6.3 Development of Quantum Theory

Behavior in the Microscopic World

We know how matter behaves in the macroscopic world—objects that are large enough to be seen by the naked eye follow the rules of classical physics. A billiard ball moving on a table will behave like a particle: It will continue in a straight line unless it collides with another ball or the table cushion, or is acted on by some other force (such as friction). The ball has a well-defined position and velocity (or a well-defined momentum, p = mv, defined by mass m and velocity v) at any given moment. In other words, the ball is moving in a classical trajectory. This is the typical behavior of a classical object.

When waves interact with each other, they show interference patterns that are not displayed by macroscopic particles such as the billiard ball. For example, interacting waves on the surface of water can produce interference patters similar to those shown on (Figure). This is a case of wave behavior on the macroscopic scale, and it is clear that particles and waves are very different phenomena in the macroscopic realm.

As technological improvements allowed scientists to probe the microscopic world in greater detail, it became increasingly clear by the 1920s that very small pieces of matter follow a different set of rules from those we observe for large objects. The unquestionable separation of waves and particles was no longer the case for the microscopic world.

One of the first people to pay attention to the special behavior of the microscopic world was Louis de Broglie. He asked the question: If electromagnetic radiation can have particle-like character, can electrons and other submicroscopic particles exhibit wavelike character? In his 1925 doctoral dissertation, de Broglie extended the wave–particle duality of light to material particles. He predicted that a particle with mass m and velocity v (that is, with linear momentum p) should also exhibit the behavior of a wave with a wavelength value λ, given by this expression in which h is the familiar Planck’s constant:

This is called the de Broglie wavelength. Unlike the other values of λ discussed in this chapter, the de Broglie wavelength is a characteristic of particles and other bodies, not electromagnetic radiation (note that this equation involves velocity [v, m/s], not frequency [ν, Hz]. Although these two symbols appear nearly identical, they mean very different things). Where Bohr had postulated the electron as being a particle orbiting the nucleus in quantized orbits, de Broglie argued that Bohr’s assumption of quantization can be explained if the electron is considered not as a particle, but rather as a circular standing wave such that only an integer number of wavelengths could fit exactly within the orbit ((Figure)).

For a circular orbit of radius r, the circumference is 2πr, and so de Broglie’s condition is:

Shortly after de Broglie proposed the wave nature of matter, two scientists at Bell Laboratories, C. J. Davisson and L. H. Germer, demonstrated experimentally that electrons can exhibit wavelike behavior by showing an interference pattern for electrons travelling through a regular atomic pattern in a crystal. The regularly spaced atomic layers served as slits, as used in other interference experiments. Since the spacing between the layers serving as slits needs to be similar in size to the wavelength of the tested wave for an interference pattern to form, Davisson and Germer used a crystalline nickel target for their “slits,” since the spacing of the atoms within the lattice was approximately the same as the de Broglie wavelengths of the electrons that they used. (Figure) shows an interference pattern. It is strikingly similar to the interference patterns for light shown in Section 6.1 for light passing through two closely spaced, narrow slits. The wave–particle duality of matter can be seen in (Figure) by observing what happens if electron collisions are recorded over a long period of time. Initially, when only a few electrons have been recorded, they show clear particle-like behavior, having arrived in small localized packets that appear to be random. As more and more electrons arrived and were recorded, a clear interference pattern that is the hallmark of wavelike behavior emerged. Thus, it appears that while electrons are small localized particles, their motion does not follow the equations of motion implied by classical mechanics, but instead it is governed by some type of a wave equation. Thus the wave–particle duality first observed with photons is actually a fundamental behavior intrinsic to all quantum particles.

The Quantum–Mechanical Model of an Atom

Shortly after de Broglie published his ideas that the electron in a hydrogen atom could be better thought of as being a circular standing wave instead of a particle moving in quantized circular orbits, Erwin Schrödinger extended de Broglie’s work by deriving what is today known as the Schrödinger equation. When Schrödinger applied his equation to hydrogen-like atoms, he was able to reproduce Bohr’s expression for the energy and, thus, the Rydberg formula governing hydrogen spectra. Schrödinger described electrons as three-dimensional stationary waves, or wavefunctions, represented by the Greek letter psi, ψ. A few years later, Max Born proposed an interpretation of the wavefunction ψ that is still accepted today: Electrons are still particles, and so the waves represented by ψ are not physical waves but, instead, are complex probability amplitudes. The square of the magnitude of a wavefunction, ψ², describes the probability of the quantum particle being present near a certain location in space. This means that wavefunctions can be used to determine the distribution of the electron’s density with respect to the nucleus in an atom. In the most general form, the Schrödinger equation can be written as:

is the Hamiltonian operator, a set of mathematical operations representing the total energy of the quantum particle (such as an electron in an atom), ψ is the wavefunction of this particle that can be used to find the special distribution of the probability of finding the particle, and E is the actual value of the total energy of the particle.

