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...that from which everything is built
I believe most folks have a general notion that if an object is pulled apart into smaller and smaller pieces, ultimately you get to a piece that can not be broken down by ordinary means, ordinary being short of what we know of as an atom smasher. Whether you knew that or not, that is the case. It is true that the atom is made of smaller pieces called protons, electrons and neutrons and those bits may be assembled from quarks, which as far as I know are fundamental particles. We will not need to go into detail of the internal workings of the parts of an atom but the parts themselves, the protons, electrons and neutrons have different properties meaningful to the nature of the atom.
Protons and neutrons are very tiny heavy particles. They are nearly equal in size and weight. The electron is a much lighter particle. Protons carry a positive electrical charge and like charges repel so they repel each other with a force that increases by a factor of four with every halving of their separation. Neutrons being electrically neutral are not subject to the repulsive force but along with protons contribute to an extremely powerful nuclear attractive force. When protons are forced very close together in the presence of some neutrons, the strong nuclear force overwhelms repulsive force and binds the protons and neutrons together to form the nucleus of an atom.
The electrons with a negative electrical charge repel one another and are attracted to protons by the same electrical force that causes the repulsion between protons. When the nucleus we have constructed encounters some electrons, the attraction between the protons in the nucleus and the electrons draws the electrons in toward the nucleus. With what we have learned to this point we would expect the electrons to go all the way into the nucleus but there is another effect in play here.
Quantum mechanics is the science that best describes the behavior of very small objects, about the size of a large of atom or smaller. The details of quantum mechanics are not accessible without a lot of study but the rules of behavior that fall out of those details are not beyond our understanding. I will invoke those rules as they apply to our subjects. In this case quantum mechanics gets us out of the collapse of the atom predicted by the force between protons and electrons. Quantum mechanics, by managing the probability of events for very small objects, makes an electron falling all the way to the nucleus so unlikely that we may neglect that possibility in our explanation of the atom. In fact quantum mechanical effects arrange the electrons in an atom into neat shells surrounding the nucleus.
Let’s take as an example oxygen atom, one of the most common on planet Earth.
Two things to notice about the image above. First the electrons occupy shells at different distances from the nucleus where the protons and neutrons lie. This is a further consequence of those pesky quantum mechanics effects mentioned earlier. Second, the scale of the drawing does not reflect the scale of things in the atom at all. One of my correspondents asked the following question:
Question:
My friend calls me an airhead. I think that peoples skulls are filled with brains. Am I right?
Answer:
Actually it is worse than your friend claims. Your head...and your friend's and mine is almost entirely filled with empty space - perfect vacuum, no air, nothing.
Consider the inside of a person's head. The matter filling the cranial cavity is made up of atoms, mostly of hydrogen, oxygen, carbon and a few other elements. Each of the atoms enclosed by the skull has more than 1999/2000 of its mass in the atomic nucleus. The rest of it is in the electrons circulating around the outside of the atom. Between the nucleus and the electrons is nothing.
A typical nucleus is only about 1e-5 the diameter of its atom, which means if the nucleus was the size of a beach ball, located in Melbourne Beach, the electron shells would contain both Cape Canaveral and Sebastian Inlet. Since the volume of a thing is proportional to the cube of its dimensions, the volume of an atom's nucleus is about 1e-15th the volume of the atom. Making allowance for the space between the outside electronic shells of the atoms, also empty, the volume filled with matter is less than 1e-15 of the total volume enclosed by the skull. Clearly "air head" is excessively generous.
Considering the preceding paragraphs a person might wonder how a bat can hit a baseball when the mass is essentially all in the nuclei and the probability of a bat nucleus finding a ball nucleus must be very small indeed. Answering that question will take us a long way in justifying some of the simplifications we will use in this practical look at how the universe works. For now we will continue in the next sections by thinking about how atoms form the objects that we see in the world around us.
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