Quantum Theory and Atomic Structure 7.1 The Nature of Light

7.2 Atomic Spectra

7.3 The Wave-Particle Duality of Matter and Energy

7.4 The Quantum-Mechanical Model of the Atom

7-1

The Wave Nature of Light Visible light is a type of electromagnetic radiation. The wave properties of electromagnetic radiation are described by three variables: - frequency (n), cycles per second - wavelength (l), the distance a wave travels in one cycle - amplitude, the height of a wave crest or depth of a trough.

The speed of light is a constant: c=nxl = 3.00 x 108 m/s in a vacuum 7-2

Figure 7.1 The reciprocal relationship of frequency and wavelength.

7-3

Figure 7.2

7-4

Differing amplitude (brightness, or intensity) of a wave.

Figure 7.3

7-5

Regions of the electromagnetic spectrum.

Sample Problem 7.1

Interconverting Wavelength and Frequency

PROBLEM: A dental hygienist uses x-rays (l= 1.00Å) to take a series of dental radiographs while the patient listens to a radio station (l = 325 cm) and looks out the window at the blue sky (l= 473 nm). What is the frequency (in s-1) of the electromagnetic radiation from each source? (Assume that the radiation travels at the speed of light, 3.00x108 m/s.) PLAN:

Use the equation c = nl to convert wavelength to frequency. Wavelengths need to be in meters because c has units of m/s. wavelength in units given use conversion factors 1 Å = 10-10 m

wavelength in m n= c l

frequency (s-1 or Hz)

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Sample Problem 7.1 SOLUTION: For the x-rays:

-10 l = 1.00 Å x 10 m = 1.00 x 10-10 m 1Å

3.00 x 108 m/s c = n= l 1.00 x 10-10 m

For the radio signal:

For the blue sky:

7-7

n= c = l

n= c = l

3.00 x 108 m/s 10-2 m 325 cm x 1 cm 3.00 x 108 m/s 10-9 m 473 nm x 1 nm

= 3.00 x 1018 s-1

= 9.23 x 107 s-1

= 6.34 x 1014 s-1

Figure 7.4

7-8

Different behaviors of waves and particles.

Figure 7.5

7-9

Formation of a diffraction pattern.

Energy and Frequency A solid object emits visible light when it is heated to about 1000 K. This is called blackbody radiation. The color (and the intensity ) of the light changes as the temperature changes. Color is related to wavelength and frequency, while temperature is related to energy. Energy is therefore related to frequency and wavelength: E = nhn

7-10

E = energy n is a positive integer h is Planck’s constant

Figure 7.6

Familiar examples of light emission related to blackbody radiation.

Smoldering coal

7-11

Electric heating element

Lightbulb filament

The Quantum Theory of Energy Any object (including atoms) can emit or absorb only certain quantities of energy. Energy is quantized; it occurs in fixed quantities, rather than being continuous. Each fixed quantity of energy is called a quantum. An atom changes its energy state by emitting or absorbing one or more quanta of energy. DE = Dnhn where n can only be a whole number.

7-12

Figure 7.7

7-13

The photoelectric effect.

Sample Problem 7.2

Interconverting Energy, Wavelength and Frequency

PROBLEM: A student uses a microwave oven to heat a meal. The wavelength of the radiation is 1.20 cm. What is the energy of one photon of this microwave radiation? PLAN:

We know l in cm, so we convert to m and find the frequency using the speed of light. We then find the energy of one photon using E = hn.

SOLUTION: hc E = hn = l

7-14

=

(6.626 x 10-34 J∙s)(3.00 x 108 m/s) (1.20 cm)(

10-2 m 1 cm

)

= 1.66 x 10-23 J

Figure 7.8

7-15

The line spectrum of hydrogen.

Figure 7.9 Three series of spectral lines of atomic hydrogen.

Rydberg equation

1 l

=

R

1 n12



1 n22

R is the Rydberg constant = 1.096776x107 m-1 for the visible series, n1 = 2 and n2 = 3, 4, 5, ...

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The Bohr Model of the Hydrogen Atom Bohr’s atomic model postulated the following: • The H atom has only certain energy levels, which Bohr called stationary states. – Each state is associated with a fixed circular orbit of the electron around the nucleus. – The higher the energy level, the farther the orbit is from the nucleus. – When the H electron is in the first orbit, the atom is in its lowest energy state, called the ground state.

7-17

• The atom does not radiate energy while in one of its stationary states. • The atom changes to another stationary state only by absorbing or emitting a photon. – The energy of the photon (hn) equals the difference between the energies of the two energy states. – When the electron is in any orbit higher than n = 1, the atom is in an excited state.

7-18

Figure 7.10

7-19

A quantum “staircase” as an analogy for atomic energy levels.

Figure 7.11

7-20

The Bohr explanation of three series of spectral lines emitted by the H atom.

