Module 22_Lesson 4_ The Quantum Model of the Atom

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• Physics
• Secondary Education

The Quantum Model
of the Atom

Lesson 4

Objectives

1. Describe the shortcomings of Bohr’s atomic model.

2. Describe the quantum model of the atom.

3. Explain how a laser works.

4. Describe the properties of laser light.

New Vocabulary

quantum model

electron cloud

laser

quantum mechanics

stimulated emission

Use your book to define each term.

1. ……………………………………. is a model of the atom that predicts only the probability that an electron is in a specific region

2. ……………………………………. is region in which there is a high probability of finding an electron

3. ……………………………………. is the study of the properties of matter using its wave properties

4. ……………………………………. is a process that occurs when an excited atom is struck by a photon having energy equal to the energy difference between the excited state and the ground state—the atom drops to the ground state and emits a photon with energy equal to the energy difference between the two states

5. ……………………………………. is a device that produces powerful, coherent, directional, monochromatic light that can be used to excite other atoms

From Orbits to an Electron Cloud

From Orbits to an Electron Cloud

The circumference of the Bohr orbit is a whole-number multiple of the electron’s de Broglie wavelength.

From Orbits to an Electron Cloud

• The region in which there is a high probability of finding the electron is called the electron cloud.

• Even though the quantum model of the atom is difficult to visualize, quantum mechanics makes use of this model.

• Quantum mechanics is the study of the properties of matter using its wave properties.

Lasers

💡 Coherent vs. Incoherent Light

• Incoherent light: random phases and multiple wavelengths (e.g., lightbulbs).

  • Coherent light: waves in phase and same wavelength (e.g., lasers).

  • Laser light is intense, monochromatic, directional, and can be pulsed or continuous.

Lasers

✨ Spontaneous vs. Stimulated Emission

• Spontaneous emission: excited atom emits a photon randomly.

  • Stimulated emission: incoming photon triggers emission of a second photon, both in phase.

  • This leads to an avalanche of photons, forming the basis of laser operation.

Lasers

• A laser is a device that produces powerful, coherent, directional, monochromatic light that can be used to excite other atoms.

• The acronym stands for light amplification by stimulated emission of radiation.

• The atoms in a laser can be excited, or pumped.

• The photons emitted by the lasing atoms are contained within a chamber that has parallel mirrors at each end.

Lasers

🔬 How Lasers Work

• Lasing atoms are excited by light or collisions.

  • Helium-neon lasers use electric discharge to excite helium, which transfers energy to neon.

  • Laser chamber has mirrors to reflect photons and amplify light.

  • Output is coherent, monochromatic, and highly directional.

Lasers

• Most of the photons that are emitted in the direction of the ends of the tube will be reflected and will strike more atoms, releasing more photons with each pass between the mirrors.

• As the process continues, a high-intensity beam of photons builds.

Lasers

• The small percentage of photons that exit the tube through the partially reflecting mirror produce the laser beam.

  • Light leaving the laser is coherent.

  • The light may also be monochromatic.

  • The laser light is highly directional.

  • The beam does not diverge much as it travels.

Laser Applications

• Lasers have many applications:

• CD, DVD, and Blu-ray players

  • Surveying

  • Plate tectonics research

  • Lunar research

  • Fiber-optic communications

  • Laser printers

Problem Solving

Easy (Level 1/Tier 3 Question) CD Players Gallium arsenide lasers are commonly used in CD players. If such a laser emits at 840 nm, what is the difference in eV between the two lasing energy levels?

Problem Solving

Moderate (Level 2/Tier 2 Question) A GaInNi laser lases between energy levels that are separated by 2.90 eV.

1. What wavelength of light does it emit?

2. In what part of the spectrum is this light?

Problem Solving

Difficult (Level 3/Tier 1 Question) The power in a laser beam is equal to the energy of each photon times the number of photons per second that are emitted. a. If you want a laser at 840 nm to have the same power as one at 427 nm, how many times more photons per second are needed? b. Find the number of photons per second in a 5.0-mW 840-nm laser.

Problem Solving

Moderate (Level 2/Tier 2 Question) A carbon-dioxide laser emits very high-power infrared radiation. What is the energy difference in eV between the two lasing energy levels? Consult Table 28-1.

Problem Solving

Moderate (Level 2/Tier 2 Question) HeNe Lasers The HeNe lasers used in many classrooms can be made to lase at three wavelengths: 632.8 nm, 543.4 nm, and 1152.3 nm.

a. Find the difference in energy between the two states involved in the generation of each wavelength.

b. Identify the color of each wavelength.

Quiz

How does the modern quantum model of the atom describe electrons in an atom?

1.

CORRECT

Quiz

Laser light has all of the characteristics listed except one. Which is NOT a characteristic of laser light?

2.

CORRECT

Quiz

What happens to most of the photons that are emitted by the lasing atoms in the direction of the ends of the tube?

3.

CORRECT

Quiz

What is the term for the region in which there is a high probability of finding an electron in an atom?

4.

CORRECT

Quiz

Which describes the energy of the photon that strikes excited atoms in a laser?

5.

CORRECT