An electron can be liberated from a surface due to particle collisions an electron and a photon.

Size: px

Start display at page:

Download "An electron can be liberated from a surface due to particle collisions an electron and a photon."

Brittany Parsons
5 years ago
Views:

1 Quantum Theory and the Atom the Bohr Atom The story so far Einstein argued that light is a photon (particle) and each photon has a discrete amount of energy associated with it governed by Planck's equation. An electron can be liberated from a surface due to particle collisions an electron and a photon. Extended to the atom all electron must have the ability to be freed from the atom if bombarded with enough energy to "free" the electron!!!!! 2. Dilemma about Rutherford's model of the atom. What are the electrons doing? *any accelerating charge releases energy and should therefore lose speed and should collapse to the positive nucleus 3. Spectral analysis of various materials demonstrated distinct energies emitted from objects. AtomicPhysics/AtomicPhysics.htm YYBCNQnYNM&feature=related Great video outlining the whole story Aside Balmer, a mathematician, created a function which predicts the various wavelengths seen with no attempt to explain the science behind the results. λ = 364.5n2 (n 2 4) Bohr took a "quantum" approach to the hydrogen atom and made the folowing claim. Electrons can only exist in discrete orbits where they don't radiate energy they only radiate when they jump from one level to another Energy of an electron in orbit E Total = Force of an electron in Equilibrium ΣF i = F net = ma Bohr made an argument about quantizing (fitting into the quantum theory by limiting their values to discrete levels) based on conserving angular momentum. He derived the equation n 2 r n = h 2 where h m e ke 2 = h 2π based on some minimal angular momentum related to Planck's equation Subbing in the various constants, the following 2 equations work for the Hydrogen atom r n = 5.29 x n 2 E n = 2.18 x n 2 1

2 2

In 1923, de Broglie makes an argument for Bohr's conclusion based on wave principles. Recall that de Broglie proposed matter waves wave behaviour of matter.

3 In 1923, de Broglie makes an argument for Bohr's conclusion based on wave principles. Recall that de Broglie proposed matter waves wave behaviour of matter. If the electron has a wave property, then the electron does not give off energy if it is "vibrating" in its fundamental frequency. The circumference of the orbit would have to be in agreement with its wavelength And if quantized then 2πr = λ 2πr n = nλ According to de Broglie, the wavelength of the electron is λ = h mv Note the energy equations comes from E Total = ke2 2r Complete the following table for the various possible values of an electron in a Hydrogen atom n, the quantum number radius (m) Energy (J) Energy (ev) Absorption and Emission Spectrum Bohr's theory is that the electron absorbs energy by jumping to higher "orbtals" before "falling" back to its ground state. During the fall, many jumps are possible. Each jump corresponds with a specific energy either by absorbing that much energy (absorption spectrum) or by emitting that energy (emission spectrum). These equations correctly predict the observed values. Which jumps would produce visible light? 3

4 Possible energy transformations that produce light. Colour (nm) Energy range (J) Energy range (ev) red ( ) orange( 4

5 Failure of the Bohr model (excerpt from astr.gsu.edu/hbase/bohr.html#c1) While the Bohr model was a major step toward understanding the quantum theory of the atom, it is not in fact a correct description of the nature of electron orbits. Some of the shortcomings of the model are: 1. It fails to provide any understanding of why certain spectral lines are brighter than others. There is no mechanism for the calculation of transition probabilities. 2. The Bohr model treats the electron as if it were a miniature planet, with definite radius and momentum. This is in direct violation of the uncertainty principle which dictates that position and momentum cannot be simultaneously determined. The Bohr model gives us a basic conceptual model of electrons orbits and energies. The precise details of spectra and charge distribution must be left to quantum mechanical calculations, as with the Schrodinger equation. Read pages

Physical Electronics. First class (1)

Physical Electronics. First class (1) Physical Electronics First class (1) Bohr s Model Why don t the electrons fall into the nucleus? Move like planets around the sun. In circular orbits at different levels. Amounts of energy separate one