In this case, there is a gravitational force attracting the planet which produces orbital motion. This will work anywhere in the solar system. Early physicist thought of the electron in an atom a lot like a planet orbiting the Sun. The key difference is that the electron in the Bohr model orbits due to an electric interaction and not a gravitational interaction. Well, the other difference in the Bohr model is that the electron can not orbit if it does orbit, which it doesn't at any distance and any energy.
Here is the essence of the Bohr model. The Bohr model depends on a connection between the frequency of light and the energy of the level change. If light of a frequency corresponding to the energy change interacts with the atom, the electron can absorb the light and jump up a level.
If an excited electron jumps down a level, it looses energy. The energy the electron loses becomes light with a frequency corresponding to a the change in energy. The Bohr model can be quite confusing to introductory students, but the important point is that this model agrees with the following evidence. There is a key point about the Bohr model that is no longer accepted in current models of the atom. In the Bohr model, the electrons are still thought to orbit the nucleus just like planets orbit the sun.
Actually, this is something that we can not say is true. The problem with atoms and electrons is that we humans except them to obey the same rules as things like baseballs and planets. Actually, the rules are the same, but baseballs and planets follow the rules of quantum mechanics without us humans even noticing. It turns out that we can't really say anything about the trajectory or position of electrons in an atom.
What we can say is all about probabilities. We can say what regions an electron is likely to be. Here is a diagram that might help. These are probability distributions for the different energy levels in an atom from wikipedia.
I totally forgot that I made a video lecture for this same stuff. If you like to listen and watch instead of read, check this out. View Iframe URL. Instead, Rutherford found that some of the nuclei were deflected at large angles. A few were even deflected back to where they had come from. It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you.
On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive center, carrying a charge.
This problem would be solved by Niels Bohr in discussed in Chapter Brownian motion is named after the British botanist Robert Brown who, in , looked at pollen grains under a microscope and saw that they move around in the water in a similar way to how dust particles move in sunlight. In , Einstein predicted that Brownian motion is caused by molecules of water converting heat into kinetic energy, causing them to move.
The water molecules hit each other as well as the much larger pollen molecules but because the water molecules can't be seen, even under a microscope, it looks like the pollen is moving on its own.
Einstein showed that by studying the pollen grains you could calculate how many water molecules were colliding with them and their speed.
Ernest Solvay was not present when the photo was taken and his portrait was pasted on before the picture was released. Copyright Privacy Disclaimer Search Sitemap. I Pre 20th Century theories 1. Atoms and Waves 2. Reflection, Refraction, and Diffraction 3. Newton's theory of Light 4. Measuring the Speed of Light 5. As expected, most alpha particles went right through the gold foil but to his amazement a few alpha particles rebounded almost directly backwards.
These deflections were not consistent with Thomson's model. Rutherford was forced to discard the Plum Pudding model and reasoned that the only way the alpha particles could be deflected backwards was if most of the mass in an atom was concentrated in a nucleus. His many contributions to the development of atomic physics and quantum mechanics; his personal influence on many students and colleagues; and his personal integrity, especially in the face of Nazi oppression, earned him a prominent place in history.
For decades, many questions had been asked about atomic characteristics. From their sizes to their spectra, much was known about atoms, but little had been explained in terms of the laws of physics. One big puzzle that the planetary-model of atom had was the following.
Because the electron would lose energy, it would gradually spiral inwards, collapsing into the nucleus. This atom model is disastrous, because it predicts that all atoms are unstable. Also, as the electron spirals inward, the emission would gradually increase in frequency as the orbit got smaller and faster. This would produce a continuous smear, in frequency, of electromagnetic radiation.
However, late 19th century experiments with electric discharges have shown that atoms will only emit light that is, electromagnetic radiation at certain discrete frequencies.
To overcome this difficulty, Niels Bohr proposed, in , what is now called the Bohr model of the atom. He suggested that electrons could only have certain classical motions:. The significance of the Bohr model is that the laws of classical mechanics apply to the motion of the electron about the nucleus only when restricted by a quantum rule. Therefore, his atomic model is called a semiclassical model. In previous modules, we have seen puzzles from classical atomic theories e.
