Introduction to Simple Atomic Structure

To figure how many protons are in He 8 is easy, we just use the atomic number, 2 protons. To figure how many neutrons are in He 8, we would use the following equation: Mass number - atomic number = neutrons (8 - 2 = 6).

The term nuclide generally refers to atoms of different elements. We already know what isotopes are. The term isotope generally refers to atoms of the same element, having the same number of protons. So when we talk of isotopes we are referring to varieties of a specific element, varieties of Carbon for example are isotopes of Carbon. Now when we talk of nuclides, we are referring to varieties of atoms of all types. So we use the word nuclides when referring to varieties of elements.

Below is a small nuclide chart. I am using an old chart (1966) from the Knolls Atomic Power Laboratory to keep the chart small for teaching purposes. There is a more up to date nuclide chart available on the internet from Japan.

Lets look at the chart. The names of the stable atoms are red and the names of all the radioactive elements that breakdown are blue. In each box are two pieces of information. The top symbol indicates the name of the nuclide. The bottom symbol for the stable elements indicate the percent of natural occurrence. If we had a sample of Lithium, we can see that it would be made of 7.42% of Li 6 and 92.58% of Li 7. The bottom symbol for the radioactively unstable elements gives the half-life for that nuclide. Looking at Carbon 15, we can see that it has a half-life of 2.25 seconds. If you had a sample of Carbon 15, it would take 2.25 seconds for half of it to turn to something else. It would be changing right before your eyes!

Notice that the stable elements are organized into a line where there are approximately equal numbers of protons and neutrons. On either side of this line are the unstable nuclides. As a general rule, the half-life gets shorter the further a nuclide is away from the stable group of nuclides. The atoms with the longest half-lives are those nuclides that are next to the stable elements. These atoms are more stable however they still break down.

There are exceptions, looking at the chart, Helium 5 is one of the most glaring exceptions. It has one of the shortest half-life of all the nuclides on the chart, yet it is right next to the stable group of nuclides (He 4 and Li 6) but also there are other Helium isotopes further away from the stable nuclides with longer half-lives.

This chart to the left is like the chart above except that each nuclide is reduced to a dot. No information for each nuclide can be given, we just want to see the pattern or distribution of stable nuclides. So, only the stable nuclides are indicated on this chart.

There are some very interesting things to see and notice in this chart. Only nuclides with certain characteristics are stable. Other nuclides that do not match these characteristics are not stable. Let's note some of these characteristics to try to understand what it is that make these atoms stable.

There are around 2000 known nuclides but only 270 are stable nuclides. Bismuth with 83 protons is the last stable nuclide. All nuclides with more than 83 protons are unstable. Tin with 50 protons has the highest number of stable isotopes. There are 10 isotopes of tin.

You can see that the ratio of neutrons to protons changes as we look at larger and still larger atoms. H 1 has no neutrons at all. Up to about Ca 40 (20 protons) the ratio is about 1 to 1. However even then we can see the line start to shift away from a strict 1 to 1 ratio. Beyond Calcium, the ratio strays away from the 1 to 1 ratio to maybe 1 to 1.4. Bismuth has a ratio of 1 to 1.5.

From the chart we can see that nuclides are more likely to have even numbers of protons and neutrons. It is extremely rare to have odd numbers of both protons and neutrons in a stable nuclide. Also elements that have an odd atomic number usually have only one or two stable isotopes. Elements having an even atomic number have more stable isotopes.

In addition nuclides having 2, 8, 20, 50, 82 protons or neutrons are usually more stable than other nuclides. For example, there are five stable calcium isotopes that have 20 protons. There are only two stable isotopes of potassium having 19 protons, and only one stable scandium nuclide having 21 protons.

We can also see the same kind of phenomena with neutrons. There are four stable nuclides that have 20 neutrons. However there are no stable nuclides that have either 19 or 21 neutrons. Tin (Sn) having 50 protons is even more dramatic with 10 stable isotopes.

So, why are some nuclides stable and others, the majority, are not stable? Why are all the super large nuclides (above 83 protons) radioactive?

Most of us know that opposite charges attract and that like charges repel. From the start it might seem that the nucleus of the atom might have problems with stability since there are all these positively charged protons. What is it that keeps the nucleus from flying apart. Many think that neutrons help to keep the nucleus together by providing a nuclear force that is able to keep the protons and neutrons together in the nucleus. Without the neutrons, the positive charged protons would cause a repulsion force that would result in the nucleus flying apart.

For small nuclides it takes approximately equal numbers of neutrons and protons for it to be stable. Apparently, having too many neutrons is just as unstable as not having enough neutrons. When we look at larger and larger nuclides having more and more positive charge within the nucleus, we see that a higher percentage of neutrons are needed. Finally, there comes a point where no amount of neutrons can make the nuclide stable. There are just too many positive charges.

So the nucleus is held together by very strong forces called nuclear binding energy, however we do not understand the nature of this force. But we do know that enormous amounts of energy are needed to separate the nucleus into separate protons and neutrons.

If you have ever taken a chemistry class in college, you might remember that a major assumption you take is that mass is conserved. Matter is not added or lost, it does not change. This is especially important when you try to balance chemical equations, an exercise called Stoichiometry. Chemistry deals with the interactions of the outside shell of the electron cloud. Electrons have an extremely small mass so the undetectable mass changes that occur in chemical reactions can be ignored. However, when we start looking at nuclear reactions its another matter. Nuclear reactions tend to have changes in mass that are 50,000 times larger than what occurs in chemical reactions.

In the 1930s, it was discovered that the nucleus of an atom is always less than the individual protons and neutrons that make up that nucleus. This mass that is lost is converted into nuclear binding energy that holds the nucleus together. The graphic to the left or above shows the story of the mass that is lost in the nucleus of He 4.

Large amounts of energy are needed to tear the nucleus apart. When this happens, the protons and neutrons retain the mass that they lost when they became part of the atom. So the mass that is lost in the atom is converted to energy to become the binding force that keeps the atom together. We can calculate the amount of energy that is created by measuring how much mass (mass defect) was lost when the atom was formed. We know the conversion: 1 atomic mass unit (amu or u) = 931.5 MeV (million electron-volt), so it is an easy calculation. Look at the graphic to the left or above for the specifics of the calculation. The nuclear binding energy per nucleon (nuclear particle which are protons and neutrons) has a direct relationship with predicting the stability of an atom as you will see in the next introduction page.

On the next web page we will see that some nuclides have a higher nuclear binding energy than others. The higher the nuclear binding energy is, the more stable the nuclide is and will determine if the structure will be stable or will be radioactive. In addition, some nuclides are more likely to undergo a fusion reaction, and others are more likely to undergo a fission reaction.