What is the nuclues?

Answer 1

Any atom is mostly empty space. However, it contains one or more electrons and a nucleus, each of which is much smaller than the atom. Typically, each atom is about

10-10 meters across, which is scientific notation for 0.0000000001 meters. If you divide a meter into 1000 equal pieces, each one is a millimeter. If you divide a millimeter into 1000 equal pieces, each one is a micrometer (just a little larger than a wavelength of visible light). If you divide each micrometer into 1000 equal pieces, each piece is a nanometer. A nanometer is still 10 times the diameter of a hydrogen atom, which has a diameter of about 10-10 meters.

However, the nucleus is at least 10,000 times smaller than the atom, and some of them are closer to 100,000 times smaller. The electron is even smaller than that. So the mass of the atom only takes up a very small volume of it.

Almost all of that mass is in the nucleus, which consists of a mixture of protons and neutrons. The proton has nearly the same mass as the neutron, and each of these has about 1830 times the mass of the electron.

Each electron contains one negative elementary charge (sometimes called -e). Although the electron is small and light, its size is sort of a funny concept in quantum theory. They are, in fact, impossible to locate precisely; therefore they seem to occupy relatively large regions of space in the vicinity of the nucleus. In this context, "large" means a large chunk of the atom - still pretty small in human terms. In some ways each electron acts like a cloud of charge, which can surround or nearly surround the nucleus. Chemistry books usually contain detailed pictures of these "clouds". They sometimes look spherical, sometimes like a lot of intersecting figure eights, and sometimes have even stranger shapes.

At the center of this cloud resides the nucleus with its protons and neutrons. The position of a proton or a neutron can be found to a higher degree of accuracy than that of an electron, something that is related to the greater mass of the proton and the neutron. These positions still cannot be determined exactly, though, which is a fundamental part of quantum theory.

These are protons, each with a charge of +e

These are neutrons with no net charge

Therefore the popular picture of an atom as a small solar system with electrons moving smoothly around the nucleus like little planets is wrong. The "cloud" idea is better. The picture shown here of a nucleus full of protons and neutrons is less inaccurate. In many ways a nucleus acts like a little liquid drop composed of individual particles sliding around over each other like molecules in a drop of water.

The nucleus shown here is a helium nucleus with two protons and two neutrons. Each proton has one positive elementary charge, often called +e, equal and opposite to the charge on the electron. It is called helium 4 because of the total of 4 particles.

Both the proton and the neutron are made of smaller parts each of which is called a quark. There are six kinds of quark, but we only need to know about two of them to describe protons and neutrons. One of these is called the "up" quark, and the other is, of course, the "down" quark.

This is an up quark with a charge of 2/3 times the proton charge of e.

We will represent the up quark in this way:

This is a down quark with a negative charge of -1/3 times the proton charge of e.

We will represent the down quark this way:

So here is a picture of the "works" inside of a proton:

It consists of two up quarks and a down quark.

The total charge is 2/3 + 2/3 - 1/3 = 1 proton charge.

Here is a picture of the "works" inside of a neutron.

It consists of one up quark and two down quarks.

The total charge is 2/3 - 1/3 - 1/3 = 0.

It would be possible to turn a neutron into a proton if there were some way of turning a down quark into an up quark. Likewise it would be possible to turn a proton into a neutron if an up quark could turn into a down quark. Both of these processes sometimes actually happen, as we will see below.

Hydrogen 1 consisting of just a proton and represented

1

by 1H.

The reason why we know the nucleus pictured above is helium is that it has two protons. Each chemical element has its own number of protons. For example, any atom with just one proton in the nucleus is hydrogen. Here is the simplest form of hydrogen, called hydrogen 1. It is called that, of course, because there is just one particle, a proton, in the nucleus.

Hydrogen 2 consisting of one proton and one neutron and represented

2

by 1H.

Here is another form of hydrogen. It is called hydrogen 2 because it has two particles in the nucleus, one proton and one neutron. This form of hydrogen is also called deuterium.

There is one other form of hydrogen, called hydrogen 3. It is also called tritium. The three particles in the nucleus are one proton and two neutrons. All are hydrogen because of the one proton in each nucleus. We have called them different forms of hydrogen, but the proper term is "isotope". Hydrogen 1, hydrogen 2, and hydrogen 3 are called the three isotopes of hydrogen. Different isotopes have the same number of protons but different numbers of neutrons.

