Neils Bohr Tried to use the emission spectrum of hydrogen to further explain the atom Proposed that electrons orbit the nucleus in circular paths of fixed energy (energy levels) Ground state: when all electrons are in the lowest possible energy levels Excited state: at least one electron has absorbed energy and moved to a higher energy level
What happens when electrons jump? An electron can increase in energy level by absorbing energy Lower Higher energy level = increase in potential energy Endothermic process An electron releases energy in the form of light when it falls from a higher to lower energy level Higher Lower energy level = decrease in potential energy Exothermic process Photons are bundles of light energy that are emitted by electrons as they go from higher lower energy levels.
Hydrogen Emission Spectrum Only certain lines of color Only certain wavelengths Only certain energies White light separates into all these different wavelengths But when we tried this to see what types of light certain elements give off if their electrons are excited, we find that they only give off certain colors. They don’t give off the entire rainbow of colors. http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/linesp16.swf
Emission Spectra Visible Light Spectrum White light separates into all these different wavelengths But when we tried this to see what types of light certain elements give off if their electrons are excited, we find that they only give off certain colors. They don’t give off the entire rainbow of colors.
Emission Spectra - Each discrete line in an emission spectra corresponds to one exact frequency of light emitted by the atom The emission spectrum of each element is like a person’s fingerprint; no two are alike We can identify elements based on their spectral lines Remember how sunlight spreads out into a rainbow, or a continuous spectrum, when you pass it through a prism? Well, when you pass light from a sample of glowing hydrogen through a prism, it doesn't spread out into a continuous spectrum. Instead, it spreads out into a discontinuous spectrum, with only four lines of colored light. A similar thing happens when you pass the light from a sample of glowing neon, or argon, or even sodium through a prism. Instead of getting a continuous spectrum, you get a discontinuous spectrum composed of a series of colored lines. The particular series of colored lines that you get out of any specific element is called the element's atomic spectrum or emission spectrum. Each element has an emission spectrum that is characteristic to that element. In other words, the emission spectrum from sodium is always the same and is different than the emission spectrum from any other element, like calcium or helium, or gold. First, because an element's emission spectrum is characteristic of the element, scientists can often use emission spectra to determine which elements are present or absent in an unknown sample. If the emission spectrum from the sample contains lines of light that correspond to sodium’s emission spectrum, then the sample contains sodium. You may have heard or read about scientists discussing what elements are present in some distant star, and after hearing that, wondered how scientists could know what elements are present in a place no one has ever been. Scientists determine what elements are present in distant stars by analyzing the light that comes from stars and finding the atomic spectrum of elements in that light. If the emission spectrum from the sample contains lines of light that correspond to helium's emission spectrum, then the sample contains helium. Second, and perhaps more importantly, the existence of atomic spectra and the fact that atomic spectra are discontinuous, can tell us a lot about how the atoms of each element are constructed. In general, an element's atomic spectrum results from the interaction between the electrons and protons within an atom of that element. The relationship between atomic spectra and the components of the atom will be the topic of the next lesson.
Absorption vs. Emission spectra An absorption spectrum is produced when the electrons of an atom absorb energy (low high) A bright-line (emission) spectrum is produced when excited electrons fall from a higher to a lower energy level (highlow)
Hydrogen series
Hydrogen Series, cont. Emission or absorption processes in hydrogen give rise to series: sequences of lines corresponding to atomic transitions, each ending or beginning with the same atomic state in hydrogen. Lyman series: involve jumps to or from the ground state, n=1 (UV emission) Balmer series: corresponds to n=2 (visible wavelengths) Paschen series: corresponds to n=3 (IR band)
The Flame Test Electrons absorb heat energy As they “fall” back down to lower energy levels, energy is emitted in the form of light
Why isn’t the planetary model our modern model? Calculations worked great for hydrogen, but poorly for multi-electron systems. Because the calculations for e- orbits did not work for all atoms, it must be that this model is incorrect.