The Structure of Atoms
As we saw in an earlier unit, atoms are mostly empty space with a dense nucleus at the centre and clouds of electrons surrounding. This is shown in the diagram here, which is expanded twice to show the nucleus on the right-hand side -- a nucleus that contains a number of positively charged protons, and a number of uncharged neutrons. One implication of the empty space in atoms is that they can be crushed down to much denser states. We saw this when we introduced white dwarfs, which are a million times as dense as water, but as we'll discuss in an ASTR 102, they are neutron stars, which are a trillion times that density.
In the nuclei of atoms, there are two important players: neutrons, which are neutral; and protons, which are positively charged. These two are just about equally as massive, and individually about 2,000 times as heavy as the electrons that we find distributed around the outskirts of the atom. By the way, the actual structure of neutrons and protons is more complex than that and involves quarks, but we will not explore the physics at that level.
When we consider a particular element, the most important attribute is given by what is called the atomic number symbolized by a capital Z.
This is the number of protons in the nucleus of the atom. For example, hydrogen consists of one proton; helium has two; carbon, six; oxygen, eight; uranium, 92, and so on for all the known elements. Although the atomic number Z tells us about something deep in the nucleus of the atom, it matters chemically because the number of protons determines the number of electrons that will in general be surrounding that atom.
This in turn determines how the atom bonds and behaves with other atoms.
In the diagram at the right here, we see one oxygen combined with two hydrogens to form a molecule of water. The reason hydrogen behaves as hydrogen chemically is because it has that one electron surrounding.
And that reflects the fact that it has one proton in the nucleus. But the atomic number Z does not tell the whole story. A nucleus of an element can also contain neutrons, which means it will be more massive, and yet have the same positive charge and the same number of electrons.
The atomic mass number, then, is given by the sum of the number of protons and the number of neutrons.
Isotopes
Nuclei that differ in the number of neutrons are known as isotopes of one another, because adding the neutrons changes the mass but not the fundamental type of the atom.
Thus, for example, ordinary hydrogen has one proton. Heavy hydrogen, otherwise known as deuterium, has one proton plus one neutron; this is what is found in heavy water. There's another isotope called tritium, in which we have one proton and two neutrons. You might call that “even heavier hydrogen.” Because of the one proton in each case, chemically all three of these behave as hydrogen.

Here in three separate pictures we see three isotopes of hydrogen, which we just discussed; three isotopes of carbon, which has six protons in each case; and two isotopes of uranium, which has 92 protons in each case. How many isotopes might there be? Hydrogen has one proton; could we add arbitrary numbers of neutrons to it? Could there be for example a super-heavy hydrogen with one proton and perhaps as many as 50 neutrons? Conversely, could there be an ultralight uranium that has 92 protons (which means it's uranium) but just five or six neutrons, so it'd be very lightweight?
The answer is evidently no. Nature does not seem to work that way.
Typically, nuclei contain more neutrons than protons, but not usually twice as many. So for example, one of the isotopes of uranium has 92 protons and 146 neutrons, shown by the red arrow. Carbon 14 has six protons and eight neutrons. But you can see that arbitrary of neutrons are apparently not allowed.
Instability and Radioactivity
Interestingly, some isotopes of some nuclei are unstable, especially the heavy ones -- but not only those. Unstable nuclei spontaneously break up, spitting out smaller pieces and releasing energy. And as we'll see, this is important in four ways.
First of all, it's a great tool for age dating, which is the point of the current discussion.
It is also very useful in certain medical applications.
But on the other hand a health hazard in other circumstances.
And finally, as we'll see in a subsequent unit, it's important in determining a planet's internal heat budget.
When this radioactivity takes place, energy is emitted in two forms. It can be emitted in the form of kinetic energy, the energy of motion of moving particles; and in the form of radiant energy, the energy carried by penetrating radiation: gamma rays.
Naturally radioactive material, such as uranium, is used to create electricity, as in the Pickering Power Plant on the shores of Lake
Ontario for example. Radioactive material in the core of the reactor heats water, which is enclosed; but through a series of heat exchangers, it creates steam that drives turbines and produces electricity which flows into the grid. Water from the lake is brought in to cool the system and discharge back out into the lake. But notice that the water that is in contact with the reactor core never has an escape to the outside air. The link at the bottom of the page takes you to a discussion of the behaviour of such reactors and a nice animation.
Radioactivity was discovered serendipitously in the late 1800s and not understood for many years until the development of our modern understanding of the atom. In the early days, it was recognized there were three kinds of radioactivity, but as they were mysterious, they were referred to as alpha, beta and gamma ray s from the first three letters of the Greek alphabet. (Indeed the word “alphabet” comes from the first two of those.)

