Electrons and Atoms -- LiveWire Teen Forums

i think one helpful way to explain this might be to look at the historical context of studying the atom. So we will start from Rutherford's model of the atom, which consisted of having protons (and the neutrons were added later after they were discovered by Chadwick) and neutrons in the nucleus, and then electrons orbiting this nucleus.

Now this raised two problems for the science. The first was, if the nucleus had all positive charges, then why don't the protons repel each other like all other positive charges (keep in mind that gravity is completely negligible compared to electromagnetic forces, especially when dealing with such small masses as protons and neutrons)?

The second question requires a little knowledge of electromagnetic waves. If electrons had no charge, but somehow there was an attractive force between the electron and the nucleus then it would be conceivable that electrons orbited the nucleus in a similar manner to the planets orbiting the sun, which is that they have enough velocity to keep the acceleration perpendicular to the direction of motion and thus forming an approximately circular orbit; however, there was a problem with this model, which is that electrons are charged. When you oscillate or move a charged particle, it changes the electric and magnetic fields in space, and thus this change propagates through space as a wave, specifically and electromagnetic wave, which light is a type of. These waves carry energy with them, and thus if an electron is orbiting the nucleus, it would be moving all the time and constantly radiating electromagnetic energy, which would in turn cause it to lose all its energy, slowing it down and causing it to spiral into the nucleus.

These problems existed inherently due to the assumptions of Classical Physics, which most notably includes the Continuum Hypothesis, which is the idea that physical quantities are continuous. Now for the world we deal with on a daily basis, the world of atoms is so small, that we can treat tables, cars, chairs, and even insects and continuous objects, and we have an amazingly strong approximation; however, at the level of an atom, obviously this assumption no longer holds.

The first problem was due to the lack of knowledge about two of the fundamental forces. For Classical Mechanics, everything came down to Electromagnetism or Gravity. Since the resolution of the first problem is not as interesting (as well as quite frankly i still do not know much of the theory behind it) a force later known as the strong force or Strong Nuclear Force was found that exists between the protons and neutrons that is much stronger than electromagnetism, but it decays much faster with distance and thus does not exist beyond the nucleus.

The second problem was studied by looking at a phenomena known as Blackbody Radiation. A Blackbody is called a blackbody because the idea is it absorbs all light that is shone on it and thus appears black when not heated. These objects allow us to observe how atoms absorb and then give off light. The idea is when you heat a blackbody, it gives off light because the atoms in it gain energy and so oscillate back and forth and as stated above, when charged particles oscillate they give off electromagnetic radiation. The original classical physics prediction was that these bodies would give off all kinds of light if heated to the right temperature because energy transmission and the light spectra should be continuous. This turned out to be experimentally wrong, thus a new theory was necessary. This is what was known as the "Ultraviolet Catastrophe". Objects it turns out will not go violet and then ultraviolet, but when heated they stay around red and orange and white.

A scientist name Max Planck then hypothesized that instead, things are discrete or are "quantized", meaning that energy, especially light is carried in packets of energy called "quanta" or nowadays photons and that these come in a fixed energy level. This new hypothesis was able to explain the blackbody experiment as well as another known as the photoelectric effect. Now keep in mind, these quanta are very small and thus as far as we are concerned at the macroscopic level, they are continuous.

Niels Bohr then took this quanta idea to create the model of the atom you are discussing, where the electrons orbit the nucleus in circles, but they cannot collapse into the nucleus, because they can only lose energy if they lose that energy all at once in that fixed amount, and that energy is large for the electrons. While this explanation is nice and still decently intuitive, it is wrong, though on the right track. What i am going to discuss next was partially integrated into Bohr's model of the atom by Bohr, but he did not take it far enough.

You may have noticed that i seem to have said to things in contradiction in what i have written. i said that light is an electromagnetic wave but that it also travels as a photon, thus i have said light acts both as a particle and as a wave. This is known as the Wave-Particle Duality, and a scientist named De-Broigle proposed that this is true of all things and even came up with an approximate equation describing this. If we let h be plancks constant, which is about 6.626*10^-34 Joules*seconds, L be the wavelength, p be the momentum, m be the mass, and v be the velocity, we have that:
L=h/p and since p=mv we have L=h/(mv)
Thus the higher the mass, the smaller the wavelength and if you use this to approximate, you will see that even bacteria have pretty insignificant wavelengths (note the significance should be judged relative to the geometric dimensions of the object). For an electron this is significant, however, and implies that electrons act as waves.

Another thing, known as the Uncertainty Principle was also derived, which stated that the momentum and position of an object could not be known at the same time beyond a certain accuracy and this allows for a wave function or probability distribution to exist for the electron. In other words, what this means is that the electron is not a particle, but is instead a diffuse probability distribution of negative charge that has no position or orbit. In fact, they can be model by a wave equation derived by Schrodinger and thus in quantum mechanics, things become even more complicated than Bohr expected.

Thus the modern view of the electron is that it is a sort of cloud of charge distributed around the nucleus that does not have well defined boundaries and instead is just modeled by a probability function. These are often called orbitals by Chemistry students and if you look up orbitals, you can see some drawings. Note, however, that these drawings show the boundary within which you have a 90% chance of finding the electron, but 10% of the time, the electron will be somewhere else. Some of this may have been confusing, but i hope it helps. If not, please ask about anything that confused you and i will discuss it more.

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