atoms (Exercises)
- These are homework exercises to accompany Chapter 8 of McQuarrie and Simon's "Physical Chemistry: A Molecular Approach" Textmap.