Quantum Theory

Claimed by Chris Cooper

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Current model of the atom.

The Main Idea

Quantum theory is the accepted modern explanation of the observed behaviors of matter based upon atomic energy and particle interactions. After many notable physicists had hypothesized and disproved various theories to describe the structure of the atom, scientists arrived at the Bohr Model, which currently has the most support from other work and theories from quantum mechanics. After the Rutherford's Gold Foil Experiment, the idea came about that the atom actually exists as many particles held together or near each other by electromagnetic force, which is the attraction or repulsion of charged particles, or the strong force, which holds protons and neutrons together at the nucleus of an atom, and that between these particles there is nothing but empty space. Why these particles stay together in certain configurations and their reactions to incidence with energy or other other particles is explained by quantum physics.

History

The theory and all of its applications, much like any other scientific development of the 20th century, comes from contributions of multiple notable scientists over the course of many years. Initially, Newtonian Laws dominated physics, but the atom was represented by the plum pudding model, which developed after the discovery of the electron and the idea that atom must be made from more particles than previously suspected. None of the leading theories at the time, though, could explain electric discharges or the phenomenon of black lines appearing in the spectra from light passed through various materials. Some scientists were unsure of whether electrons existed as particles or if the electrons themselves were the energy and radiation observed from interactions with atoms. One of the earliest elements that lead to the current model was Max Planck's idea that energy could be quantified or defined by smaller units, which he called "quanta". Later, Albert Einstein applied Planck's work to radiation via what is called the photoelectric effect, where he determined that the results of electron particle interaction with incident radiation, not just energy, depended specifically upon the frequency of the radiation. Niels Bohr determined his model of atomic structure in 1913; rejecting that idea that electrons orbiting the nucleus eventually lose energy and fall into the nucleus, he proposed that electrons were held in fixed orbits by electromagnetic forces and that they could shift to other orbits, or other energy levels, by absorption or emission of energy. Werner Heisenberg also suggested that electrons simply could not possibly be defined by an exact location or momentum by physicists, not without applying some radiation incident to the electron and measuring the disturbance of the system in effect- this idea known as the uncertainty principle. All of these ideas come together to form our current understanding of quantum physics, which greatly impacts the practice of modern physics.

Mathematical Application

From the development of the quantum theory, we obtain fundamental equations and others which are very useful in introductory physics problems.

• As Einstein determined, the incident energy that may be absorbed or emitted from electrons (or any particle for this case) depends on the frequency of the radiation:

[math]\displaystyle{ {E} = {hν} }[/math] in units of joules (J)

where Planck's constant (h) = 64985 Joules/Coloumb and ν(nu) is the frequency of the radiation, which is also [math]\displaystyle{ {ν} = {\frac{c}{λ}} }[/math]

• The radius of an electron's orbit may be determined from:

[math]\displaystyle{ {r} = {\frac{Nλ}{2π}} }[/math] or [math]\displaystyle{ {r} = {\frac{Nh}{2π|\vec{p}|}} }[/math] where N is the energy level in which the electron is orbiting and λ is the wavelength

• From the derivation of the orbit's radius, we can find the angular momentum of the electron:

[math]\displaystyle{ {\vec{L}} = {\vec{r}x\vec{p}} = {\frac{Nh}{2π}} }[/math]

• The centripetal force holding the electron in circular motion is the electromagnetic force produce from the positive charges of the protons in the nucleus and negative charges of the electrons:

[math]\displaystyle{ {F_{perpendicular}} = {\frac{mv^2}{r}} }[/math] [math]\displaystyle{ {F_{electromagnetic}} = {\frac{1}{4πε_0}\frac{q_e^2}{r^2}} }[/math]

from this radius of the orbit may also be found with [math]\displaystyle{ {r} = {\frac{N^2h^2}{ke^24π^2m}} }[/math] where [math]\displaystyle{ {k} = {\frac{1}{4πε_0}} }[/math]

• The total energy of an electron, specifically in the case of the hydrogen atom:

[math]\displaystyle{ {E} = {\frac{-13.6}{N^2}} }[/math] in units of electron volts (eV) where [math]\displaystyle{ {1eV} = {1.6x10^{-19} J} }[/math]

• Other energy calculations for an electron orbiting a hydrogen nucleus:

Potential Energy [math]\displaystyle{ {U} = {{-}\frac{1}{4πε_0}\frac{q_e^2}{r}} }[/math] may also be found with [math]\displaystyle{ {U} = {\frac{-27.2}{N^2}} }[/math]

Kinetic Energy [math]\displaystyle{ {K} = {\frac{1}{2}\frac{kq_e^2}{r}} }[/math]

Total Energy [math]\displaystyle{ {E_T} = {{U}+{K}} = {{U} + {\frac{-U}{2}}} = {\frac{U}{2}} }[/math]

Examples

Excitation of Hydrogen's Electron

If energy is imparted on the orbiting electron of a hydrogen atom, the resulting transfer of energy will raise the energy level of the electron. Since the electron's only applying force prior to the incident energy is the electromagnetic force holding it to the atom, its total energy is negative. Adding energy increases the value of its total energy by an amount equal to that energy adsorbed; furthermore, the only amounts of energy that the electron will take in are those exactly equal to the amount required to completely move it one or more energy levels (meaning it cannot orbit between energy levels, as that event is not stable and the particle will shift immediately to change it). Although electrons are known to move up in energy levels (excited states), it will always release the energy almost immediately after in order to transition back down to a lower energy state (the lowest level known as the ground state E1) where the atom will be more stable and balanced. Applying the full energy that binds the electron to the atom will be a resulting level greater than the extent of the nucleus' attractive force, and the electron will be released from orbit, effectively ionizing the atom.

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Adsorption and Emission of Energy by Electrons

Important to note: If another particle such as an electron collides with the electron of our system, then the amount of energy imparted to our system's electron may any amount required to move up by one or more energy level up to a maximum equal to the total kinetic energy of the colliding electron. If our system's electron gains energy from radiation, such as a photon, then the electron will absorb it completely; therefore, this instance may only occur if the total energy of the photon is equal to the amount required to move up by one or more energy levels.

Connectedness

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See also

• Atomic Theory
• Bohr Model
• Electronic Energy Levels and Photons
• Rutherford Experiment and Atomic Collisions

Further reading

• Chabay R., Sherwood B. Matter and Interactions. 4th ed. Hoboken, NJ: Wiley, 2015. Print.

External links

• History and Explanation of Quantum Theory
• Defining "What is quantum theory?"

References

• Chabay R., Sherwood B. Matter and Interactions. 4th ed. Hoboken, NJ: Wiley, 2015. 323-340,445-450. Print.
• "The Fundamental Forces of Nature." Web. Nd. [1]