bohr atom

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How does the Bohr atom improve over Rutherford's? What do the spectral lines in emission spectra prove to us?

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The Bohr Atom : 

The Bohr Atom Experiments and Models Fixing the problems in the Rutherford model ++

Rutherford’s Atom : 

Rutherford’s Atom Power and Problems Power A nuclear atom Demonstrated that the atom was 99.9%+ empty space Identified the electrons as the “engines” of chemical change Properly positioned the sites of positive and negative charge ++

Rutherford’s Atom : 

Rutherford’s Atom Power and Problems Problems The 360° swing of the electrons ignored the fact that molecules have a definite shape, so atoms seem to be bonded in definite configurations The atom was unstable according to classical electrodynamics Accelerating charged particles lose energy ++

Rutherford’s Catastrophe : 

Rutherford’s Catastrophe Electrons can’t behave like this ++

Rutherford’s Catastrophe : 

Rutherford’s Catastrophe Accelerating charged particles lose energy ++ - + attracts - ++

Rutherford’s Catastrophe : 

Rutherford’s Catastrophe Accelerating charged particles lose energy ++ - But electron must move about nucleus in planetary motion ++

Rutherford’s Catastrophe : 

Rutherford’s Catastrophe Accelerating charged particles lose energy ++ - Yet as electron accelerates toward the center it must lose energy ++

Rutherford’s Catastrophe : 

Rutherford’s Catastrophe Accelerating charged particles lose energy ++ - And as it loses energy it gets closer & closer to the nucleus ++

Rutherford’s Catastrophe : 

Rutherford’s Catastrophe Accelerating charged particles lose energy ++ Eventually the electron spirals into the nucleus <10-9 sec ++ While it collapses, the atom would emit all frequencies of light, like a Doppler whine

However : 

However Electrons appear to remain in “orbit” indefinitely Clearly, the laws of classical electrodynamics do not hold in the vicinity of the nucleus When atoms lose energy, they don’t give off all frequencies of light, or all kinds of energy They give off discrete and predictable frequencies (colors) of light ++

Hot gases emit light : 

Hot gases emit light You know from observation that every hot gas, like neon or hydrogen, emits only certain frequencies of light Neon Hydrogen Krypton

Examples : 

Examples For more of this, wait until physics next year Wavelength High voltage passes through a gas like hydrogen or neon or argon, giving off a distinct series of light frequencies ++

Examples : 

Examples The mixture of colors from the emission spectrum gives the distinctive color of each lamp Neon emission spectrum (colors separated) Colors mixed From Toms River School District

Meaning : 

Meaning Only certain energy changes are “allowed” when electrons lose energy This is the hydrogen absorption spectrum: the dark lines represent the only allowed energy changes ++ From Brad McCormick

Bohr’s interpretation : 

Bohr’s interpretation In the vicinity of the nucleus, electrons don’t obey classical electrodynamics Bohr atom Electrons may only “orbit” at certain distances from the nucleus These correspond to particular energy levels Electrons may jump from one energy level to the next, but may not inhabit any level in between allowed levels These levels are called “shells”

An energy-level diagram : 

An energy-level diagram Energy given in electron-volts Electrons are normally at lowest energy n=1

Exciting an electron : 

Exciting an electron Electrons can absorb energy and jump “up” Electron absorbs enough energy to jump from n=1 to n=2

Electron “relaxing” : 

Electron “relaxing” Electrons can release energy Electron may then release energy as light & fall back to n=1 This energy of light is in the ultraviolet range, not visible to the human eye

Other transitions : 

Other transitions Electrons can be at higher states, n=3 or 4 or more In this case, an electron at n=3 falls to n=2; this would release light in the visible range

Slide 20: 

Intensity Color Wavelength (nm) strong Red 656.3 strong Cyan 486.1 strong Indigo 434.0 weak Violet 410.2 Each of these lines comes from zillions of energy level changes in billions of hydrogen atoms

A Simplified Bohr “picture” : 

A Simplified Bohr “picture” Shows the nuclear atom & the allowed energy levels The energy level is also known as the first quantum number, n

Beyond Bohr : 

Beyond Bohr 20th century physics has refined this “quantum model” of the atom. You will study this more in physics and AP chemistry (2nd year)