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CRC Press eBooks

The Hydrogen-Like Atom

Ahmad Amer

Published Nov 5, 2025• 0 citations
Publisher

Abstract

This chapter presents a detailed analysis of the hydrogen-like atom within the framework of quantum mechanics. The radial Schrödinger equation is solved for the Coulomb potential, yielding the quantized energy spectrum, energy eigenstates, and the associated degeneracy of levels. The expectation value of the electron's radius is calculated, and a comparison is drawn between quantum predictions and classical circular orbits. The quantum two-body problem is introduced to account for the motion of the nucleus, revealing its influence on the energy levels of the electron. This correction underpins the operation of the mass spectrometer and supports the spectroscopic identification of hydrogen isotopes such as deuterium and tritium. The analysis also explains how similar effects confirm the presence of helium in the solar spectrum through astrophysical observations. Metadata/Keywords: hydrogen-like atom, Coulomb potential, energy spectrum, quantum orbits, nucleus motion, two-body problem, mass spectrometer, hydrogen isotopes, helium, solar spectrum

DOI
10.1201/9781003677109-7

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•Mar 3, 2017•0 citations
This chapter presents a detailed analysis of the hydrogen-like atom within the framework of quantum mechanics. The radial Schrödinger equation is solved for the Coulomb potential, yielding the quantized energy spectrum, energy eigenstates, and the associated degeneracy of levels. The expectation value of the electron's radius is calculated, and a comparison is drawn between quantum predictions and classical circular orbits. The quantum two-body problem is introduced to account for the motion of the nucleus, revealing its influence on the energy levels of the electron. This correction underpins the operation of the mass spectrometer and supports the spectroscopic identification of hydrogen isotopes such as deuterium and tritium. The analysis also explains how similar effects confirm the presence of helium in the solar spectrum through astrophysical observations. Metadata/Keywords: hydrogen-like atom, Coulomb potential, energy spectrum, quantum orbits, nucleus motion, two-body problem, mass spectrometer, hydrogen isotopes, helium, solar spectrum
The mere existence of stable atoms, and their arrangement into many-atom nanoscale structures, can only be explained by the laws of quantum mechanics. Indeed, the accumulated evidence regarding the internal structure of atoms motivated the formulation of quantum mechanics at the early twentieth century. Here we focus on single-electron (hydrogen-like) atoms. The dominance of the Coulomb central potential between the electron and nucleus, translates into a simple Schrödinger equation for their relative position distribution. Solving rigorously this equation reveals quantized energy levels, which explains the emission spectra of hydrogen-like atoms (Rydberg formula). The corresponding stationary probability density distributions for the electron around the nucleus are reminiscent of the Bohr model, where classical trajectories (orbits) are replaced by single-electron wave functions (orbitals). The degeneracies of different orbitals, associated with three quantum numbers, will later turn out to be critical for understanding many-electron atoms and the periodic table of the elements.
This chapter begins with the simple solution of the Schrödinger equation for hydrogen atom for which the energy value has been derived. The Bohr radius of H atom has also been computed. After this, this chapter provides the generalised solution of the Schrödinger equation for H atom/hydrogen-like species in which F equation, R equation, and T equations have been derived in a simple and lucid way. This chapter also deals with the Laguerre differential equation, Laguerre polynomials, Rodrigues formula for the Laguerre polynomials, Laguerre associated equation and its solution, associated Laguerre polynomials, Rodrigues formula for the associated Laguerre polynomials, and solution of the radial equation. It also provides normalisation of the radial wave function and complete wave function; for the hydrogen atom, hydrogenic atomic orbital; electron density of 1 s electron; mathematical background of shape of orbitals; the most probable distance of electron from nucleus of hydrogen atom, and average distance of electron from the nucleus of hydrogen atom. The end of this chapter describes the references and adequate computational problems (solved problems and question on concepts).
