5.2: The Quantum Mechanical Model
Lesson Objectives
- Understand the de Broglie wave equation and how it illustrates the wave nature of the electron.
- Explain the difference between quantum mechanics and classical mechanics.
- Understand how the Heisenberg uncertainty principle and Schrödinger’s wave equation led to the idea of atomic orbitals.
- Know the four quantum numbers and their significance to an atom’s electron arrangement.
- Describe the interrelationships of principal energy level, sublevel, orbital and electron spin and how they relate to the number of electrons of an atom.
Lesson Vocabulary
- angular momentum quantum number
- Heisenberg uncertainty principle
- magnetic quantum number
- orbital
- principal quantum number
- spin quantum number
- quantum mechanical model
- quantum mechanics
- quantum numbers
Wave Nature of the Electron
Bohr’s model of the atom was valuable in demonstrating how electrons were capable of absorbing and releasing energy and how atomic emission spectra were created. However, the model did not really explain why electrons should exist only in fixed circular orbits rather than being able to exist in a limitless number of orbits all with different energies. In order to explain why atomic energy states are quantized, scientists needed to rethink the way in which they viewed the nature of the electron and its movement.
de Broglie Wave Equation
Planck’s investigation of the emission spectra of hot objects and the subsequent studies into the photoelectric effect had proven that light was capable of behaving both as a wave and as a particle. It seemed reasonable to wonder if electrons could also have a dual wave-particle nature. In 1924, French scientist Louis de Broglie (1892-1987) derived an equation that described the wave nature of any particle. Particularly, the wavelength () of any moving object is given by:
In this equation, is Planck’s constant, is the mass of the particle in kg, and is the velocity of the particle in m/s. The problem below shows how to calculate the wavelength of the electron.
Sample Problem 5.4: de Broglie Equation
An electron of mass 9.11 × 10^{-31} kg moves at nearly the speed of light. Using a velocity of 3.00 × 10^{8} m/s, calculate the wavelength of the electron.
Step 1: List the known quantities and plan the problem.
Known
- mass () = 9.11 × 10^{-31} kg
- Planck's constant () = 6.626 × 10^{-34} J•s
- velocity () = 3.00 × 10^{8} m/s
Unknown
- wavelength ()
Apply the de Broglie wave equation to solve for the wavelength of the moving electron.
Step 2: Calculate
Step 3: Think about your result.
This very small wavelength is about 1/20^{th} of the diameter of a hydrogen atom. Looking at the equation, as the speed of the electron decreases, its wavelength increases. The wavelengths of everyday large objects with much greater masses should be very small.
- Calculate the wavelength of a 0.145 kg baseball thrown at a speed of 40 m/s.
The above practice problem results in an extremely short wavelength on the order of 10^{−34} m. This wavelength is impossible to detect even with advanced scientific equipment. Indeed, while all objects move with wavelike motion, we never notice it because the wavelengths are far too short. On the other hand, particles with measurable wavelengths are all very small. However, the wave nature of the electron proved to be a key development in a new understanding of the nature of the electron. An electron that is confined to a particular space around the nucleus of an atom can only move around that atom in such a way that its electron wave “fits” the size of the atom correctly (Figure below). This means that the frequencies of electron waves are quantized. Based on the equation, the quantized frequencies means that electrons can only exist in an atom at specific energies, as Bohr had previously theorized.
The circumference of the orbit in (A) allows the electron wave to fit perfectly into the orbit. This is an allowed orbit. In (B), the electron wave does not fit properly into the orbit, so this orbit is not allowed.
The study of motion of large objects such as baseballs is called mechanics, or more specifically classical mechanics. Because the quantum nature of the electron and other tiny particles moving at high speeds, classical mechanics is inadequate to accurately describe their motion. Quantum mechanics is the study of the motion of objects that are atomic or subatomic in size and thus demonstrate wave-particle duality. In classical mechanics, the size and mass of the objects involved effectively obscures any quantum effects so that such objects appear to gain or lose energies in any amounts. Particles whose motion is described by quantum mechanics gain or lose energy in the small pieces called quanta.
