Emission of Energy by Atoms

Consider the results of the experiment shown on page 328. This experiment is run by dissolving compounds containing the Li ion, the Cu ion, and the Na ion in separate dishes containing methyl alcohol (with a little water added to help dissolve the compounds). The solutions are then set on fire. 

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An excited lithium atom emitting a photon of red light to drop to a lower energy state. 

When salts containing Li and Na+ dissolved in methyl alcohol are set on fire, brilliant colors result Li+, red Cu +, green and Na yellow. 

OBJECTIVE To understand how the emission spectrum of hydrogen demonstrates the quantized nature of energy. 

As we will see in more detail in the next section, the colors of these flames result from atoms in these solutions releasing energy by emitting visible light of specific wavelengths (that is, specific colors). The heat from the flame causes the atoms to absorb energy we say that the atoms become excited. 

Why do we see colors when salt solutions in methanol are burned  

O Copy and label the atom showing the locations O What is electromagnetic radiation Provide three of the electrons, protons, neutrons, and nucleus. examples. 

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This is what accounts for the discrete values of frequency v in emission spectra of atoms. Absorption spectra are correspondingly associated with the annihilation of a photon of the same energy and concomitant excitation of the atom from En to Em Fig. 1.9 is a schematic representation of the processes of absorption and emission of photons by atoms. Absorption and emission processes occur at the same set frequencies, as is shown by the two types of line spectra in Fig. 1.7. 

The selection rules of the new quantum theory allow electric dipole transitions between levels of the same n, but since the probability of spontaneous transition depends on the cube of the frequency, such transitions in hydrogen are exceedingly improbable. Stimulated transitions, on the other hand, may take place under quite small alternating electric fields of the appropriate frequency. Absorption and emission of energy by the atom are equally probable, so that a change in an assembly of atoms may only be detected if the two states between which transitions are taking place are unequally populated at the outset. 

Absorption or emission of energy by an atom occurs when the electron changes from one stationary energy state to another. One photon of energy is absorbed or emitted as a result. 

A third pumping method (Fig. Ic) uses an electrical discharge in a mixture of gases. It relies on electronic excitation of the first component of the gas mixture, so that those atoms are raised to an upper energy level. The two components are chosen so that there can be a resonant transfer of energy by collisions from the upper level of the first component to level 3 of the second component. Because there are no atoms in level 2, this produces a population inversion between level 3 and level 2. After laser emission, the atoms in the second component return to the ground state by collisions. 

One of the most important and exciting advances in modern biochemistry has been the application of spectroscopic methods, which measure the absorption and emission of energy of different frequencies by molecules and atoms. Spectroscopic studies of proteins, nucleic acids, and other biomolecules are providing many new insights into the structure and dynamic processes in these molecules. 

The absorption or emission of radiation by matter involves the exchange of energy and in order to understand the principles of this exchange it is necessary to appreciate the distribution of energy within an atom or molecule. 

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