Monday, May 6, 2013

History of Science -- Part Twenty: Anatomy of an Atom

We now understand the three basic particles that make up the atom. There’s the electrons in orbit around a tiny yet massive nucleus made up of protons and neutrons, collectively called nucleons. The electrons each carry a charge of minus one and the protons balance that out with their individual charge of plus one. The Neutrons are electrically neutral and carry no charge.

The number of protons determines the type of element the atom is. For example, one proton is hydrogen and two protons is helium. In the middle of the table of elements is iron with 26 and lead with 82, while the very large atoms such as plutonium have 94 protons.

Typically, the number of electrons matches the number of protons, and the electric charge of the atom is balanced, although there can be local electric effects. It is also possible for an atom to lose or gain an electron or two giving the atom a net electric charge and these are called ions.

There are usually about the same number of neutrons in the nucleus as there are protons, but the number can vary slightly. The overall mass of the atom is given by the number of nucleons since the electrons add very little to the overall mass. Although the mass of an atom does affect the overall weight or interaction with gravity, that is a macro effect depending on the billions and billions of atoms in an object of “human” size.

Besides the atoms and molecules and substances they make up, the weight is also affected by the organization of the atoms and their spacing. More important at the atomic level, mass has an impact on inertia and momentum. Since the heavier elements are fifty to one hundred times as massive as hydrogen, the larger elements such as iron and lead tend to weight more in large quantities than smaller elements such as hydrogen or carbon.

Isotopes are variants of a particular chemical element. While all isotopes of a given element share the same number of protons and electrons, each isotope differs from the others in its number of neutrons. For example there is carbon-12, carbon-13, and carbon-14, each with a different number of neutrons. Isotopes are indicated by the number counting the total protons and neutrons.

Carbon has 6 protons, and it commonly occurs with 6, 7, or even 8 neutrons. These different forms of the element differ greatly by the their total mass since neutrons are just a little more massive than protons. More significantly, these different isotopes vary in their nuclear stability. Some isotopes are stable, while others are prone to spontaneous decay. That’s a great hint of what is holding the nucleus together and this instability is what gives us radioactivity and also nuclear power and bombs.

The size of an atom is difficult to describe because atoms have no definite outer boundary. To overcome this problem, the size of an atom is estimated by describing its radius. In metals, this is done by measuring the distance between two nuclei in the solid state and dividing this distance by 2. For nonmetallic elements, that exist in pure form as molecules, measurements can be made of the distance between nuclei for two atoms covalently bonded together. The size of a typical atom is on the order of 10-8cm. Atoms can be viewed with modern instruments and IBM Research has even produced a recent movie where they created an animated story line by moving and photographing atoms.

The nucleus, on the other hand, is very small and located in the center of the structure. The compact nucleus is between about 2 and 15 x 10-12 cm. That means that the nucleus is between ten to one hundred thousandths the size of the overall atom. Atoms are mostly just empty space with a hard and massive core at the center. If an atom was blown up to the size of a large living room, the nucleus would only be about the size of a spec of dust … mostly empty space for sure.

The size of the actual particles echoes this in reverse with electrons very small while protons and neutrons are much larger particles.

The mass given in the table is the “rest” mass, since relativity states that energy and mass are interchangeable (times c2), so accelerated particles have additional mass. For the same reason, mass of particles is often given in electron-volts which is a measure of energy. (For “atomic mass units” or “amu” we assume that the neutron is exactly 1.)

(Again, although a large collection of atoms such as a rock or a mountain or a planet are affected by gravity, the tiny atoms are not affected by gravity because the electrical force and other forces in the atom are much, much more powerful. You ignore gravity when calculating for atoms.)

Name Symbol Mass (g) Mass (amu) Size
Proton P+ 1.6726231 x 10-24 0.99862349 10-13 cm
Neutron n0 1.6749286 x 10-24 1 10-13 cm
Electron e- 9.1093897 x 10-28 0.000543867 < 10-15

Notice from the table that the mass of the proton is 99.86% the mass of the neutron and the electron is 0.054% the mass of the neutron.

Recall that, before neutrons were discovered, scientists thought that the neutral particles in the nucleus were atoms of hydrogen: a single proton with a single electron in orbit. Notice that you can’t really tell by the mass, although the mass of a neutron is slightly more than the mass of a proton plus the mass of an electron. Before the neutron was discovered, the measurement of these particle masses was not as accurate, so that did not disprove the “nuclear electrons” theory. It was quantum formulas that first indicated electrons can’t be contained in the nucleus, even if they are attracted by the positive charge of the proton. Trapping an electron in such a small space wold require a tremendous amount of energy — much more energy than is in the atom.

As you can calculate from the table, it takes nearly 2,000 electrons to equal the mass of a single nucleon. Also, electrons are very small compared to neutrons and protons. The actual diameter of an electron has never been measured and some consider it may be a mathematical “point,” although that would imply an infinite electric field at that point.

As we will soon learn, the electron is an elementary particle and can’t be broken down, while the neutron and the proton are actually made up of more elementary particles called “quarks.” Both the nucleons are made of a combination of three quarks, but there are different kinds of quarks.

It is possible to make a comparison between our solar system and an atom. You can compare the nucleus to the sun and the electron orbits to the orbits of the planets. However, there are significant differences. For example, the planets in the solar system are primarily in a single plane, while atoms are more “three dimensional.” Also, the electron orbits are not defined by Kepler’s laws, but rather by Bohr’s description using Planck’s constant. Of course, we learned that, in quantum theory, the actual location of the electrons in the orbit is a probabilistic wave that sort of indicates the electrons are simultaneously “everywhere” at once.

But those differences aside, it is true that the atom, like the solar system, is mostly empty space. Although the concentration of mass in an atom is much more intense than the concentration of mass in the sun, you can visualize the large central object of the solar system to be like the nucleus.

So that’s what an atom looks like. I mentioned that modern instruments, analogous to a microscope, have directly observed atoms. However, the “magnification” required to see the nucleus is beyond our current technology. Instead, we “observe” the nucleus and the nucleons using indirect methods. We fire particles at atoms in “atom smashers” called “particle accelerators” and determine facts by observing what particles fly out of the rare collisions. (Since atoms are mostly empty space, we rarely strike the nucleus when we fire particles, so we have to fire a lot in order to get results.)

The energy of the particles being fired determines the resolution or “magnification” capability. Modern accelerators can resolve the nucleus and the protons and neutrons. Getting deeper into the particles and observing the quarks and other related particles is the current state of the art. Our theories extend even deeper to the components of quarks and similar particles which is the realm of modern “string theory,” but we have yet to explore that deep into atoms with our current experimental technology.

Since string theory is the very frontier of science, we won’t address it. We will discuss quarks as we search for the force that holds the nucleus together … next time.

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