Heisenberg Uncertainty Principle
Another feature that is unique to quantum mechanics is the uncertainty principle. The Heisenberg Uncertainty Principle states that it is impossible to determine simultaneously both the position and the velocity of a particle. The detection of an electron, for example, would be made by way of its interaction with photons of light. Since photons and electrons have nearly the same energy, any attempt to locate an electron with a photon will knock the electron off course, resulting in uncertainty about where the electron is located (Figure below). We do not have to worry about the uncertainty principle with large everyday objects because of their mass. If you are looking for something with a flashlight, the photons coming from the flashlight are not going to cause the thing you are looking for to move. This is not the case with atomic-sized particles, leading scientists to a new understanding about how to envision the location of the electrons within atoms.
Heisenberg Uncertainty Principle: The observation of an electron with a microscope requires reflection of a photon off of the electron. This reflected photon causes a change in the path of the electron.
You can see a funny, animated explanation of Heisenberg's Uncertainty Principle at http://video.pbs.org/video/18121247.
Quantum Mechanical Model
In 1926, Austrian physicist Erwin Schrödinger (1887-1961) used the wave-particle duality of the electron to develop and solve a complex mathematical equation that accurately described the behavior of the electron in a hydrogen atom. The quantum mechanical model of the atom comes from the solution to Schrödinger’s equation. Quantization of electron energies is a requirement in order to solve the equation. This is unlike the Bohr model, in which quantization was simply assumed with no mathematical basis.
Recall that in the Bohr model, the exact path of the electron was restricted to very well defined circular orbits around the nucleus. The quantum mechanical model is a radical departure from that. Solutions to the Schrödinger wave equation, called wave functions, give only the probability of finding an electron at a given point around the nucleus. Electrons do not travel around the nucleus in simple circular orbits.
The location of the electrons in the quantum mechanical model of the atom is often referred to as an electron cloud. The electron cloud can be thought of in the following way. Imagine placing a square piece of paper on the floor with a dot in the circle representing the nucleus. Now take a marker and drop it onto the paper repeatedly, making small marks at each point the marker hits. If you drop the marker many, many times, the overall pattern of dots will be roughly circular. If you aim toward the center reasonably well, there will be more dots near the nucleus and progressively fewer dots as you move away from it. Each dot represents a location where the electron could be at any given moment. Because of the uncertainty principle, there is no way to know exactly where the electron is. An electron cloud has variable densities: a high density where the electron is most likely to be and a low density where the electron is least likely to be (Figure below).
An electron cloud: the darker region nearer the nucleus indicates a high probability of finding the electron, while the lighter region further from the nucleus indicates a lower probability of finding the electron.
In order to specifically define the shape of the cloud, it is customary to refer to the region of space within which there is a 90% probability of finding the electron. This is called an orbital, the three-dimensional region of space that indicates where there is a high probability of finding an electron.
Atomic Orbitals and Quantum Numbers
Solutions of the Schrödinger wave equation give the energies that an electron is allowed to have. The mathematical representation of those energies results in regions of space called orbitals, which, as we will soon see, can have different sizes and shapes. In order to completely describe the orbitals and the electrons which occupy them, scientists use quantum numbers. Quantum numbers specify the properties of the atomic orbitals and the electrons in those orbitals. Understanding quantum numbers is helped by an analogy. Let’s say you are attending a basketball game. Your ticket may specify a gate number, a section number, a row, and a seat number. No other ticket can have the same four parts to it. It may have the same gate, section, and seat number, but if so it would have to be in a different row. There are also four quantum numbers which describe each and every electron in every atom. No two electrons in a given atom can have the same four quantum numbers. We will describe each of these quantum numbers separately.
