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The insides of an atom
Now that we know how all this matter stuff is formed and we have become familiar with electrons, we can get on with taking a look at the inside of atoms. By the turn of the century it was known that the nucleus of an atom contained protons. These are particles about 2000 times heavier than an electron, each of which carries a positive charge that exactly matches (and cancels) the negative charge of an electron. A hydrogen atom is the simplest of all atoms having just one proton and one electron.
A carbon atom has six protons and six electrons. The protons are all bunched together in the nucleus and because they have all have positive charges, there is a tendency for them to fly apart. To help compensate for this "fly apart" tendency, all atoms larger than hydrogen also have neutrons in their nucleus. A neutron comes close to being a proton without its positive charge. Their presence helps to stabilize the nucleus.
Is that all there is to this atom story? It was thought so for a long time, but anomalies arose that could not be explained by current theories. By the 1970's, it had been realized that the proton and neutron were not fundamental particles but rather were composed of particles called quarks which are accompanied by other particles known as "virtuals" that keep popping in and out of existence. In fact, if the total momentum (product of mass and motion) of a proton or a neutron is measured, the quarks account for only about half. Other particles so far identified in protons and neutrons are the gluons and pions. These are known to play a vital role in maintaining nuclear stability.
For whom the bell tolls
The first ringing of the bell that tolled the end of materialism occurred at the turn of the century when Max Planck, a German professor of physics, suggested that electromagnetic radiation comes in indivisible packets called "quanta."
We will meet this and other key concepts in more detail later. But first we need an outline of what scientists have been doing during this century that has so drastically revised the work of earlier times.
The first mental picture of the atom that emerged was that the electrons were embedded in its nucleus like currents in a pudding. The work of Rutherford showed the incorrectness of the picture and the "Bohr" picture of the atom gradually emerged with electrons circulating around the nucleus like planets around the sun. [Neils Bohr was a pioneer of quantum theory.]
Let there be light
When we use a prism to break up sunlight into the colors of the rainbow, we see many dark lines that have been identified as being due to the absorption of specific parcels of light energy by various atoms in the sun itself.
Atoms can emit as well as absorb light. For example, if a hot, glowing iron bar from a blacksmith's forge is looked at through a prism, we see bright bands instead of dark ones.
Eventually it was realized that both kinds of bands were due to electrons absorbing or emitting light in specific energy packets, the "quanta" or quantity of energy required to promote the electron to a higher energy "planetary orbit" or the "quanta" emitted when an electron dropped back to a lower energy level. These packets get the name "photons."
Perpetual motion??
If electrons actually circulated around a nucleus like planets around the sun and if they can lose energy by emitting photons of light, why do they not get closer and closer to the nucleus and finally collapse onto it?
From this vexing question, a new picture of the atom emerged in which electrons behaved more like stationary waves that occupy "shells" centered upon the atomic nucleus, the shells representing the various energy levels an electron can occupy. An electron absorbing a quantum of light energy through collision with a photon of light would be promoted to a higher energy level. It could then re-emit a photon of light in a quantum "jump" that would drop it back to its former level.
Where did that go?
A key point in this new picture is that the electron is either in one allowed orbit or another. It is never found anywhere in between. The energy levels for all orbits are fixed--quantized. How does the electron jump from one level to another without ever being in the space-time in between those levels? Good question.
We have noted above that the electron fits better to the description of a stationary wave in the vicinity of the nucleus than to an object circulating in planetary orbit. So is it a wave or is it a particle? The same question can be asked for the nature of light, the atom, and even larger "particles." In fact it has been shown that even you and I have a wave associated with us.
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