Modern Atomic Theory Part One. Wave-Particle Duality

Transcription

1 Modern Atomic Theory Part One Wave-Particle Duality

2 The Beginnings of Quantum Mechanics Quantum mechanics forms the foundation of chemistry Explaining the periodic table The behavior of the elements in chemical bonding Provides the practical basis for lasers, computers, and countless other applications

3 The Behavior of the Very Small Electrons are incredibly small. A single speck of dust has more electrons than the number of people who have ever lived on Earth. Electron behavior determines much of the behavior of atoms. Directly observing electrons in the atom is impossible; the electron is so small that observing it changes its behavior. Even shining a light on the electron would affect it.

4 A Theory That Explains Electron Behavior The quantum-mechanical model explains the manner in which electrons exist and behave in atoms. It helps us understand and predict the properties of atoms that are directly related to the behavior of the electrons: Why some elements are metals and others are nonmetals Why some elements gain one electron when forming an anion, whereas others gain two Why some elements are very reactive, while others are practically inert Why in other periodic patterns we see in the properties of the elements

5 The Nature of Light: Its Wave Nature Light: a form of electromagnetic radiation Composed of perpendicular oscillating waves, one for the electric field and one for the magnetic field An electric field is a region where an electrically charged particle experiences a force. A magnetic field is a region where a magnetized particle experiences a force. All electromagnetic waves move through space at the same, constant speed m/s = the speed of light

6 Wave Characteristics

7 Characterizing Waves The amplitude is the height of the wave. The distance from node to crest or node to trough The amplitude is a measure of light intensity the larger the amplitude, the brighter the light. The wavelength (l) is a measure of the distance covered by the wave. The distance from one crest to the next The distance from one trough to the next, or the distance between alternate nodes

8 The Relationship Between Wavelength and Frequency For waves traveling at the same speed, the shorter the wavelength, the more frequently they pass. This means that the wavelength and frequency of electromagnetic waves are inversely proportional. Because the speed of light is constant, if we know wavelength we can find the frequency, and vice versa.

9 Example 1 Wavelength and Frequency Calculate the wavelength (in nm) of the red light emitted by a barcode scanner that has a frequency of s 1. Solution You are given the frequency of the light and asked to find its wavelength. Use the equation which relates frequency to wavelength. You can convert the wavelength from meters to nanometers by using the conversion factor between the two (1 nm = 10 9 m). For Practice 1a A laser dazzles the audience in a rock concert by emitting green light with a wavelength of 515 nm. Calculate the frequency of the light. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro 2014 Pearson Education, Inc.

10 Color The color of light is determined by its wavelength or frequency. White light is a mixture of all the colors of visible light. A spectrum Red Orange Yellow Green Blue Indigo Violet When an object absorbs some of the wavelengths of white light and reflects others, it appears colored; the observed color is predominantly the colors reflected.

11 Amplitude and Wavelength

12 The Electromagnetic Spectrum Visible light comprises only a small fraction of all the wavelengths of light, called the electromagnetic spectrum. Shorter wavelength (high-frequency) light has higher energy. Radio wave light has the lowest energy. Gamma ray light has the highest energy. High-energy electromagnetic radiation can potentially damage biological molecules. Ionizing radiation

13 Electromagnetic Spectrum

14 Continuous Spectrum

15 The Photoelectric Effect It was observed that many metals emit electrons when a light shines on their surface. This is called the photoelectric effect. Classic wave theory attributed this effect to the light energy being transferred to the electron. According to this theory, if the wavelength of light is made shorter, or the light wave s intensity made brighter, more electrons should be ejected. Remember that the energy of a wave is directly proportional to its amplitude and its frequency. This idea predicts if a dim light were used there would be a lag time before electrons were emitted. To give the electrons time to absorb enough energy

16 The Photoelectric Effect: The Problem Experimental observations indicate the following: A minimum frequency was needed before electrons would be emitted regardless of the intensity called the threshold frequency. High-frequency light from a dim source caused electron emission without any lag time.

17 Einstein s Explanation Einstein proposed that the light energy was delivered to the atoms in packets, called quanta or photons. The energy of a photon of light is directly proportional to its frequency. Inversely proportional to its wavelength The proportionality constant is called Planck s Constant, (h), and has the value J s.

18 Example 2 Sort You are given the wavelength and total energy of a light pulse and asked to find the number of photons it contains. Given: Find: number of photons Strategize In the first part of the conceptual plan, calculate the energy of an individual photon from its wavelength. In the second part, divide the total energy of the pulse by the energy of a photon to get the number of photons in the pulse. Conceptual Plan Photon Energy A nitrogen gas laser pulse with a wavelength of 337 nm contains 3.83 mj of energy. How many photons does it contain? Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro 2014 Pearson Education, Inc.

19 Example 2 Continued Photon Energy Relationships Used E = hc/λ Solve To execute the first part of the conceptual plan, convert the wavelength to meters and substitute it into the equation to calculate the energy of a 337 nm photon. To execute the second part of the conceptual plan, convert the energy of the pulse from mj to J. Then divide the energy of the pulse by the energy of a photon to obtain the number of photons. Solution Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro 2014 Pearson Education, Inc.

20 Example 2 Continued Photon Energy Check The units of the answer, photons, are correct. The magnitude of the answer (10 15 ) is reasonable. Photons are small particles and any macroscopic collection should contain a large number of them. For Practice 2a A 100-watt light bulb radiates energy at a rate of 100 J/s (The watt, a unit of power, or energy over time, is defined as 1 J/s.) If all of the light emitted has a wavelength of 525 nm, how many photons are emitted per second? (Assume three significant figures in this calculation.) For More Practice 2b The energy required to dislodge electrons from sodium metal via the photoelectric effect is 275 kj/mol. What wavelength in nm of light has sufficient energy per photon to dislodge an electron from the surface of sodium? Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro 2014 Pearson Education, Inc.

21 Example 3 Solution Examine the Figure and note that X-rays have the shortest wavelength, followed by visible light and then microwaves. a. wavelength X rays < visible < microwaves Wavelength, Energy, and Frequency Arrange the three types of electromagnetic radiation visible light, X-rays, and microwaves in order of increasing a. wavelength. b. frequency. c. energy per photon. Since frequency and wavelength are inversely proportional the longer the wavelength the shorter the frequency the ordering with respect to frequency is the reverse of the ordering with respect to wavelength. b. frequency microwaves < visible < X rays FIGURE The Electromagnetic Spectrum The right side of the spectrum consists of high-energy, high-frequency, short-wavelength radiation. The left side consists of low-energy, low-frequency, long-wavelength radiation. Visible light constitutes a small segment in the middle. Energy per photon decreases with increasing wavelength but increases with increasing frequency; therefore, the ordering with respect to energy per photon is the same as for frequency. c. energy per photon microwaves < visible < X rays Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro 2014 Pearson Education, Inc.

22 Example 3 Continued Wavelength, Energy, and Frequency For Practice 3a Arrange these three colors of visible light green, red, and blue in order of increasing a. wavelength. b. frequency. c. energy per photon. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro 2014 Pearson Education, Inc.

23 Spectra When atoms or molecules absorb energy, that energy is often released as light energy, Fireworks, neon lights, etc. When that emitted light is passed through a prism, a pattern of particular wavelengths of light is seen that is unique to that type of atom or molecule the pattern is called an emission spectrum. Noncontinuous Can be used to identify the material Flame tests

24 Exciting Gas Atoms to Emit Light with Electrical Energy

25 Identifying Elements with Flame Tests Na K Li Ba

26 Emission Spectra

27 Examples of Spectra