Hydrogen 3 with one proton and two neutrons and represented by

3

1H.

It is usually difficult to keep drawing the different nuclei with all of the circles and plus signs and so forth. So there is a shorthand notation to describe the numbers of protons and neutrons in a nucleus as well as its total mass and the chemical element that it represents. For the three hydrogen isotopes described above they are shown along with the pictures of the nuclei.

4

Here is another example: 2He represents helium 4. The lower number is called the atomic number, and it represents the number of protons. The symbol He is the standard chemical symbol for helium, which is appropriate because, as mentioned, anything with 2 protons is helium. The upper number is called the mass number, and it represents the total number of protons and neutrons together. You can always find the number of neutrons by subtracting the upper number minus the lower number.

3

The symbol 2He represents another isotope of helium, helium 3. There are two protons and the total of the neutrons plus the protons comes to 3. The number of neutrons is the upper number minus the lower number, or 3-2, which is one neutron.

12

The most common carbon isotope by far is 6C, which is called carbon 12. It has 6 protons and 12-6=6 neutrons.

14

Another isotope of carbon is carbon 14 symbolized by 6C. It is the one used for carbon dating of archeological objects and it is radioactive. Anything with 6 protons is carbon. This isotope has 14-6=8 neutrons.

238 235

Uranium has 92 protons. Here are two of its isotopes: 92U and 92U. Respectively, they have 238-92=146 neutrons and 235-92=143 neutrons. The lighter of these, uranium 235, is the one that can be "split" in such a way that it can liberate large amounts of energy in nuclear explosions. Another pattern of "splitting" this isotope can liberate energy in a slower, more controlled way in a nuclear reactor. The name for this splitting process is nuclear fission. By far most of the natural uranium is uranium 238 with less than one percent uranium 235 mixed in. The two isotopes must be partially separated, a difficult process known as enrichment, before a process involving fission can become practical.

Some of these nuclei are stable and some are not. All of them are full of positive charge, which would tend to blow them apart. However there is another nuclear force, called the strong nuclear force or - often - the strong interaction that can make protons and neutrons attract one another. This force acts only over very short distances within a nucleus, and because of it protons can attract protons, neutrons can attract neutrons, and these two particles can attract one another. This is enough to hold stable nuclei together.

It can also hold other nuclei together temporarily. However, many of these nuclei cannot last forever; they are unstable and will eventually break apart into two or more pieces. There is no way to predict precisely when an unstable nucleus will break apart, because this process - like all quantum mechanical processes - is governed by probability. If you had a very large number of unstable nuclei, you could use probability laws to predict fairly accurately how many would break down in each second, but you could not say specifically which ones would decay. The word "decay" by the way is usually used to describe this process of "breaking apart". The word "disintegrate" is also often used.

Each type of unstable nucleus has its own standard method of disintegrating. Sometimes there are two methods of decay that compete with one another. The common unstable nuclei have been investigated, and their decay modes have been cataloged.

For example, Hydrogen 3 (also called tritium) can decay into helium 3. In other words,

one of these:

Can become one of these:

Look at these pictures, and you will notice that a neutron must turn into a proton for this to work. This can happen if the neutron suddenly disintegrates into a proton and an electron, which at least means that the total amount of charge comes out right. It is known that electric charge cannot be created out of nothing, and it cannot disappear into nothing. If a negative charge comes into being, then a balancing positive charge must also be created. That is what happens in the process being discussed; the neutron disappears and in its place an electron and a proton come into existence:

+

+

"neutron" becomes "electron" + "proton" + "antineutrino"

The antineutrino is not something that we need to discuss for our present purposes, so we will not worry about it except to say that one is produced in this reaction.

Inside the neutron a down quark changed into an up quark by emitting an electron and the antineutrino. However, talking about quarks is not going to help what we want to discuss, so from now on we are going to leave them alone as well.

The electron is much lighter than the proton, so it flies out of the nucleus at a high rate of speed while the proton stays behind. So the effect of the process is that the nucleus loses a neutron and gains a proton. The high-speed electron that is ejected is called a "beta particle" or a "beta ray". This name was given to it before its nature was understood. If there were a sample of tritium consisting of a very large number of atoms (most samples of anything have at least ten to the twenty-something atoms), then each second a certain number of tritium nuclei decay in this manner ejecting electrons. No one can say in advance, however, exactly which tritium atoms will do this. So the tritium gradually changes into helium 3, and there is a steady stream of beta rays coming out of it. In other words, it is radioactive because beta rays are a form of radioactivity. The beta particles can damage organic cells when they pass through, so such a sample of tritium might be dangerous, depending on how much of it is present and how long you are exposed to it. The antineutrinos are not dangerous; they pass right through ordinary matter without damaging anything. That is why we do not need to say much about them.

