Ch2 Bohr’s atomic model Four puzzles –Blackbody radiation –The photoelectric effect –Compton effect –Atomic spectra Balmer formula Bohr’s model Frank-Hertz.

Slides:



Advertisements
Similar presentations
Ch. 13 Electrons in Atoms Ch Models of the Atom
Advertisements

Happyphysics.com Physics Lecture Resources Prof. Mineesh Gulati Head-Physics Wing Happy Model Hr. Sec. School, Udhampur, J&K Website: happyphysics.com.
1 My Chapter 27 Lecture. 2 Chapter 27: Early Quantum Physics and the Photon Blackbody Radiation The Photoelectric Effect Compton Scattering Early Models.
Chapter 27: Early Quantum Physics and the Photon
Black body radiation BBR is the radiation emitted by a non-reflecting solid body. A perfect black body is one which absorbs all the radiations falling.
1 Light as a Particle The photoelectric effect. In 1888, Heinrich Hertz discovered that electrons could be ejected from a sample by shining light on it.
2. The Particle-like Properties Of Electromagnetic Radiation
Electromagnetic Radiation
The Electronic Structures of Atoms Electromagnetic Radiation
Electronic Structure of Atoms
Light. Photons The photon is the gauge boson of the electromagnetic force. –Massless –Stable –Interacts with charged particles. Photon velocity depends.
Quantum Theory of Light A TimeLine. Light as an EM Wave.
1 Light as a Particle In 1888, Heinrich Hertz discovered that electrons could be ejected from a sample by shining light on it. This is known as the photoelectric.
Physics at the end of XIX Century Major Discoveries of XX Century
Introduction to Quantum Physics
Classical vs Quantum Mechanics Rutherford’s model of the atom: electrons orbiting around a dense, massive positive nucleus Expected to be able to use classical.
Quantum Physics. Black Body Radiation Intensity of blackbody radiation Classical Rayleigh-Jeans law for radiation emission Planck’s expression h =
Cutnell/Johnson Physics 7 th edition Classroom Response System Questions Chapter 39 More about Matter Waves Reading Quiz Questions.
Early Quantum Theory and Models of the Atom
Chapter 38.
Electron Configurations & the Periodic Table Chapter 7.
Electronic Structure of Atoms Chapter 6 BLB 12 th.
Particle Nature of Light
Chapter 39 Particles Behaving as Waves
Quantum Physics Study Questions PHYS 252 Dr. Varriano.
Young/Freeman University Physics 11e. Ch 38 Photons, Electrons, and Atoms © 2005 Pearson Education.
Midterm results will be posted downstairs (by the labs) this afternoon No office hours today.
Chapter 6: Electronic Structure of Atoms Pages
Quantum Mechanics. Planck’s Law A blackbody is a hypothetical body which absorbs radiation perfectly for every wave length. The radiation law of Rayleigh-Jeans.
As an object gets hot, it gives Off energy in the form of Electromagnetic radiation.
Quantum Physics. Quantum Theory Max Planck, examining heat radiation (ir light) proposes energy is quantized, or occurring in discrete small packets with.
Blackbody Radiation: The light from a blackbody is light that comes solely from the object itself rather than being reflected from some other source. A.
Wave-Particle Duality: The Beginnings of Quantum Mechanics.
Thompson’s experiment (discovery of electron) + - V + - Physics at the end of XIX Century and Major Discoveries of XX Century.