A tabletop analogy for defining the energy of a system.

DE = Efinal – Einitial =

7-21

–2.18x10-18

J

1 – n2final

1 n2initial

Sample Problem 7.3

Determining DE and l of an Electron Transition

PROBLEM: A hydrogen atom absorbs a photon of UV light (see Figure 7.11) and its electron enters the n = 4 energy level. Calculate (a) the change in energy of the atom and (b) the wavelength (in nm) of the photon. PLAN: (a) The H atom absorbs energy, so Efinal > Einitial. We are given nfinal = 4, and Figure 7.11 shows that ninitial = 1 because a UV photon is absorbed. We apply Equation 7.4 to find DE. (b) Once we know DE, we find frequency and wavelength.

7-22

Sample Problem 7.3 SOLUTION: (a) DE =

–2.18x10-18

=

–2.18x10-18

J

J

1



1

n2final

n2initial

1

1

16



1

=

-2.18x10-18

7-23

42



1 12

= 2.04 x 10-18 J

-34 8 (b) l = hc = (6.626x10 J∙s)(3.00x10 m/s) DE 2.04x10-18 J

9.74x10-8 m x 1 nm 10-9 m

J

1

= 97.4 nm

= 9.74 x 10-8 m

Tools of the Laboratory Figure B7.1

Flame tests and fireworks.

strontium 38Sr

copper 29Cu

The flame color is due to the emission of light of a particular wavelength by each element.

7-24

Fireworks display emissions similar to those seen in flame tests.

Tools of the Laboratory Figure B7.2

7-25

Emission and absorption spectra of sodium atoms.

Tools of the Laboratory Figure B7.3

7-26

Components of a typical spectrometer.

Tools of the Laboratory Figure B7.4 Measuring chlorophyll a concentration in leaf extract.

7-27

The Wave-Particle Duality of Matter and Energy Matter and Energy are alternate forms of the same entity. E = mc2 All matter exhibits properties of both particles and waves. Electrons have wave-like motion and therefore have only certain allowable frequencies and energies. Matter behaves as though it moves in a wave, and the de Broglie wavelength for any particle is given by: l= 7-28

h mu

m = mass

u = speed in m/s

Figure 7.12

7-29

Wave motion in restricted systems.

Table 7.1

Mass (g)

Slow electron

9x10-28

1.0

7x10–4

Fast electron

9x10-28

5.9x106

1x10–10

Alpha particle

6.6x10-24

1.5x107

7x10–15

1-gram mass

1.0

0.01

7x10–29

142

40.0

1x10–34

3.0x104

4x10–63

Earth

6.0x1027

Speed (m/s)

l (m)

Substance

Baseball

7-30

The de Broglie Wavelengths of Several Objects

Sample Problem 7.4

PROBLEM:

PLAN:

Find the deBroglie wavelength of an electron with a speed of 1.00x106 m/s (electron mass = 9.11x10-31 kg; h = 6.626x10-34 kgŸm2/s).

We know the speed and mass of the electron, so we substitute these into Equation 7.5 to find l.

SOLUTION:

l= l=

7-31

Calculating the de Broglie Wavelength of an Electron

h mu 6.626x10-34 kg∙m2/s 9.11x10-31 kg x 1.00x106 m/s

= 7.27 x 10-10 m

Figure 7.13

Diffraction patterns of aluminum with x-rays and electrons.

x-ray diffraction of aluminum foil

7-32

electron diffraction of aluminum foil

Figure 7.14 False-color scanning electron micrograph of blood cells.

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Figure 7.15

CLASSICAL THEORY Matter particulate, massive

Energy continuous, wavelike

Major observations and theories leading from classical theory to quantum theory

Since matter is discontinuous and particulate, perhaps energy is discontinuous and particulate. Observation Blackbody radiation

7-34

Theory Planck:

Photoelectric effect

Energy is quantized; only certain values allowed. Einstein: Light has particulate behavior (photons).

Atomic line spectra

Bohr:

Energy of atoms is quantized; photon emitted when electron changes orbit.

Figure 7.15 continued Since energy is wavelike, perhaps matter is wavelike. Observation Davisson/Germer: Electron beam is diffracted by metal crystal.

Theory deBroglie: All matter travels in waves; energy of atom is quantized due to wave motion of electrons. Since matter has mass, perhaps energy has mass.

Observation

Theory

Compton: Einstein/deBroglie: Mass and energy are equivalent; particles have Photon’s wavelength increases (momentum wavelength and photons have decreases) after colliding momentum. with electron. QUANTUM THEORY

7-35

Energy and Matter particulate, massive, wavelike

Heisenberg’s Uncertainty Principle Heisenberg’s Uncertainty Principle states that it is not possible to know both the position and momentum of a moving particle at the same time. Dx∙m Du ≥

h 4p

x = position u = speed

The more accurately we know the speed, the less accurately we know the position, and vice versa.