This example of "beta decay" is normally expressed in symbols as follows:

3 3

1H 2He + e- + n

The symbol at the end, n, is the standard symbol for the antineutrino. The symbol e- stands for an electron, which when emitted from a nucleus in this way is also called a beta particle.

Potassium 42 is another example of an isotope that decays by beta decay. About 93% of naturally occurring potassium is potassium 39, which is stable. But potassium 42 is unstable and therefore radioactive. In beta decay, a neutron changes into a proton. Therefore the potassium nucleus, which has 19 protons, will change into something with 20 protons. Anything with 20 protons is calcium. On the other hand, the total number of particles, 42, will stay the same, because a proton appears in the nucleus to balance the neutron that is lost. In symbols, this is

42 42

19K 20Ca + e- + n

Another type of decay is alpha decay. In this case, a relatively heavy nucleus emits a combination of two protons and two neutrons. Such a combination is a particularly stable one, and it is called an alpha particle. It got this name before anyone realized what it was. Of course, it is also a helium 4 nucleus, as discussed above in this article. Anything with two protons is helium.

There are many examples of nuclei that emit an alpha particle. One such example is radon 222. Radon is a gas under ordinary temperatures and pressures, and there have been many recent news stories about how it can sometimes seep through the soil and build up in houses. Since it is radioactive, this might be a problem. Radon has 86 protons, and the alpha particle that it emits carries away takes two of them. The nucleus left over therefore has 84 protons, which makes it Polonium. Since the alpha particle also contains two neutrons, then a total of four particles are carried away. So the Polonium nucleus that is created has 218 particles in it. In symbols:

222 4 218

86Rn 2He + 84Po

The helium nucleus only has 4/218 of the mass of the other particle, so it is emitted at a higher speed. With a large sample of radon 222, there would be a fairly steady stream of these helium nuclei, or alpha particles, coming out of it. Each second some of the radon nuclei would decay, but as before there would be no way to predict which ones. Alpha particles are easily absorbed by paper, skin, or lung linings, depending on whatever they hit first. However, they can do damage to the substance absorbing them; it is not a good idea to leave an alpha emitter next to your skin for a long period of time or to inhale it.

The polonium 218 created by this process is also an alpha emitter, and, like all alpha emitters, it loses two protons and a total of 4 particles when it decays. That means the residual nucleus will have 82 protons. Anything with 82 protons is lead. The lead nucleus that is created would have 218 - 4 = 214 total particles, so it is lead 214. In symbols, this is

218 4 214

84Po 2He + 82Pb

The other member of the "big three" modes of nuclear decay is gamma decay, which produces a gamma ray. Again, this was named before anyone knew just what it was. As discussed in the previous article on waves, a gamma ray is an example of electromagnetic radiation of a very short wavelength. It is similar to an x-ray, but on the average gamma rays have shorter wavelengths. The shortest wavelengths also have the highest frequencies and the highest energies. So gamma rays can hit hard and they are good at penetrating into matter. They do damage to whatever matter they penetrate and are therefore dangerous.

Gamma rays do not carry any protons or neutrons away from the emitting nucleus; they are basically just electromagnetic energy. So they do carry energy away from the emitting nucleus. The result of this is a nucleus of the same type and nearly the same mass as it had before emitting the gamma ray. But the resulting nucleus also has a lower energy.

Gamma rays are emitted by nuclei that have a certain amount of "excess" energy, some extra "jiggling", so to speak, of the protons and neutrons in the nucleus. A nucleus in such a state of excess energy is said to be in an "excited state". The nucleus shakes off the excess energy by emitting the gamma ray.

Usually, a nucleus acquires this extra energy as a result of some other nuclear process such as a beta decay. An example of this is Cobalt 60, which can emit a beta ray and end up in an excited state. It shakes off the extra energy by emitting one or two gamma rays. So cobalt 60 is known as a gamma emitter.

Answer 2

The nucleus is the brain of the cell, it tells the cell where to go and what job it has to do.