Interpretation of Hydrogen Emission Spectra

White Light Spectrum (Wide Range of Colors and hence Energies of light)

Hydrogen Emission Spectrum

Spectral lines of Helium (left) and Neon (right)

Mercury spectrum

Concept of Quantized Quantized – restricted values; only certain specific values are allowed. Analogies: Nonquantized: Ramp Quantized: Stairs, ladder

Concept of Absorbance vs. Emission Spectrum

Hydrogen Emission Spectra Experiment – Experimental Observations Energy added in the form of heat, electricity or light → only certain specific colors of light are emitted

Interpretation of Hydrogen Line Spectra- Experimental Observations Since ONLY CERTAIN SPECIFIC COLORS OF LIGHT ARE EMITTED, ONLY CERTAIN SPECIFIC FREQUENCIES AND WAVELENGTHS ARE EMITTED Longer wavelengths – like RED, have lower energy Shorter wavelengths – like BLUE have higher energy Specific colors = ONLY SPECIFIC ENERGIES OF LIGHT ARE EMITTED

Bohr Model (1913 – Niels Bohr) If no restrictions on electrons’ energy and motion e- would pulled into nucleus- this does not happen; therefore there must be restrictions. Quantized- means restricted: only certain, specific values are possible Analogies: Nonquantized = ramp HW 8-2 Quantized = stairs Link to Hydrogen model’s phet

Postulates of Bohr model HW 8-2 Electrons can have only certain specific values of energy. Electrons travel in fixed, circular orbits* (* = we no longer believe this postulate) (QUANTIZED ENERGIES)

Postulates of Bohr model GROUND STATE = LOWEST ENERGY STATE EXCITED STATE= HIGHER ENERGY STATE Energy absorbed Energy Released as light When an electron absorbs the exact difference in energy between 2 levels it makes a quantum leap up When an electron makes a quantum leap down it releases the exact difference in energy between two levels as light

Link to Electron Orbits

Interpretation of Experiment Energy added to atoms is absorbed by ELECTRONS. Electron absorbs specific quantity of energy, it makes a quantum leap from its lowest energy GROUND state to higher energy EXCITED state.

Quantum Leaps: Ground → Excited Add Energy electron = ↑ n = 3 ___________ excited state n=2 ___________ excited state n=1 ↑ ____________ ground state

Quantum Leaps: Ground → Excited Add Energy electron = ↑ n = 3 ___________ excited state n=2 ___________ excited state n=1 ↑ ____________ ground state

Quantum Leaps: Ground → Excited Add Energy electron = ↑ n = 3 ___________ excited state n=2 ↑ ___________ excited state n=1 ____________ ground state

Back Page Electrons CANNOT remain in an excited state for any significant time period. Electron in excited state returns to the ground state by emitting a specific amount of energy in the form of LIGHT. Because only certain specific jumps from excited to ground state are possible, only certain specific energies and hence colors of light can be emitted. HW 8-2, #6

Quantum Leaps: Excited → Ground Release of Light n = 3 ___________ excited state n=2 ↑ ___________ excited state n=1 ____________ ground state

Quantum Leaps: Excited → Ground Release of Light n = 3 ___________ excited state n=2 ___________ excited state n=1 ↑ ____________ ground state

Which color of light is associated with which Quantum Jump? We observed the lines of color in the hydrogen spectrum red, teal, and violet. These different colors were released as a result of 3 different jumps between levels: 5 → 2, 4 → 2 and 3 → 2. Which jump corresponds to which color of light emission and how do you know? Link to Hydrogen Emission Spectrum Animation

How can all of these jumps be happening at the same time? Each of the different colors in the hydrogen spectrum is the result of different quantum leaps. A hydrogen atom has one electron. How can all of these jumps being happening at once? How can all of these different colors of light be emitted at the same time?

Link to lasers phet The tube contains an enormous population of hydrogen atoms – trillions and trillions of atoms Within the population every possible transition that could take place does

Why can we see only 3 lines of color for the hydrogen spectrum? Each quantum leap from excited back to ground should release energy in the form of light e.g. 2→ 1, 3 → 1, 4 → 3, etc. Yet we only see 3 colors of light from the 5 → 2, 4 → 2 and 3 → 2 transitions. Why don’t we see more colors of light?

Link to Neon light PhEt applet Most of the transitions release light outside the visible range – in the infrared or ultraviolet range.