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Bohr atom

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Even though it’s now considered obsolete, Niels Bohr took a huge step forward in quantum physics, when in 1913, he proposed his model of the hydrogen atom. Bohr built it on the model that his mentor, Sir Ernest Rutherford, had proposed. In the Rutherford model, electrons orbit the nucleus like planets around the sun.

Rutherford and other physicists knew at the time that the “solar system model” couldn’t, in the end, be correct. However, it was a useful model as a first step. Physicists were well aware of its fatal flaw: According to Maxwell’s laws of electromagnetism, the electrons would radiate electromagnetic energy as they circled the nucleus. They would, in this way, lose energy so fast that they would almost immediately become vulnerable to the attraction of the positive nucleus and fall into it. This contrasts with what happens in reality—electrons don’t fall into the nucleus. Thus, matter sits all around us, very stably.

Bohr improved on the Rutherford model by proposing a new law that applies only to the quantum (subatomic and atomic) world. He proposed that Nature allows electrons to occupy only specific orbits. The distance of each orbit from the nucleus corresponds to the electron’s energy level. The greater the energy, the greater the distance from the nucleus.

Bohr atom, hydrogen, electron circles nucleus
Diagram of the Bohr atom as it applies to hydrogen. Hydrogen is the simplest atom and has only one proton (red) and one electron (blue). In the simplest form of hydrogen, shown here, it has no neutrons. In the Bohr visualization of the atom in 1913, the electron circles the nucleus. The electron is constrained to specific orbits. If it gains energy by, for example, absorbing a photon (green wiggly arrow), it instantaneously jumps up into the higher orbit. If it spits out a photon, it instantaneously falls down into the lower orbit. This model works for hydrogen, but was found not to fit empirical observations of atoms with more than one electron. Animation by Kurzonddddd (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons. https://en.wikipedia.org/wiki/Bohr_model retrieved May 31, 2017

According to this law, electrons do not lose energy as they circle the nucleus but are constrained within whichever orbit they happen to occupy. Only when they are fed energy, as when they absorb a photon, can they jump to an orbit more distant from the nucleus. This is the famous “quantum leap.” The mathematical equations indicate that the jump from one orbit to another is instantaneous; it does not involve crossing the distance in between. The same is true when an electron falls to a closer-in, lower-energy orbit—it must spit out energy, for example, in the form of a photon. Again, it does not travel across the distance to the closer-in orbit; it makes an instantaneous quantum leap. First it is in one orbit; the next moment it’s in another.*

By this law of Nature, applicable only in the quantum world, electrons are kept from spiraling into the nucleus. Bohr’s model of the atom was one of the first steps in the realization that the quantum world operates on different laws from the everyday world that we live in. However, Bohr and other physicists did not yet fully appreciate how different the laws of the two worlds are.

Bohr’s model worked only for hydrogen atoms, not for any other element. It was replaced by two different mathematical models, one by Heisenberg (Matrix Mechanics) and one by Schrodinger (the Schrodinger Wave Equation). The two models appear to be different, but are mathematically equivalent.

These models describe an electron that does not adopt either a position or momentum in the physical universe until the electron is detected—that is, until it interacts with something else in the physical universe. So, until then, it can’t circle the nucleus in a definite path. It’s in a state that we don’t experience in everyday reality—it’s in a probabilistic state of being anywhere and everywhere at once within a certain region around the nucleus. This state is called a superposition. All quantum particles are in superpositions until they interact with another particle and, thus, are “detected.”

Most physicists do not attempt to assign physical universe meaning to the Heisenberg and Schrodinger models of the atom. In other words, while the equations do a good job of describing and predicting the results of experiments, many physicists do not try to come up with physical mechanisms that the equations are describing.

Many physicists would say that the only way to accurately describe the electron prior to detection is that it has a certain energy level. All you can do is write down the amount of energy it has. If you try to describe in words a physical representation of the electron, for example, by saying it’s an “electron cloud” vibrating around the nucleus, you’re really speaking only in analogies.

In this analogy, the region that the “cloud” occupies is called an “orbital.” An orbital is visualized as having a certain shape and size. Depending on the amount of energy that the electron has at the moment, the shape and size of the orbital could be a small sphere, a large sphere, a dumbbell shape, a donut shape, etc. These visualizations, though considered no more than analogies, aid physicists in refining quantum theory and aid students in understanding it.

In my explanation, such as it is, I have relied on the Copenhagen Interpretation, the one developed largely by Bohr, Schrodinger, and Heisenberg in the 1920’s and 1930’s as they continued to develop quantum theory. This is the “Orthodox Interpretation,” but it has been losing ground among physicists in recent years. There are many alternative interpretations of quantum physics.

Another interpretation, the Transactional Interpretation, is somewhat more understandable to me. It was developed by John Cramer in the 1980’s and was refined and described by Ruth Kastner in her book written for lay audiences, Understanding Our Unseen Reality, Solving Quantum Riddles. In this interpretation, quantum behavior occurs in a level of reality below the reality that we can observe. It is quite lawful behavior, but the laws are not those of our everyday world. In this interpretation, it’s possible to assign reality to quantum behavior; we’re not limited to mathematical equations that have no physical correlates. But, it’s not the kind of reality that we’re used to.

*The energy levels of the orbits are called “discrete” or “discontinuous.” A hallmark of the quantum world is that properties of its energy and matter are discrete. They increase or decrease in quanta, that is, discrete, discontinuous lumps.

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