Principal Quantum Number
The principal quantum number is symbolized by the letter n and is the principal or main energy level occupied by the electron. The value of n begins with n = 1, which is the lowest in energy and is located closest to the nucleus. As the n value increases to n = 2, 3, and so on, the distance from the nucleus increases. The principal quantum number is essentially the same as the energy levels in the Bohr model of the atom that were used to explain atomic emission spectra. More than one electron may occupy a given principal energy level, but the specific number varies depending on which energy level.
Angular Momentum Quantum Number
The angular momentum quantum number is symbolized by the letter l and indicates the shape of the orbital. For each given principal energy level, orbitals of different shapes exist, and these are slightly different in energy and are referred to as energy sublevels. The number of sublevels varies depending on the value of n. Specifically, the number of sublevels possible is equal to the value of n. In other words, when n = 1, there is only one sublevel. When n = 2, there are two sublevels. The quantum number l is an integer which varies from 0 up to a value equal to n - 1. In other words, if n = 1, the only possible value of l is 0. If n = 4, then l can have a value of 0, 1, 2, or 3. Finally, each of the orbitals represented by the various sublevels has a letter designation: s, p, d, or f. If l = 0, the orbital is an s orbital. If l = 1, the orbital is a p orbital. See Table below for a summary.
Principal Energy Level | Number of Possible Sublevels | Possible Angular Momentum Quantum Numbers | Orbital Designation by Principal Energy Level and Sublevel |
---|---|---|---|
n = 1 | 1 | l = 0 | 1s |
n = 2 | 2 |
l = 0 l = 1 |
2s 2p |
n = 3 | 3 |
l = 0 l = 1 l = 2 |
3s 3p 3d |
n = 4 | 4 |
l = 0 l = 1 l = 2 l = 3 |
4s 4p 4d 4f |
From the table you can see that in the 1^{st} principal energy level (n = 1) there is only one sublevel possible – an s sublevel. In the 2^{nd} principal energy level (n = 2), there are two sublevels possible – the s and p sublevels. This continues through the 3^{rd} and 4^{th} principal energy levels adding in the d and the f sublevels. In general for the n^{th} principal energy level there are n sublevels available. The order of the sublevels is always the same.
Magnetic Quantum Number
As mentioned above, each of the different orbital types has a different shape. The s orbitals are spherical in shape (Figure below), the p orbitals are dumbbell shaped (Figure below), and the d and f orbitals are more complex with multiple lobes (Figure below).
The s orbitals are spherical in shape and centered on the nucleus. The size of the orbital increases with the higher principal energy level.
The p orbitals are dumbbell-shaped. (a) shows the electron density distribution, while (b) shows the orbitals that result. The three different p orbitals are identical in shape, but oriented along the x, y, or z axis.
View the p orbitals at http://www.dlt.ncssm.edu/core/Chapter8-Atomic_Str_Part2/chapter8-Animations/P-orbitalDiagram.html.
The five d orbitals have more complex shapes. Four of them have a 4-lobed appearance with different orientations, while the d_{z}^{2} orbital has a more complex shape.
The magnetic quantum number is symbolized by the letter m_{l} and indicates the orientation of the orbital around the nucleus. Because an s orbital is spherical in shape and centered on the nucleus, it only has one possible orientation. Thus, it has only one possible value of the magnetic quantum number and that is m_{l} = 0. Within each s sublevel, there is just one s orbital. The larger is the principal energy level (n), the larger is the size of the spherical s orbital. As seen in Figure above, the dumbbell-shaped p orbitals have three possible orientations. In one orientation, called the p_{x}, the lobes of the orbital lie along the defined x-axis. In the p_{y} orbital, they lie along the y-axis, or at a 90° angle to the p_{x} orbital. Finally, the p_{z} orbital lies along the z-axis or 90° relative to the other two orbitals. In any p sublevel, there are always these three orbitals and that means that there are three possible values of the magnetic quantum number: m_{l} = −1, m_{l} = 0, and m_{l} = +1. There is no particular relationship between the coordinates (x, y, and z) and the m_{l} value.