Baby-Quiz 1.Why are diffraction effects of your eyes more important during the day than at night? 2.Will the converging lens focus blue light or red light.
Chemistry is in the electrons Electronic structure – how the electrons are arranged inside the atom Two parameters: –Energy –Position.
Chemistry 330 Chapter 11 Quantum Mechanics – The Concepts.
Blackbody A black body is an ideal system that absorbs all radiation incident on it The electromagnetic radiation emitted by a black body is called blackbody.
Physics 1C Lecture 28A. Blackbody Radiation Any object emits EM radiation (thermal radiation). A blackbody is any body that is a perfect absorber or emitter.
Radiation Protection and Safety 11/15/ Atomic Structure   Dalton – law of definite proportions   Avogadro – equal volumes of gas   Balmer –
Chapter 27- Atomic/Quantum Physics
Quantum Physics Chapter 27!.
28.3 THE BOHR THEORY OF HYDROGEN At the beginning of the 20th century, scientists were puzzled by the failure of classical physics to explain the characteristics.
Slide 1 of 38 chemistry. Slide 2 of 38 © Copyright Pearson Prentice Hall Physics and the Quantum Mechanical Model > Light The amplitude of a wave is the.
1 Electromagnetic Radiation c=  How many wavelengths pass through point P in one second? Frequency! P.
Chapter 28:Atomic Physics
Rutherford’s Model: Conclusion Massive nucleus of diameter m and combined proton mass equal to half of the nuclear mass Planetary model: Electrons.
Bohr Model and Quantum Theory
4: Introduction to Quantum Physics
Origin of Quantum Theory
Sydney Opera House Opens (1973) READING: Chapter 8 sections 1 – 2 READING: Chapter 8 sections 1 – 2 HOMEWORK – DUE TUESDAY 10/20/15 HOMEWORK – DUE TUESDAY.
The Nature of Light: Its Wave Nature Light is a form of made of perpendicular waves, one for the electric field and one for the magnetic field All electromagnetic.
1 2. Atoms and Electrons How to describe a new physical phenomenon? New natural phenomenon Previously existing theory Not explained Explained New theoryPredicts.
Unit 12: Part 2 Quantum Physics. Overview Quantization: Planck’s Hypothesis Quanta of Light: Photons and the Photoelectric Effect Quantum “Particles”:
Chapter 33 Early Quantum Theory and Models of Atom.
Chapter 5 “Electrons in Atoms”. Section 5.3 Physics and the Quantum Mechanical Model l OBJECTIVES: Describe the relationship between the wavelength and.
QUANTUM AND NUCLEAR PHYSICS. Wave Particle Duality In some situations light exhibits properties that are wave-like or particle like. Light does not show.
Light, Quantitized Energy & Quantum Theory CVHS Chemistry Ch 5.1 & 5.2.
1© Manhattan Press (H.K.) Ltd Continuous spectra Spectra Sun’s spectrum and Fraunhofer lines.
Properties of light spectroscopy quantum hypothesis hydrogen atom Heisenberg Uncertainty Principle orbitals ATOMIC STRUCTURE Kotz Ch 7 & Ch 22 (sect 4,5)
3.1 Discovery of the X-Ray and the Electron 3.2Determination of Electron Charge 3.3Line Spectra 3.4Quantization 3.5Blackbody Radiation 3.6Photoelectric.
Chapter 39 Particles Behaving as Waves
Chapter2. Elements of quantum mechanics
Chapter 39 Particles Behaving as Waves
Electrons and Light Chapter 13.3.
Chemistry 141 Monday, October 30, 2017 Lecture 23 Light and Matter
Chapter 27 Early Quantum Theory
Ch. 13 Electrons in Atoms Ch Models of the Atom
Chapter 39 Particles Behaving as Waves
c = speed of light (ms-1, constant)
Presentation transcript:

Ch2 Bohr’s atomic model Four puzzles –Blackbody radiation –The photoelectric effect –Compton effect –Atomic spectra Balmer formula Bohr’s model Frank-Hertz experiment

Blackbody Absorptivity (absorptance): the ratio of the radiation absorbed by a body to that incident on the body. Blackbody: A body with a surface having an absorptivity equal to unity. A realistic blackbody: For a cavity kept at a constant temperature with the interior wall blackened, a small hole in the wall behaves like a blackbody.

Some observations Stefan's Law states that the power radiated by a body is proportional to the 4th power of the absolute temperature. For a given temperature, the radiation forms a continuous spectrum with respect to the frequency.

Wein's Displacement Law

Reyleigh-Jeans law

Ultraviolet catastrophe

Puzzles in blackbody radiation Two puzzles: –Why were not radiation above the ultraviolet region present? –Why was there a non-uniform distribution of electromagnetic radiation being emitted?

Plank’s theory Planck made an assumption that the energy of an oscillator must be an integral multiple of the product of the constant h and the frequency of the electromagnetic radiation it emitted. His assumption resulted in a formula for the blackbody radiation that was in excellent agreement with experiment at all frequencies.

Two puzzles to be explained Radiation in the high frequency region were not emitted from the blackbodies because this required large energy changes which could not occur in the atoms. Certain energy states were more probable in the atoms and therefore frequencies associated with these energy states were more likely to be emitted.

The photoelectric effect When light of a high frequency was incident on a metallic surface, electrons were emitted from the surface.

Actual observation Intensity: The high intensity of light would not cause electrons to have high KE. The actual reaction time is very short (10 -9 s). Frequency: At a certain frequency called threshold frequency, electrons were emitted. A frequency beyond it will cause the electrons to have a greater KE. Stopping voltage: The energy of the ejected electrons was proportional to the frequency of the illuminating light & had nothing to do with intensity.

Einstein’s explanation For a photoelectron, E=hf. The minimum energy required to pull electrons from inside to outside the metal is called the work function W. W=hf 0 If an electron is given an energy E larger than W, it can escape the metal and will have a maximum KE:

The Compton effect (Compton scattering) This could be explained when X rays are regards as particles (photons). The collision between a photon and an electron is regarded as an elastic collision.

Discrete spectra Atoms emit and absorb light only at specific frequencies. –Emission lines, –Absorption lines, Balmer found that the wavelengths of visible and near ultraviolet line spectra of hydrogen obey a simple formula exactly: R H =1.097x10 7 m -1 is called the Rydberg ( 里德伯 ) constant.

Bohr model There are three postulates used in Bohr’s model: –The electron moved in a certain set of stable orbits in which classical mechanics can be used to describe motion of the electron. –Moving electrons in stable states do not radiate. An electron can make a sudden quantum jump between the orbits. –The orbital angular momenta of the electrons are quantized.

Quanta in the atom The total energy of the electron is inversely proportional to the square of n, i.e. where n is called quantum number. The total energy is also found to be negative, indicating a “bound” state. The most negative state, the most tightly bound electron, occurs for n=1, referred to as the ground state of the atom, n>=2, excited states. The angular momentum of the electron moving in a circular orbit can only take discrete values:

Line spectra of the H atom Energy levels: Lyman series: n=1; Balmer series: n=2; Paschen series: n=3; Brackett series: n=4

Improvement on the Bohr model Finite nuclear mass (motion of nucleus): When taking the nuclear mass into account, the reduced mass should replace the electron mass. Relativistic correction: The effect of the relativistic mass change m(v) should be considered. Faster  massive  decrease in energy. Sommerfeld’s extension: Electrons should have elliptical orbits with the same energies as that in circular orbits. The second quantum number should be introduced.

Frank-Hertz experiment Frank & Hertz in 1913 showed the existence of discrete energy levels in atoms.

Frank-Hertz experiment results

Explanation With the increase of grid potential, more electrons move to the plate and the current rises accordingly. For mercury atoms, when V=4.9V, the electrons make inelastic collision and leave the atom jump to a high orbit (n=2). The original electrons move off with little energy and could not reach the plate and thus reduce the current. As V is increased further, the current rises again and would drop at V=9.8V. This would make more atoms to jump to n=2 state.

Limitations of Bohr model It can not be generalised to deal with systems with two more electrons as the force between the electrons can not be easily added. It can not explain the closely spaced lines. It can not be used to calculate the rate of transitions between different energy levels. The Bohr model was eventually superseded by the quantum mechanics developed by E Schrodinger, W Heisenberg and others, following the ideas of L de Broglie.