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- 3.11 Kirchhoff's Laws Explained
3.11 Kirchhoff's Laws Explained
Pre-Lecture Reading 3.11
Video Lecture
Supplementary Notes
Elements Other Than Hydrogen
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Every element has different absorption and emission lines, and series of lines, because:
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Every element has a different number of protons in its nucleus, which pull on the electrons differently, resulting in different orbitals and different energies between orbitals.
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Elements have different numbers of electrons.
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Inner electrons partially shield the charge of the nucleus from outer electrons, again resulting in different orbitals and different energies between orbitals.
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Each electron can transition between orbitals, resulting in different numbers of absorption and emission lines.
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Partial ionization (losing some but not all electrons) changes both the amount of shielding and the number of lines.
Absorption Line Spectra
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Absorption line spectra are caused by atoms with electrons in lower energy states transitioning to higher energy states.
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This requires that the gas be transparent (for the rest of the light to pass through) and cool (for the electrons to be in the lower energy states, and not collisionally excited to higher energy states or ionized).
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This is Kirchhoff's Third Law.
Emission Line Spectra
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Emission line spectra are caused by atoms with electrons in higher energy states transitioning to lower energy states (or by ionized atoms recombining).
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This requires that the gas be transparent (for the light to pass through, without being thermalized into a continuous spectrum) and hot or heated (for the electrons to have gotten into the higher energy states or ionized, via either collisional excitation or absorption of light).
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This is Kirchhoff's Second Law.