Niels Bohr’s model of the hydrogen and hydrogen-like atoms, in which the electron is allowed to revolve in certain circular orbits around the nucleus, was extended by Arnold Sommerfeld to allow for the possibility of elliptical orbits. Each elliptical orbit was constrained by two quantization conditions specified by a positive integer nφ and a non-negative integer nr. If, in the presence of the attractive Coulomb force between the electron and the nucleus, the dynamics of the atom were taken to be governed by the Newtonian laws of motion, the energy Ɛ of the two-particle system was found to be a function of the positive integer n = nr + nφ. The atomic energy levels thus exhibited an n-fold degeneracy, which would be lifted in the presence of external electric or magnetic fields, thus accounting for the fine-structure of the spectral lines emitted by the atom—phenomena observed, respectively, in the Stark effect and the Zeeman effect. Sommerfeld also examined the behavior of his elliptical orbits by allowing the revolving electron to obey Einstein’s relativistic dynamics. In this case, the energy-level degeneracies were lifted (even in the absence of external perturbations), and the elliptical orbits were found to precess around the nucleus—similarly to the precession of, say, the orbit of the planet Mercury around the Sun. What is astonishing about Sommerfeld’s relativistic expression of the energy Ɛ as a function of nφ and nr is that it is precisely the same expression as obtained for hydrogen and hydrogen-like atoms from Dirac’s equation, which came more than a decade later. Recall that the spin of the electron was not yet discovered when Sommerfeld developed his model, and that in Dirac’s fully-relativistic quantum mechanics, the spectral fine-structure arises from the coupling of the electron’s spin to its orbital motion. The present paper provides a streamlined yet detailed account of Sommerfeld’s elliptical orbits in hopes of bringing about a deeper appreciation for the miraculous coincidence of Sommerfeld’s result with that derived from Dirac’s equation.
The hydrogen atom is the simplest of all atoms consisting of proton of charge e and the electron of charge -e. It is a two particle system. A two-body problem can always be reduced to a one-body problem by choosing the centre of mass coordinate and the relative coordinates. The existence of degenerate energy eigenvalues implies that linear combination of the corresponding eigenfunctions are solutions of the wave equation with the same energy. The isotopes of atomic hydrogen, deuterium and tritium also provide examples of hydrogenic systems. Since the mass of deuterium is md˜2mp and that of tritium is mt˜3mp, the reduced masses of them will be slightly different from that of hydrogen. In addition to deuterium and tritium, there exists other hydrogenic systems in which the role of proton is played by another positive particle.
Because of considerable mathematical difficulties the Schrödinger equation can be solved exactly for only a few simple quantum systems. In particular for atoms with at least two electrons no such exact solutions exist. The most important system where an exact solution is indeed possible consists of a single electron with charge q 1 = - e in motion under the influence of a Coulomb potential about a nucleus with a positive charge q 2 = Ζe , for Ζ ≥ 1 an integer. Such a system is called a hydrogenic or hydrogen-like atom , with atomic number Z . One may view the nucleus as consisting of Ζ protons. For Ζ = 1 we obtain the hydrogen atom. For Ζ = 2, for example, the system is singly ionized helium He + ; this is helium with its two protons as usual, but stripped of one of its two electrons. Since it has only one electron but two protons it is positively charged, as indicated by the plus sign +. Similarly the lithium atom Li consists of three electrons and three protons (and four neutrons). If we strip off two electrons we obtain the ion Li ++ —doubly ionized lithium, which is the case Ζ = 3. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
This chapter illustrates Niels Bohr's ideas in some detail. In the formulation of his model, Bohr assumed that the electron in the hydrogen atom moves under the influence of the Coulomb attraction between it and the positively charged nucleus, as assumed in classical mechanics. The Bohr model was very successful in explaining the discrete atomic spectra of one-electron atoms. The Bohr model is, of course, a theoretical model of an atom. Using Bohr's quantization postulate, find the linear velocity of the electron's sphere. To find the radius of the orbit, Bohr postulated that the angular momentum of the electron is quantized, i.e., it can only take values that are integer multiples of ħ. The chapter begins with considering the classical equation of motion for the electron in a circular orbit, which is based on Newton's laws of motion and Coulomb's law of electric force.