Atomic PhysicsIntroduction:Although the hydrogen atom is the simplest atomic system, it’s

especially important for several reasons:• The quantum numbers used to characterize the allowed states of

hydrogen can also be used to describe (approximately) the allowed states of more complex atoms. This enables us to understand the periodic table of the elements, one of the greatest triumphs of quantum mechanics.

• The hydrogen atom is an ideal system for performing precise comparisons of theory with experiment and for improving our overall understanding of atomic structure.

• Much of what is learned about the hydrogen atom with its single electron can be extended to such single-electron ions as He+ and Li2+.

28.2 ATOMIC SPECTRASuppose an evacuated glass tube is filled with

hydrogen (or some other gas) at low pressure. If a voltage applied between metal electrodes in the tube is great enough to produce an electric current in the gas, the tube emits light having a colour that depends on the gas inside. (This is how a neon sign works.)

When the emitted light is analyzed with a spectrometer, discrete bright lines are observed, each having a different wavelength, or colour. Such a series of spectral lines is commonly called an emission spectrum.

• The wavelengths contained in such a spectrum are characteristic of the element emitting the light

• Because no two elements emit the same line spectrum, this phenomenon represents a marvellous and reliable technique for identifying elements in a gaseous substance

• The emission spectrum of hydrogen shown in Figure 28.4 (Balmer series) includes four prominent lines that occur at wavelengths of 656.3 nm, 486.1 nm, 434.1 nm, and 410.2 nm, respectively. In 1885 Johann Balmer (1825–1898) found that the wavelengths of these and less prominent lines can be described by the simple empirical equation

• where n may have integral values of 3, 4, 5, . . . , and RH is a constant, called the Rydberg constant. If the wavelength is in meters, RH has the value

• In addition to the Balmer series of spectral lines, a Lyman series was subsequently discovered in the far ultraviolet, with the radiated wavelengths described by a similar equation.

• In addition to emitting light at specific wavelengths, an element can absorb light at specific wavelengths.

• The spectral lines corresponding to this process form what is known as an absorption spectrum.

• An absorption spectrum can be obtained by passing a continuous radiation spectrum (one containing all wavelengths) through a vapour of the element being analyzed.

• Each line in the absorption spectrum of a given element coincides with a line in the emission spectrum of the element

• The various absorption lines observed in the solar spectrum have been used to identify elements in the solar atmosphere, including one that was previously unknown. the new element was named helium.

28.3 THE BOHR THEORY OF HYDROGENWhy did atoms of a given element emit only certain

lines? Further, why did the atoms absorb only those wavelengths? That they emitted?

In 1913 Bohr provided an explanation of atomic spectra that includes some features of the currently accepted theory.

His model of the hydrogen atom contains some classical features, as well as some revolutionary postulates that could not be justified within the framework of classical physics. The basic assumptions of the Bohr theory as it applies to the hydrogen atom are as follows:

1. The electron moves in circular orbits about the proton under the influence of the Coulomb force of attraction, as in Figure 28.5. The Coulomb force produces the electron’s centripetal acceleration.

2. Only certain electron orbits are stable. These are orbits in which the hydrogen atom doesn’t emit energy in the form

of electromagnetic radiation. Hence, the total energy of the atom remains constant, and classical mechanics can be used to describe the electron’s motion

3. Radiation is emitted by the hydrogen atom when the electron “jumps” from a more energetic initial state to a less energetic state. The “jump” can’t be visualized or treated classically. In particular, the frequency f of the radiation emitted in the jump is related to the change in the atom’s energy and is independent of the frequency of the electron’s orbital motion. The frequency of the emitted radiation is given by

where Ei is the energy of the initial state, Ef is the energy of the final state, h is Planck’s constant, and Ei > Ef .

4. The size of the allowed electron orbits is determined by a condition imposed on the electron’s orbital angular momentum: the allowed orbits are those for which the electron’s orbital angular momentum about the nucleus is an integral multiple of ħ (pronounced “h bar”), where

With these four assumptions, we can calculate the allowed energies and emission wavelengths of the hydrogen atom.

• We use the model pictured in Figure 28.5, in which the electron travels in a circular orbit of radius r with an orbital speed v.

The electrical potential energy of the atom is

where ke is the Coulomb constant. Assuming the nucleus is at rest, the total energy E of the atom is the sum of the kinetic and potential energy

28.5

• We apply Newton’s second law to the electron. We know that the electric force of attraction on the electron, ke e 2/r 2, must equal mear, where ar =v 2/r is the centripetal acceleration of the electron. Thus,

• From this equation, we see that the kinetic energy of the electron is

• We can combine this result with Equation 28.5 and express the energy of the atom as

• where the negative value of the energy indicates that the electron is bound to the proton.

• An expression for r is obtained by solving Equations 28.4

and 28.6 for v and equating the results:

This equation is based on the assumption that the electron can exist only in certain allowed orbits determined by the integer n.

The orbit with the smallest radius, called the Bohr radius, a 0, corresponds to n= 1 and has the value

A general expression for the radius of any orbit in the hydrogen atom is obtained by substituting Equation 28.10 into Equation 28.9:

The first three Bohr orbits for hydrogen are shown in Active Figure 28.6

Equation 28.9 may be substituted into Equation 28.8 to give the following expression for the energies of the quantum states:

• If we insert numerical values into Equation 28.12, we obtain

• The lowest energy state, or ground state, corresponds to n= 1 and has an energy E1 =-meke

2 e 4/2ħ2 = -13.6 eV. • The next state, corresponding to n= 2, has an energy E2 = E1/4= -3.40

eV, and so on. An energy level diagram showing the energies of these stationary states and the correspo-nding quantum numbers is given in Active Figure 28.7.

