The Bohr model of the atom is often called the planetary model. As shown in Figure 1 for a hydrogen atom, the Bohr model envisions the nucleus of the atom occupying a fixed position at the center of an atomic system with the electron revolving around the nucleus in the same way that a planet revolves around the sun.
18/1 180/1 1,800/1 18,000/1
Exercise 2 We describe the motion of the planets around the fixed position of the sun. Is the sun's position really fixed?
yes no
Exercise 3 What fact allows us to assume that the position of the nucleus is fixed in a hydrogen atom?
gravity attracts the nucleus more than it does the electron the mass of the electron is much smaller than that of the protonthe nucleus is positively charged the electron is too small to locate
Bohr knew that the emission of light was the way the atoms released the energy they had absorbed when the high voltage was applied. He made a connection between the discrete colors emitted and the discrete energy levels that were available to the electron. He postulated that each color corresponded to the transition of an electron from one energy state to another, lower energy state. He indexed the energy states of the electron with the letter n, the value of n being 1 for the state where the electron had the lowest energy (the ground state), 2 for the next higher energy state, 3 for the third electronic energy state, etc. Bohr's index n became what we now refer to as the principal quantum number.
Figure 3 animates the changes that occur during the measurement of the emission spectrum of hydrogen.
Bohr's success in rationalizing the emission spectrum of hydrogen led to the general acceptance of the planetary model of the atom. It has since been shown that this model is overly simplistic. Despite its shortcomings, the simplicity of the Bohr model appeals to organic chemists because it lends itself to a simple pictorial description of molecular structure.
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