Absorption Emission of Photons and Conservation of Energy
Absorption / Emission of Photons and Conservation of Energy hv hv Ef - Ei = hv Ei - Ef = hv
Energy Levels of Hydrogen
Electron jumping to a higher energy level E = 12. 08 e. V
Spectrum of Hydrogen, Emission lines Bohr’s formula:
Hydrogen is therefore a fussy absorber / emitter of light It only absorbs or emits photons with precisely the right energies dictated by energy conservation
Electron in a Hydrogen Atom • The three quantum numbers: – n = 1, 2, 3, … – l = 0, 1, …, n-1 – m = -l, -l+1, …, l-1, l • For historical reasons, l = 0, 1, 2, 3 is also known as s, p, d, f
1 s Orbital
Density of the cloud gives probability of where the electron is located
2 s and 2 p Orbitals
Another diagram of 2 p orbitals Note that there are three different configurations corresponding to m = -1, 0, 1
3 d Orbitals Now there are five different configurations corresponding to m = -2, -1, 0, 1, 2
4 f Orbitals There are seven different configurations corresponding to m = -3, -2, -1, 0, 1, 2, 3
• The excited atom usually de-excites in about 100 millionth of a second. • The subsequent emitted radiation has an energy that matches that of the orbital change in the atom. • This emitted radiation gives the characteristic colors of the element involved.
Emission Spectra Continuous Emission Spectrum Slit White Light Source Prism Photographic Film
Emission Spectra of Hydrogen Discrete Emission Spectrum Slit Film Low Density Glowing Hydrogen Gas Prism Photographic Film
Portion of the Absorption Spectrum of Hydrogen Discrete Emission Spectrum Discrete Absorption Spectrum Slit Hot Hydrogen Gas White Light Source Film Prism Photographic Film
Absorption Spectra • Frequencies of light that represent the correct energy jumps in the atom will be absorbed. • When the atom de-excites, it may emit the same kinds of frequencies it absorbed. • However, this emission can be in any direction.
Emission and Absorption Continous Spectrum Hot Gas Portion of the Emission Spectrum Cold Gas Absorption Spectrum
Absorption spectrum of Sun Emission spectra of various elements
Usually the Emission spectrum has more “features” of the absorption spectrum Atom excitation, Absorption lines from the ground state (n=1) Atom de-excitation, Emission lines from the excited states
Schrodinger equation for one electron atoms Coulomb potential
Radial and angular part
What is the physical meaning of the wave function? BORN POSTULATE The probability of finding an electron in a certain region of space is proportional to y 2, the square of the value of the wavefunction at that region. y can be positive or negative. y 2 is always positive y 2 is called the “electron density”
E. g. , the hydrogen ground state y 1 s = y 21 s = 1 1 3/2 ao p 1 3 ao e -r/ao e (ao: first Bohr radius=0. 529 Å) -2 r/ao y 21 s r
Radial electron densities The probability of finding an electron at a distance r from the nucleus, regardless of direction The radial electron density is proportional to r 2 y 2 Dr Surface = 4 pr 2 Volume of shell = 4 pr 2 Dr
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