• The uppermost level shown, corresponding to E = 0 and n→∞, represents the state for which the electron is completely removed from the atom.

• In this state, the electron’s KE and PE are both zero, which means that the electron is at rest infinitely far away from the proton.

• The minimum energy required to ionize the atom—that is, to completely remove the electron—is called the ionization energy. The ionization energy for hydrogen is 13.6 eV.

• Equations 28.3 and 28.12 and the third Bohr postulate show that if the electron jumps from one orbit with quantum number ni to a second orbit with quantum number, nf , it emits a photon of frequency f given by

• Finally, to compare this result with the empirical formulas for the various spectral series, we use Equation 28.14 and the fact that for light,λf= c, to get

• A comparison of this result with Equation 28.1 gives the following expression for the Rydberg constant:

• If we insert the known values of me , ke , e, c, and ħ into this expression, the resulting theoretical value for RH is found to be in excellent agreement with the value determined experimentally for the Rydberg constant. When Bohr demonstrated this agreement, it was recognized as a major accomplishment of his theory

• In order to compare Equation 28.15 with spectroscopic data, it is convenient to express it in the form

• We can use this expression to evaluate the wavelengths for the various series in the hydrogen spectrum. For example, in the Balmer series, nf= 2 and ni = 3, 4, 5, . . . (Eq. 28.1). For the Lyman series, we take nf =1 and ni = 2, 3, 4, . . . .

• the Bohr Theory successfully predicts the wavelengths of all the observed spectral lines of hydrogen.

EXAMPLE:• The Balmer series for the hydrogen atom corresponds to

electronic transitions that terminate in the state with quantum number n = 2, as shown in Figure 28.8.

• Find the longest-wavelength photon emitted in the Balmer series and determine its frequency and energy. (b) Find the shortest-wavelength photon emitted in the same series.

Solution(a) Find the longest wavelength photon emitted in the

Balmer series, and determine its energy.• Substitute into Equation 28.17, with ni = 3 and nf = 2:

• Take the reciprocal and substitute, finding the wavelength:

• Now use c = fλ to obtain the frequency:

• Calculate the photon’s energy by substituting into Equation 27.5:

• Find the shortest wavelength photon emitted in the Balmer series.Substitute into Equation 28.17, with ni= ∞ and nf =2.

• Take the reciprocal and substitute, finding the wavelength:

• Bohr’s Correspondence Principle

• we found that Newtonian mechanics cannot be used to describe phenomena that occur at speeds approaching the speed of light.

• Newtonian mechanics is a special case of relativistic mechanics and applies only when v is much smaller than c.

• Similarly, quantum mechanics is in agreement with classical physics when the energy differences between quantized levels are very small. This principle, first set forth by Bohr, is called the correspondence principle.

• For example, consider the hydrogen atom with n > 10000. For such large values of n, the energy differences between adjacent levels approach zero and the levels are nearly continuous, as Equation 28.13 shows. Calculations show that for n > 10000, this frequency is different from that predicted by quantum mechanics by less than 0.015%

28.4 MODIFICATION OF THE BOHR THEORYThe Bohr Theory of the hydrogen atom was a

tremendous success in certain areas because 1. it explained several features of the hydrogen

spectrum that had previously defied explanation2. It accounted for the Balmer series and other series3. it predicted a value for the Rydberg constant that

is in excellent agreement with the experimental value

4. it gave an expression for the radius of the atom; and it predicted the energy levels of hydrogen

5. the Bohr Theory gave us a model of what the atom looks like and how it behaves.

• The analysis used in the Bohr Theory is also successful when applied to hydrogen-like atoms.

• An atom is said to be hydrogen-like when it contains only one electron. Examples are singly ionized helium, doubly ionized lithium, triply ionized beryllium, and so forth.

• The results of the Bohr theory for hydrogen can be extended to hydrogen- like atoms by substituting Ze2 for e2 in the hydrogen equations, where Z is the atomic number of the element.

28.18And 28.19Although many attempts were made to extend the Bohr Theory to

more complex, multi-electron atoms, the results were unsuccessful. Even today, only approximate methods are available for treating multi-electron atoms.

• EXAMPLE 28.2• Goal Apply the modified Bohr Theory to a hydrogen-like atom.• Problem Singly ionized helium, He+, a hydrogen-like system, has

one electron in the 1s orbit when the atom is in its ground state. Find (a) the energy of the system in the ground state in electron volts, and (b) the radius of the ground-state orbit.

• Strategy Part (a) requires substitution into the modified Bohr model, Equation 28.18. In part (b), modify Equation 28.9 for the radius of the Bohr orbits by replacing e 2 by Ze2, where Z is the number of protons in the nucleus.

• Solution• Find the energy of the system in the ground state. Write Equation

28.18 for the energies of a hydrogen-like system:

• Substitute the constants and convert to electron volts:

• Substitute Z =2 (the atomic number of helium) and n=1 to obtain the ground state energy:

Find the radius of the ground state. Generalize Equation 28.9 to a hydrogen-like atom by substituting Ze 2 for e 2:

For our case, n = 1 and Z = 2:Notice that for higher Z the energy of a hydrogen-

like atom is lower, which means that the electron is more tightly bound than in hydrogen. This results in a smaller atom, as seen in part (b).

• The number of protons in an atom’s nucleus is the atomic number for that particular element

• The same element may have different numbers of neutrons in its nucleus

• These slightly different kinds of the same elements are called isotopes