ATOMIC+SPECTRA+AND+ATOMIC+ENERGY+STATES

IB PHYSICS > QUANTUM AND NUCLEAR PHYSICS
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ATOMIC SPECTRA AND ATOMIC ENERGY STATES


EMISSION SPECTRA: An element which is given enough energy emits light at particular frequencies which form a characteristic line spectrum which can be used to identify the element. ABSORPTION SPECTRA: White light shone through a hot gas will have dark lines in its continuous spectrum due to absorption at its characteristic frequencies.

Outline a laboratory procedure for producing and observing atomic spectra.



Students should be able to outline procedures for both emission and absorption spectra. Details of the spectrometer are not required. Explain how atomic spectra provide evidence for the quantization of energy in atoms. An explanation in terms of energy differences between allowed electron energy states is sufficient.

EVIDENCE FOR QUANTISATION: Electrons in an atom exist only at fixed energy levels. Any transition from one level to another involves a fixed amount of energy which is emitted or absorbed as a quantum with an associated frequency.

Calculate wavelengths of spectral lines from energy level differences and vice versa. Aim 7: Computer simulations showing the link between energy level transitions and spectral lines assist understanding.

EXAMPLE: If an electron moves from a level with energy -3.39 eV to another at -13.6 eV, what is the wavelength of the emitted photon? CALCULATE ENERGY DIFFERENCE IN JOULES FIND FREQUENCY OF PHOTON CALCULATE WAVELENGTH SOLUTION: 122nm

BOHR MODEL OF HYDROGEN ATOM: The energy of electron orbits is quantized and the energy of the spectral lines of hydrogen are predicted correctly.

Explain the origin of atomic energy levels in terms of the “electron in a box” model.

ELECTRON IN A BOX: A simplified version of electrons in the atom considers them as standing waves in one dimension. with wavelength l = 2L/n

The model assumes that, if an electron is confined to move in one dimension by a box, the de Broglie waves associated with the electron will be standing waves of wavelength 2L/n, where L is the length of the box and n is a positive integer. Students should be able to show that the kinetic energy Ek of the electron in the box is  USING 2L/n; KE = p^2/m; wavelength = h/p

Outline the Schrödinger model of the hydrogen atom.

SCHRODINGER MODEL: Electrons can be considered as wavefunctions. For the wavefunction, (amplitude)2 is proportional to probability of finding an electron at a given point. The model assumes that electrons in the atom may be described by wavefunctions. The electron has an undefined position, but the square of the amplitude of the wavefunction gives the probability of finding the electron at a particular point.

[|PHET: MODELS OF HYDROGEN FROM BILLIARD BALL TO SCHRODINGER:]

Outline the Heisenberg uncertainty principle with regard to position–momentum and time–energy. media type="custom" key="25090872" Students should be aware that the conjugate quantities, position–momentum and time–energy, cannot be known precisely at the same time. They should know of the link between the uncertainty principle and the de Broglie hypothesis. For example, students should know that, if a particle has a uniquely defined de Broglie wavelength, then its momentum is known precisely but all knowledge of its position is lost.

HEISENBERG UNCERTAINTY PRINCIPLE: It is impossible to measure exactly the momentum and position of a particle simultaneously. This is also true of energy and time.

 [|POEM ABOUT SCHROEDINGER'S CAT]