There are five different d orbitals within each d sublevel. The corresponding magnetic quantum numbers are m_{l} = −2, m_{l} = −1, m_{l} = 0, m_{l} = +1, and m_{l} = +2. Finally, the pattern continues with the f sublevel containing seven possible f orbitals and m_{l} values ranging from −3 to +3. Ground states of all known elements can be described with these four sublevels.
Spin Quantum Number
Experiments show that electrons spin on their own internal axis, much as Earth does. The spinning of a charged particle creates a magnetic field. The orientation of that magnetic field depends upon the direction that the electron is spinning, either clockwise or counterclockwise. The spin quantum number is symbolized by the letter m_{s} and indicated the direction of electron spin. The values are and . Each orbital, no matter its shape or orientation can hold a maximum of two electrons, which must have opposite spins. Table below shows a summary of energy levels and sublevels and the number of electrons that can possibly exist in the various states.
Principal Quantum Number (n) | Allowable Sublevels | Number of Orbitals per Sublevel | Number of Orbitals per Principal Energy Level | Number of Electrons per Sublevel | Number of Electrons per Principal Energy Level |
---|---|---|---|---|---|
1 | s | 1 | 1 | 2 | 2 |
2 |
s p |
1 3 |
4 |
2 6 |
8 |
3 |
s p d |
1 3 5 |
9 |
2 6 10 |
18 |
4 |
s p d f |
1 3 5 7 |
16 |
2 6 10 14 |
32 |
Notice that the total number of allowable orbitals in each principal energy level (n) is equal to n^{2}. That is, when n = 1, there are 1^{2} = 1 orbital possible. When n = 2, there are 2^{2} = 4 orbitals possible, etc. Since each orbital holds two electrons, the number of electrons that can exist in a given principal energy level is equal to 2n^{2}.
Lesson Summary
- de Broglie showed that electrons have both a wave nature and a particle nature.
- The behavior of atomic and subatomic sized particles is explained by quantum mechanics, where energy is gained and lost in small discrete amounts.
- The Heisenberg uncertainty principal showed that is not possible to know the location of an electron at any precise moment.
- The Schrödinger wave equation proved mathematically that the energy of an electron must be quantized.
- The quantum mechanical model of the atom describes the probability that an atom’s electrons will be located within certain regions called orbitals.
- Electrons arrangements are governed by four quantum numbers. These correspond to the principal energy level, energy sublevel, orbital, and electron spin.
Lesson Review Questions
Reviewing Concepts
- How is the wavelength of a moving object related to its mass?
- Why is the de Broglie wave equation meaningful only for submicroscopic particles such as atoms and electrons and not for larger everyday objects?
- How does the Heisenberg uncertainty principle affect the way in which electron locations are viewed in the quantum mechanical model as compared to the Bohr model?
- What is an atomic orbital?
- How many quantum numbers are used to describe each electron in an atom?
- In what two ways is an electron that occupies the n = 2 principal energy level different than an electron which occupies the n = 1 principal energy level?
- Identify which quantum number describes each of the following.
- the orientation of an orbital in space
- the direction of electron spin
- the main energy of an electron
- the shape of an orbital
Problems
- What is the wavelength (in nm) of an electron moving at 250 m/s? In what region of the electromagnetic spectrum is this electron wave?
- What are the possible values of l that an electron in the n = 3 principal energy level can have? Which sublevel does each of those l values represent?
- What are the possible values of m_{l} that an electron in the p sublevel can have?
- Which of the following combinations of principal energy level and sublevel cannot exist?
- 4d
- 3f
- 1p
- 2s
- 3p
- 2d
- How is a p_{z} orbital different from a p_{x} orbital? How are they the same?
- How many orbitals are found in each of the following?
- any s sublevel
- any f sublevel
- the n = 2 principal energy level
- the 4p sublevel
- the n = 3 principal energy level
- How many electrons in each of the following?
- any orbital
- the n = 2 principal energy level
- the 3d sublevel
- the n = 4 principal energy level
- the 2p sublevel