A bit of experimentation reveals that these radioactive decay products have different penetrating powers. Alpha particles, for example, can be stopped by an ordinary sheet of paper. Beta radiation will pass through the paper but be stopped by your hand. Gamma radiation will pass through the paper, your hand, a pot of water and be stopped finally in concrete blocks. (Neutrons, which are sometimes emitted in radioactive decay, can be stopped in water but are otherwise fairly penetrating.)
To figure out what alpha, beta and gamma rays are, physicists looked at the nucleus that was producing them and then asked what was left over after the emission of the radiation. What's left behind is not the same as it was originally, as the nucleus is transmuted to a new element, or new isotope! This helps us to understand the bits that have come out.
Alpha Particles
Here for example is alpha particle decay. An isotope of uranium, U238, can turn into thorium by giving off an alpha particle. What has the uranium nucleus lost? Notice that it's gone from a mass of 238 to a mass of 234. So it has lost four units of atomic mass . But uranium has
92 protons; Thorium has 90. So the alpha particle has carried away four mass units, two of which are protons, and two of which are neutrons.
The alpha particle then is a helium nucleus.
We see this elsewhere. Here's an example where an isotope of plutonium turns into uranium by giving off an alpha particle. Plutonium had 94 protons; the uranium that's left has only 92. And the alpha particle has carried off two of those protons, and again, two neutrons.
You may remember that when we talked about the formation of the solar system, we indicated that helium would not condense in the inner parts of the solar system. You may have wondered where the helium on earth came from. In fact, most of it is radiogenic -- that is to say, it's formed by the radioactive decay of rocks, and it is trapped underground in the form of helium gas. Most of the helium on earth comes from gas fields in Texas. Here we see a picture of a person floating underneath a host of helium-filled balloons.
Beta Particles
The second form of radioactive decay is through the emission of beta particles. Here we see an example. Thorium, with a mass of 234, turns into protactinium, again with a mass of 234, with the loss of a beta particle. No mass appears to be lost here! -- but protactinium has one extra proton: Thorium has 90; Protactinium has 91. How does this happen? This sounds like magic. Where did the extra proton come from?
The answer comes, interestingly, with a recognition of yet another conservation law, namely that electric charge is conserved.
Within the nucleus of thorium, a brand new proton appeared with a positive charge.
That means there must be some new negative charge to go with it. What happens is that within the nucleus, a neutron has spit out an electron carrying a negative charge, and turned into a proton carrying a positive charge. This is called beta decay . So a neutron in the nucleus gives rise to a proton and an emitted electron, otherwise known as a beta particle. So the emitted beta particles are electrons , but they

are coming out of the nucleus. They are not part of the original cloud of electrons that surrounded the atom.
Gamma Rays
The third product of radioactivity is gamma rays, something we've already encountered -- and they still go by the same name. Gamma rays are light, not visible to the human eye and much higher energy and shorter wavelength than visible light. Gamma rays can cause tissue damage and disrupt DNA in the chromosomes, but they can be used for good purposes. If they're targeted they can be used to kill cancerous tumour cells. If not controlled though, this can be very bad. It can cause leukemia and radiation sickness. Here's a picture of someone undergoing radiation therapy as part of a cancer treatment process.
Decay Chains
We've talked about individual decay steps, but in fact, there can be a whole series of decay steps that take one element to a final stable form. Such a chain of decays is shown in the diagram here at the right where an isotope of uranium decays in various steps. First to thorium, then to protactinium, another isotope of the uranium, another isotope of thorium, radium, radon, and so on, down to, finally, the daughter stable isotope of lead. That's the end of the process. Once the atom has transmuted to that form, it decays no longer.
If you have a collection of say uranium atoms, they don't all decay at once. The decays take place over time, and the radioactive substance only gradually converts to a stable form. If we understand the rules that control this behaviour, we can use it as a clock, rather as an egg timer or an hourglass monitors the passage of time, the depositing of sand from the top half of the clock into the bottom in a slow, steady process. That is the subject of our next discussion.