Schrödinger’s work, as well as that of many other scientists following in their footsteps, is generally referred to as quantum mechanics.

Understanding Quantum Theory of Electrons in Atoms

In our current quantum mechanical model of the atom, we can no longer accept that electrons travel in set orbits around the nucleus.  However, we must still accept the idea of quantized energy levels, as they arose from experimental observations of actual line spectra.

Now, we interpret the energy levels as corresponding to orbitals in the atom.  An atomic orbital is interpreted as a general region in an atom within which an electron is most probable to reside.   But in truth, they are the wavefunctions that are solutions to the Schrodinger equation.  These orbital wavefunctions describe the probability of finding an electron (ψ², as described above).

As before, the energy levels are labeled with an n value, where n = 1, 2, 3, …. This number, n, is referred to as the principal quantum number. Generally speaking, the energy of an electron in an atom is greater for greater values of n. The principal quantum number is  also related to the average distance the electron is from the nucleus. The positively charged protons in the nucleus stabilize the electronic orbitals by electrostatic attraction between the positive charges of the protons and the negative charges of the electrons. So the further away the electron is from the nucleus, the greater the energy it has.

This quantum mechanical model for where electrons reside in an atom can be used to look at electronic transitions, the events when an electron moves from one energy level to another. If the transition is to a higher energy level, energy is absorbed, and the energy change has a positive value. To obtain the amount of energy necessary for the transition to a higher energy level, a photon is absorbed by the atom. A transition to a lower energy level involves a release of energy, and the energy change is negative. This process is accompanied by emission of a photon by the atom.

There are certain distances from the nucleus at which the probability density of finding an electron located at a particular orbital is zero. In other words, the value of the wavefunction ψ is zero at this distance for this orbital. Such a value of radius r is called a node.

The electron density distribution of an s orbital is spherical and a p orbital has a dumbbell shape. The d and f orbitals are more complex. These shapes represent the three-dimensional regions within which the electron is likely to be found.

(Figure) illustrates the energy levels for various orbitals. The number before the orbital name (such as 2s, 3p, and so forth) stands for the principal quantum number, n. The letter in the orbital name defines the sublevel: s orbitals, p orbitals, d orbitals, etc.  In the case of a hydrogen atom or a one-electron ion (such as He⁺, Li²⁺, and so on), energies of all the orbitals with the same n are the same. This is called a degeneracy, and the energy levels for the same principal quantum number, n, are called degenerate orbitals. However, in atoms with more than one electron, this degeneracy is eliminated by the electron–electron interactions, and orbitals that belong to different sublevels have different energies, as shown on (Figure). Orbitals within the same sublevel are still degenerate and have the same energy.

So far, we have been considering the orbitals, which we interpret is the regions in space where electrons can be found.  Now we will consider the behavior of the electrons themselves.   In an atom, each electron acts as a tiny magnet or a tiny rotating object with an angular momentum; we describe this intrinsic electron “rotation” in terms of a property called electron spin.

An electron can only “spin” in one of two quantized states. One is termed the α state (“spin up”).  The other is called the β state (“spin down”). Any electron, regardless of the atomic orbital it is located in, can only be spin up or spin down.

(Figure) illustrates this phenomenon. An electron acts like a tiny magnet. Its moment is directed up (in the positive direction of the z axis) and down (in the negative z direction). A magnet has a lower energy if its magnetic moment is aligned with the external magnetic field (the left electron on (Figure)) and a higher energy for the magnetic moment being opposite to the applied field. This is true even for an electron occupying the same orbital in an atom.

The Pauli Exclusion Principle

Now that we have looked at the orbitals and the electrons separately, we are ready to consider how electrons are placed in orbitals within an atom.   The Pauli exclusion principle, named for an Austrian physicist Wolfgang Pauli, can be formulated as follows: no two electrons in the same orbital can have the same spin.  Since there are only two spin states, any atomic orbital can be populated by only zero, one, or two electrons.