Rutherford proposed this model of an atom: WHY DON T ELECTRONS GET ATTRACTED TO THE NUCLEUS?

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2 Rutherford proposed this model of an atom: WHY DON T ELECTRONS GET ATTRACTED TO THE NUCLEUS?

3 Chapter 7

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5 Much of the understanding of quantum theory came from our understanding of electromagnetic radiation. EM RADIATION: A Wave-like oscillating magnetic and electric field perpendicular to each other. All EM Radiation moves at x 10 8 m/s

6 As waves, EM Radiation have different properties: Wavelength( ) distance between two identical points in a wave (length of 1 cycle) Amplitude (A) height of the wave.

7 As waves, EM Radiation have different properties: Frequency is the number of cycles pass thru a point at a given time (cycles per time) Speed = wavelength * frequency length time c = λ *ν = length * cycle cycle time c = *10 8 m /s

8 As waves, EM Radiation have different properties: Amplitude dictates INTENSITY of light!

9 As waves, EM Radiation have different properties: Wavelength (and frequency) dictates COLOR of light!

10 Among the many possible wavelengths and frequencies, we can only perceive of a small region (the visible region)

11 According to classical physics: LIGHT IS A WAVE REFRACTION: speed of light changes in different media (this change in speed is brought about by change in wavelength, frequency remains constant)

12 According to classical physics: LIGHT IS A WAVE DIFFRACTION: bending of waves at the edge of an object

13 According to classical physics: LIGHT IS A WAVE INTERFERENCE: happens when circular waves interact with one another.

14 PROBLEMS: A dental hygienist uses x-rays (λ = 1.00 Ǻ) to take a series of dental radiographs while the patient listens to a radio station (λ = 325 cm) and looks out the window at the blue sky (λ = 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, 3.00 x 10 8 m/s.)

15 However, during the later 19 th century, there are certain phenomena that cannot be defined by classical physics. 1. Blackbody radiation 2. The Photoelectric effect 3. Atomic Spectra

16 When an object is heated to about 1000 K, it begins to emit light (soft red glow). When you heat it further to 1500 K it begins to glow orange. At 2000 K it is very bright and whiter! This object was hypothetically called the BLACKBODY something that absorbs all types of EM radiation, but can also emit all types of EM radiation depending on the temperature

17 Temperature ENERGY But energy is related to moving particles and stored energy in particles? So then energy must also be related to light!!

18 Using classical theories, they predicted that as you heat up a blackbody, you will get lots of UV However, this is not the case, a blackbody. A blackbody only emits a certain frequency of light depending on heat, and then goes down steeply at the UV region.

19 In the 1900s, Max Planck proposed that a blackbody can only emit (or absorb) certain quantities of energy E = nhν Where n is a positive integer (also known as a quantum number) and h is Planck s constant h = x J*s

20 Planck s theory implies that if an atom can emit only certain quantities of energy then, the atom itself can only have certain quantities of energy. ENERGY IN ATOMS EXIST IN LEVELS and NOT CONTINUOUS ENERGY is QUANTIZED. ΔE atom = Δn * hν = E radiation This is your change in energy level!

21 The other phenomenon not explained by classical physicists is the photoelectric effect. When light hits a metal surface it can eject an electron and generate a current.

22 However, it is not that simple because of some observations: 1. They observed that light has to be of a certain frequency. 2. There is no time lag. The wave theory of light cannot explain these observations For waves, energy depends on amplitude and not frequency. This implies that a current should be produced when, say, high-intensity red light is used. Also, the energy of waves should be cumulative. That is, after some time, even low intensity red light should have delivered enough energy to eject an electron.

23 Using Planck s equation Albert Einstein proposed that light travels in packets/particles called photons. Each photon will have its own frequency and thus energy. A photon hitting the emitter plate will eject an electron only if it has enough energy to overcome a threshold energy (the work function of the metal) So no matter how bright your light is, if it is not in the right frequency, it will not eject an electron.

24 Using Planck s equation Albert Einstein proposed that light travels in packets/particles called photons. Each photon will have its own frequency and thus energy. The time lag is absent (even in dim light) because, it is not intensity, but rather frequency. So as long as there is 1 photon with enough energy then current will flow! Current will be weaker in dim light, but it will be there!

25 Everyday Evidence for Photons Red light is used in photographic darkrooms because it is not energetic enough to break the halogensilver bond in black and white films Ultraviolet light causes sunburn but visible light does not because UV photons are more energetic Our eyes detect color because photons of different energies trigger different chemical reactions in retina cells

26 The last phenomena is the atomic spectra.

27 This occurrence is anomalous because if electrons were moving in a continuous way, they would create a continuous spectra and not fine lines.

28 Rydberg fitted the wavelengths of each line in his equation: 1 λ = R 1 n 2 1 n R is the Rydberg constant = x 10 7 m -1 SO RUTHERFORD S MODEL OF AN ATOM, HAD TO BE REVAMPED! In comes NEILS BOHR s MODEL

29 Bohr s Model proposed that: 1. The H atom has only certain allowable energy levels (or stationary states). 2. The atom does not radiate energy while in one of its stationary states 3. The atom changes to another state by absorbing or emitting a photon whose energies equals the difference in energy between the two state

30 The H atom has only certain allowable energy levels (or stationary states).

31 The atom changes to another state by absorbing or emitting a photon whose energies equals the difference in energy between the two states E photon = E statea E stateb = hv photon E photon = E statea E stateb = h c λ photon

32 The atom changes to another state by absorbing or emitting a photon whose energies equals the difference in energy between the two state

33 For a hydrogen atom, Bohr used classical theories of electrostatic forces to calculate the energy levels of the hydrogen atom E = 2.18x statea n A 2

34 Using this, he was able to predict what wavelength of light will be absorbed or emitted by certain energy changes E statea E stateb = 2.18x10 18 J 1 2 n 1 2 A n B ΔE atom = 2.18x10 18 J 1 2 n 1 2 = E photon A n B hc λ = 2.18x10 18 J 1 2 n 1 2 A n B 1 λ = x107 m 1 2 n 1 2 A n B Rydberg s constant!!!

35 Using this, he was able to predict what wavelength of light will be absorbed or emitted by certain energy changes