7-36

Sample Problem 7.5

Applying the Uncertainty Principle

PROBLEM: An electron moving near an atomic nucleus has a speed 6x106 m/s ± 1%. What is the uncertainty in its position (Dx)? The uncertainty in the speed (Du) is given as ±1% (0.01) of 6x106 m/s. We multiply u by 0.01 and substitute this value into Equation 7.6 to solve for Δx.

PLAN:

SOLUTION:

Du = (0.01)(6x106 m/s) = 6x104 m/s Dx∙mDu ≥

Dx ≥

7-37

h 4pmDu



h 4p

6.626x10-34 kg∙m2/s 4p (9.11x10-31 kg)(6x104 m/s)

≥ 1 x 10-9 m

The Quantum-Mechanical Model of the Atom The matter-wave of the electron occupies the space near the nucleus and is continuously influenced by it. The Schrödinger wave equation allows us to solve for the energy states associated with a particular atomic orbital. The square of the wave function gives the probability density, a measure of the probability of finding an electron of a particular energy in a particular region of the atom.

7-38

Figure 7.16

7-39

Electron probability density in the ground-state H atom.

Quantum Numbers and Atomic Orbitals An atomic orbital is specified by three quantum numbers. The principal quantum number (n) is a positive integer. The value of n indicates the relative size of the orbital and therefore its relative distance from the nucleus.

The angular momentum quantum number (l) is an integer from 0 to (n –1). The value of l indicates the shape of the orbital.

The magnetic quantum number (ml) is an integer with values from –l to +l. The value of ml indicates the spatial orientation of the orbital.

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Table 7.2 The Hierarchy of Quantum Numbers for Atomic Orbitals Name, Symbol (Property) Allowed Values

Quantum Numbers

Principal, n Positive integer (size, energy) (1, 2, 3, ...)

1

Angular momentum, l 0 to n – 1 (shape)

0

0

0

0

Magnetic, ml -l,…,0,…,+l (orientation)

2

3

1

0

2

0 -1 0 +1

-1 0 +1

-2

7-41

1

-1

0

+1 +2

Sample Problem 7.6 PROBLEM:

Determining Quantum Numbers for an Energy Level

What values of the angular momentum (l) and magnetic (ml) quantum numbers are allowed for a principal quantum number (n) of 3? How many orbitals are allowed for n = 3?

PLAN: Values of l are determined from the value for n, since l can take values from 0 to (n – 1). The values of ml then follow from the values of l. SOLUTION:

For n = 3, allowed values of l are = 0, 1, and 2

For l = 0, ml = 0

For l = 1, ml = –1, 0, or +1

For l = 2, ml = –2, –1, 0, +1, or +2 There are 9 ml values and therefore 9 orbitals with n = 3.

7-42

Sample Problem 7.7

Determining Sublevel Names and Orbital Quantum Numbers PROBLEM: Give the name, magnetic quantum numbers, and number of orbitals for each sublevel with the following quantum numbers: (a) n = 3, l = 2 (b) n = 2, l = 0 (c) n = 5, l = 1 (d) n = 4, l = 3 PLAN: Combine the n value and l designation to name the sublevel. Knowing l, we can find ml and the number of orbitals. SOLUTION: n

l

sublevel name possible ml values # of orbitals

(a)

3

2

3d

(b)

2

0

2s

0

1

(c)

5

1

5p

–1, 0, 1

3

(d)

4

3

4f

7-43

–2, –1, 0, 1, 2

–3, –2, –1, 0, 1, 2, 3

5

7

Sample Problem 7.8

Identifying Incorrect Quantum Numbers

PROBLEM: What is wrong with each of the following quantum numbers designations and/or sublevel names? n l ml Name (a)

1

1

0

1p

(b)

4

3

+1

4d

(c)

3

1

–2

3p

SOLUTION: (a) A sublevel with n = 1 can only have l = 0, not l = 1. The only possible sublevel name is 1s. (b) A sublevel with l = 3 is an f sublevel, not a d sublevel. The name should be 4f. (c) A sublevel with l = 1 can only have ml values of –1, 0, or +1, not –2.

7-44

Figure 7.17

7-45

The 1s, 2s, and 3s orbitals.

Figure 7.18

7-46

The 2p orbitals.

Figure 7.19

7-47

The 3d orbitals.

Figure 7.19 continued

7-48

Figure 7.20

7-49

The 4fxyz orbital, one of the seven 4f orbitals.

Figure 7.21

7-50

Energy levels of the H atom.

7-1 Quantum Theory and Atomic Structure

7.4 The Quantum-Mechanical Model of the Atom ... An atom changes its energy state by emitting or ... When the H electron is in the first orbit, the atom is in.

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