James T. Shipman Jerry D. Wilson Charles A. Higgins, Jr. Atomic Physics Chapter 9

James T. ShipmanJerry D. WilsonCharles A. Higgins, Jr.

Atomic Physics

Chapter 9

Atomic Physics

• Classical physics (Newtonian physics) – development of physics prior to around 1900

• Classical physics was generally concerned with macrocosm – the description & explanation of large-scale phenomena – cannon balls, planets, wave motion, sound, optics,

electromagnetism• Modern physics (after about 1900) – concerned with the

microscopic world – microcosm – the subatomic world is difficult to describe with classical physics

• This chapter deals with only a part of modern physics called Atomic Physics – dealing with electrons in the atom.

Sec 9.1Early Concepts of the Atom

• Atom: Atomos (Gr)• Dalton’s model:

Atoms are like tiny ball bearings

Section 9.1


• J. J. Thompson discovered the electron

• Experiments by R. A. Milliken showed the electron had charge– The charge on the

electron is 1.6x10-19 Coulombs

• Thompson’s “Plum-pudding model”


• Rutherford’s experiment (ch 10) showed that the atom was mostly empty space– Electrons circulate

around a positively charged core


• This is how the models evolved in the early years

Section 9.1

Sec 9.2 The Dual Nature of Light-Classical Wave Theory of Light

• The key to further understanding of the atom is to study how it interacts with light– What is a blackbody?– My definition: Anything that is warm that glows

• A warm burner on an oven• A fire• The Sun• A star• The filament of a light bulb (incandescent)• A HUMAN BEING!

Section 9.2


• Red Hot Steel• The radiation

component of maximum intensity determines a hot solid’s color.

Section 9.2


• A blackbody’s temperature determines the wavelength of it’s brightest color– When heated, a

blackbody glows brighter and more blue

– When cooled, a blackbody glows dimmer and more red

Section 9.2


• At shorter wavelengths (blue), classical wave theory says blackbodies should be bright.

• This is NOT what is actually observed.

Section 9.2


• Max Planck’s model of a blackbody:• He proposed that black bodies should be

composed of a very large number of single cavity resonators.– Each resonator was like the standing waves on a

vibrating string• His radical assumptions:

– Energy was transferred between blackbody and wave essentially instantaneously

– Energy is transformed not continuously, but in discreet amounts (packets) he called quanta

– The energy contained in each quanta was proportional to the frequency of the wave: E = hf


• Planck’s constant, h = 6.63x10-23 Joule-sec• This basically said that energy does not

transform from one object to another, or from one form to another continuously, as one pours water from one cup to the another, but in discreet amounts that depended on the frequency of the wave.– We say that energy is thus “quantized”.

Section 9.2


Continuous Energy

Section 9.2

Quantized Energy


Another discovery was the photo-electric effect• Certain metals emit electrons when exposed to

light– The photoelectric effect • This direct conversion of light into electrical

energy is the basis for photocells– Automatic door openers, light meters, solar energy

applications• Once again classical wave theory could not

explain the photoelectric effect

Section 9.2


• Einstein took Planck’s concept of quanta and quantization one step further– The metal converted energy from the wave into

energy of free electrons by the absorption of “packets”– Einstein called these packets of energy “photons”– Higher frequency, higher the energy absorbed.

• So what is light? Is it a wave or a particle?It is both

Section 9.2


Section 9.3 Bohr Theory of the Hydrogen Atom

• Recall from chapter 7 that white light can be dispersed into a spectrum of colors by a prism– Due to differences in

refraction for the specific wavelengths

– This is a continuous spectrum


• In the late 1800’s experimental work with gas-discharge tubes revealed two other types of spectra– Line emission spectra

displayed only bright spectral lines of certain frequencies

– Line absorption spectra displays dark lines of missing colors


• In the late 1800’s experimental work with gas-discharge tubes revealed two other types of spectra– Line emission spectra

displayed only bright spectral lines of certain frequencies

– Line absorption spectra displays dark lines of missing colors

Sec 9.3 Bohr Theory of the Hydrogen Atom

• In 1913 an explanation of the observed spectral line phenomena was advanced by the Danish physicist Niels Bohr

Section 9.3


• Bohr predicted that the single hydrogen electron would only be found in discrete orbits with particular radii– Bohr’s possible

electron orbits were given whole-number designations, n = 1, 2, 3, …

Section 9.3


• Bohr predicted that the single hydrogen electron would only be found in discrete orbits with particular radii– “n” is called the

principal quantum number

– The lowest n-value, (n = 1) has the smallest radius

Section 9.3


• In the Bohr model, each allowed orbit has a certain energy.– n = 1 is the lowest

energy: It is the ground state

– n = 2, 3, 4, … are the ‘excited states’.

– All orbits are trapped in a ‘well’.


• A transition to a lower energy level results in the emission of a photon.

• A transition to a higher energy level results in the absorption of a photon.

Section 9.3


• Transitions among discrete energy orbit levels give rise to discrete spectral lines within the UV, visible, and IR wavelengths

Section 9.3

Sec 9.6 Matter Waves - de Broglie Waves

• With the development of the dual nature of light it became apparent that light “waves” sometime act like particles.

• Could the reverse be true?• Can particles have wave properties?• In 1925 the French physicist de Broglie

postulated that matter has properties of both waves and particles. This is de Broglie’s hypothesis.

Section 9.6

Skip section 9.4, and 9.5


De Broglie’s hypothesis: • Any moving particle has a wave associated with

it whose wavelength is given by the following formula

• l = h/mv• l = wavelength of the moving particle• m = mass of the moving particle• v = speed of the moving particle• h = Planck’s constant (6.63 x 10-34 J.s)

Section 9.6


• In 1927 two U.S. scientists, Davisson and Germer, experimentally verified that particles have wave characteristics.

• These two scientists showed that a bean of electrons (particles) exhibits a diffraction pattern (a wave property.)

• Recall Section 7.4 – appreciable diffraction only occurs when a wave passes through a slit of approximately the same width as the wavelength

Section 9.6


X-Ray pattern

Section 9.6

Diffraction pattern of electrons

Sec 9.7 Electron Cloud Model of an Atom

• Recall that Bohr chose to analyze the hydrogen atom, because it is the simplest atom

• It is increasingly difficult to analyze atoms with more that one electron, due to the myriad of possible electrical interactions

• In large atoms, the electrons in the outer orbits are also partially shielded from the attractive forces of the nucleus

Section 9.7


• With the discovery of the dual natures of both waves and particles …

• A new kind of physics was developed, called quantum mechanics or wave mechanics– Developed in the 1920’s and 1930’s as a synthesis of

wave and quantum ideas• Quantum mechanisms also integrated

Heisenberg’s uncertainty principle– The concept of probability replaced the views of

classical mechanics in describing electron movement

Section 9.7


• In 1926, the Austrian physicist Erwin Schrödinger presented a new mathematical equation applying de Broglie’s matter waves.

• Schrödinger’s equation was basically a formulation of the conservation of energy

• The simplified form of this equation is …• (Ek + Ep)Y = EY

– Ek, Ep, and E are kinetic, potential, and total energies, respectively

– Y = wave function

Section 9.7


• For the electron wave to be stable, the circumference of the orbit must be a whole number of wavelengths

Section 9.7

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James T. Shipman Jerry D. Wilson Charles A. Higgins, Jr. Atomic Physics Chapter 9