Principles of Chemistry Molecular Approach 3rd Edition, What is an emission spectrum? Essay answers are limited to about words characters maximum, including spaces.
Why does absorbance increase with concentration? What is happening on the atomic level when a photon of light is absorbed? Use Beer's Law to explain why you should ideally use the same cuvette for all absorbance measurements. Hint: remember path length 6. Determine the value of e including units for the solution. In Niels Bohr proposed the Bohr model of the atom. When the electrons return to lower energy levels, they emit energy in the form of light.
The colour of the light depends on the difference in energy between the two levels. For example, the red, green, and blue lines in the spectrum of hydrogen arise when the electron drops to level 2 from levels 3, 4, and 5. Every element has a different number of electrons and a different set of energy levels.
Thus, each element emits its own set of colours. See, for example, mercury and neon above. Those colours are as distinctive to each element as fingerprints are to people.
Why do elements emit colors when heated? This is the source of light emitted by neon signs and is also the source of light in a fire. The light frequencies emitted by atoms are mixed together by our eyes so that we see a blended color.
Several physicists, including Angstrom in and Balmer in , passed the light from energized atoms through glass prisms in such a way that the light was spread out so they could see the individual frequencies that made up the light. The emission spectrum or atomic spectrum of a chemical element is the unique pattern of light obtained when the element is subjected to heat or electricity.
When hydrogen gas is placed into a tube and electric current passed through it, the color of emitted light is pink. But when the color is spread out, we see that the hydrogen spectrum is composed of four individual frequencies. The pink color of the tube is the result of our eyes blending the four colors.
Every atom has its own characteristic spectrum; no two atomic spectra are alike. The image below shows the emission spectrum of iron. Because each element has a unique emission spectrum, elements can be defined using them. You may have heard or read about scientists discussing what elements are present in the sun or some more distant star, and after hearing that, wondered how scientists could know what elements were 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 exact four lines that compose hydrogen's atomic spectrum are present in the light emitted from the star, that element contains hydrogen.
By , the concept of the atom had evolved from Dalton's indivisible spheres idea, to J. Thomson's plum pudding model, and then to Rutherford's nuclear atom theory. Rutherford, in addition to carrying out the brilliant experiment that demonstrated the presence of the atomic nucleus, also proposed that the electrons circled the nucleus in a planetary type motion. The solar system or planetary model of the atom was attractive to scientists because it was similar to something with which they were already familiar, namely the solar system.
Unfortunately, there was a serious flaw in the planetary model. It was already known that when a charged particle such as an electron moves in a curved path, it gives off some form of light and loses energy in doing so.
This is, after all, how we produce TV signals. If the electron circling the nucleus in an atom loses energy, it would necessarily have to move closer to the nucleus as it loses energy, and would eventually crash into the nucleus. Furthermore, Rutherford's model was unable to describe how electrons give off light forming each element's unique atomic spectrum. These difficulties cast a shadow on the planetary model and indicated that, eventually, it would have to be replaced.
In , the Danish physicist Niels Bohr proposed a model of the electron cloud of an atom in which electrons orbit the nucleus and were able to produce atomic spectra.
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