Most importantly, classical electrodynamics predicts that an atom described by a classical planetary model would be unstable. To explain the puzzle, Bohr proposed what is now called the Bohr model of the atom in Here, Bohr explained the atomic hydrogen spectrum successfully for the first time by adopting a quantization condition and by introducing the Planck constant in his atomic model.
According to Bohr, electrons can only orbit stably, in certain orbits, at a certain discrete set of distances from the nucleus. Danish Physicist Neils Bohr was clever enough to discover a method of calculating the electron orbital energies in hydrogen.
This was an important first step that has been improved upon, but it is well worth repeating here, as it correctly describes many characteristics of hydrogen. At the time, Bohr himself did not know why angular momentum should be quantized, but using this assumption he was able to calculate the energies in the hydrogen spectrum, something no one else had done at the time.
Below is an energy-level diagram, which is a convenient way to display energy states—the allowed energy levels of the electron as relative to our discussion. Energy is plotted vertically with the lowest or ground state at the bottom and with excited states above. Given the energies of the lines in an atomic spectrum, it is possible although sometimes very difficult to determine the energy levels of an atom. Energy-level diagrams are used for many systems, including molecules and nuclei.
A theory of the atom or any other system must predict its energies based on the physics of the system. Energy-Level Diagram Plot : An energy-level diagram plots energy vertically and is useful in visualizing the energy states of a system and the transitions between them.
Based on his assumptions, Bohr derived several important properties of the hydrogen atom from the classical physics. Apply proper equation to calculate energy levels and the energy of an emitted photon for a hydrogen-like atom. We start by noting the centripetal force causing the electron to follow a circular path is supplied by the Coulomb force. To be more general, we note that this analysis is valid for any single-electron atom. The spectra of hydrogen-like ions are similar to hydrogen, but shifted to higher energy by the greater attractive force between the electron and nucleus.
The tacit assumption here is that the nucleus is more massive than the stationary electron, and the electron orbits about it. This is consistent with the planetary model of the atom.
Equating these:. This means that it takes energy to pull the orbiting electron away from the proton. Using this equation, the energy of a photon emitted by a hydrogen atom is given by the difference of two hydrogen energy levels:. Fig 1 : A schematic of the hydrogen spectrum shows several series named for those who contributed most to their determination. Part of the Balmer series is in the visible spectrum, while the Lyman series is entirely in the UV, and the Paschen series and others are in the IR.
Values of nf and ni are shown for some of the lines. Atomic and molecular emission and absorption spectra have been known for over a century to be discrete or quantized.
Maxwell and others had realized that there must be a connection between the spectrum of an atom and its structure, something like the resonant frequencies of musical instruments.
But, despite years of efforts by many great minds, no one had a workable theory. It was a running joke that any theory of atomic and molecular spectra could be destroyed by throwing a book of data at it, so complex were the spectra. In some cases, it had been possible to devise formulas that described the emission spectra.
As you might expect, the simplest atom—hydrogen, with its single electron—has a relatively simple spectrum. The hydrogen spectrum had been observed in the infrared IR , visible, and ultraviolet UV , and several series of spectral lines had been observed.
The observed hydrogen-spectrum wavelengths can be calculated using the following formula:. These series are named after early researchers who studied them in particular depth. The Paschen series and all the rest are entirely IR. Electron transitions and their resulting wavelengths for hydrogen. While the formula in the wavelengths equation was just a recipe designed to fit data and was not based on physical principles, it did imply a deeper meaning.
Bohr was the first to comprehend the deeper meaning. Again, we see the interplay between experiment and theory in physics. Experimentally, the spectra were well established, an equation was found to fit the experimental data, but the theoretical foundation was missing. The wave-like properties of matter were subsequently confirmed by observations of electron interference when scattered from crystals.
Electrons can exist only in locations where they interfere constructively. How does this affect electrons in atomic orbits? When an electron is bound to an atom, its wavelength must fit into a small space, something like a standing wave on a string. Waves on a String : a Waves on a string have a wavelength related to the length of the string, allowing them to interfere constructively. Allowed orbits are those in which an electron constructively interferes with itself. Not all orbits produce constructive interference and thus only certain orbits are allowed i.
As previously stated, Bohr was forced to hypothesize this rule for allowed orbits. We now realize this as the condition for constructive interference of an electron in a circular orbit.
Accordingly, a new kind of mechanics, quantum mechanics, was proposed in
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