AT_Bohr's Atomic Theory Lesson

Bohr's Atomic Theory

The failures of Dalton's atomic theory and some other discoveries (that will be mentioned below) opened the door to the development of another atomic theory. Before we learn about Bohr's atomic theory, we need some background information.

Electromagnetic Spectrum

When atoms interact with each other, it is the electrons that are in contact with each other. The nuclei of the atoms do not come into contact with nuclei of other atoms. The electrons can gain energy and become excited. When they are excited, light is emitted. This light is electromagnetic energy. This electromagnetic energy is carried through matter or space through electromagnetic radiation, which is commonly known as a light wave. The speed of a wave is found by the wave equation.

Watch the following video on waves.

Electromagnetic energy exists in a wide range of frequencies called the electromagnetic spectrum. These range from radio waves to gamma rays. In the middle of this spectrum are the different colors of visible light, known as the visible spectrum.   It represents a very small portion of the entire spectrum that is visible to the human eye. The diagram below shows a representation of the electromagnetic spectrum.

1. What do you notice about the relationship between wavelength and frequency? 
   • As wave length decreases, frequency increases
2. What kind of mathematical relationship does this show? 
   • Wavelength is inversely proportional to frequency

Quantum Theory

In 1900, a German physicist named Max Planck (shown below) proposed the radical idea that electromagnetic radiation is emitted, not in waves, but in tiny packet of energy, called quanta. We now call them photons. Planck proposed that the energy associated with this photon was proportional to its frequency. This is known as the quantum theory.

What this means is that the frequency of light is what is related to the energy, not the intensity or amount of light as one might expect.

An analogy would be getting candy from a vending machine that only recognizes dollar bills. Let's say that a candy bar costs a dollar. If you put in one dollar, you get out one candy bar. If you put in two dollars, you don't expect to get a larger candy bar; you expect to get two bars. But what if you put in one quarter? Do you expect to get ¼ of a candy bar? No. You get nothing. What if you put in four quarters, to equal one dollar? You still get nothing. Remember that this candy bar only recognizes dollar bills. You can put in quarters all day, but you still won't get any candy. Getting a candy bar requires a specific exchange. The same is true for light energy. If a certain reaction requires a specific frequency of light in order to take place, only that frequency will work. You can expose the material to a large intensity of another frequency of light, but it will not work.

Planck's idea that energy is quantized seemed to correspond to another interesting phenomenon. Think back to the continuous spectrum discussed above. A very different kind of spectrum is seen when electricity is passed through a gas, such as hydrogen. Elements are able to give off an electromagnetic spectrum that is not continuous. Electrons of an element are excited by adding an energy source, such as electricity. Those electrons jump to a higher energy level. When they fall back down to their ground state energy level, the atoms emit the absorbed energy in the form of light. Only a select few colors are visible. This is known as an atomic spectrum. Each element has a unique atomic spectrum, and elements can be identified through their atomic spectra.

The first success in explaining atomic spectra quantitatively was made with an equation from J. J. Balmer.   This equation calculated the wavelengths of the lines of the atomic spectrum of hydrogen. This equation was expanded and became the Rydberg equation.

Putting the Pieces Together

Through atomic spectra, we know that electrons gain and lose energy in very specific amounts. These energy amounts are related to energy levels. An electron can only gain or lose energy in quantized amounts. This is a very important concept in the development of the atomic theory.  In 1913, A Danish physicist, Niels Bohr (photo at right), created a model of the atom that had the nucleus at the center and electrons orbiting around the nucleus, similarly to the planets orbiting the sun. For this reason, the Bohr atomic theory is known as the planetary model of the atom. The electrons were on fixed orbits in the Bohr model. Bohr assigned quantum numbers to each electron, which defined the properties of that electron. Bohr was able to use the Rydberg equation successfully for hydrogen, but it fell apart when more complicated elements were used.

Bohr's model of the atom had both successes (earning him the Nobel Prize for his work) and failures. Bohr was able to account for the atomic spectra of hydrogen and the Rydberg equation. But, his calculations did not work for any other element. Another theory was needed. However, the concepts of quantum numbers and fixed energy levels were extremely important in the development of the modern theory of the atom.

Remember to work on the module practice problems as you complete each section of content.  

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