Nuclear radiation can be incredibly dangerous, but it can also be incredibly useful to us. The Shedding Light on Nuclear Radiation series teaches students what nuclear radiation is and how humans have harnessed its awesome power.
In Shedding Light on Nuclear Radiation Episode 1: Atomic Structure, we lay down the foundations on which the series is built: what atoms are made of and how atoms differ. We look at protons, neutrons, and electrons and at isotopes and nuclides.
A 5-minute excerpt followed by a 1-minute trailer.
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Contents:
Part A: Introduction
Part B: The Building Blocks of Atoms (Protons, Neutrons, and Electrons)
Part C: Isotopes and Nuclides
Transcript (more or less)
Part A: Introduction
In this series, we’re going to look at nuclear radiation. Nuclear radiation is all around us, though in very small quantities. Certain atoms in these rocks are emitting nuclear radiation and the air we breathe also contains tiny amounts of nuclear-radiation-emitting substances. Any substance that emits nuclear radiation is said to be radioactive.
This Geiger counter is detecting tiny tiny amounts of nuclear radiation being emitted by a tiny tiny speck of radioactive cobalt that is housed in the steel ring. As it emits the radiation, the cobalt is slowly turning into nickel. Atoms can literally change into different atoms. The process takes decades for radioactive cobalt, but every radioactive element is different.
Humans have learned to identify and to concentrate naturally occurring radioactive substances so that the radiation can be used in medicine and in industry. We’ve even learned how to actually make atoms that are radioactive, using pre-existing atoms of course. It’s fascinating.
Too much nuclear radiation can be very harmful and deadly because it can upset the chemical reactions that are going on in our bodies, but nuclear radiation can also be very useful. In hospitals, for example, certain substances that give off nuclear radiation are saving people’s lives and radioactive substances are also powering space probes that are exploring distant planets.
The three main types of nuclear radiation are alpha radiation, beta radiation, and gamma radiation but in order to understand what nuclear radiation is, we need to understand basic atomic structure.
So, let’s begin this unit with a quick look at atoms and at the three subatomic particles that atoms are made of.
Part B: The Building Blocks of Atoms
Atoms are made of protons and of neutrons, which form the nucleus of the atom, and of electrons, which surround the nucleus. The plural of nucleus is either nucleuses or nuclei. The protons have a positive charge and the electrons have a negative charge, while the neutrons are neutral, they have zero electrical charge. Since protons and neutrons both occupy the nucleus, they are collectively called nucleons. The strength of the positive charge on all protons is exactly the same and the strength of the negative charge on all electrons is also exactly the same AND the positive charge of each proton is equal to but opposite to the negative charge of each electron. Protons, neutrons, and electrons are collectively called sub-atomic particles.
Now though protons and electrons have the same-sized, but opposite charge, their masses are completely different. The mass of an electron is only about 1/2000th of the mass of a proton, so electrons make up only a very small percentage of the mass of an atom. The mass of a neutron is about the same as the mass of a proton; neutrons are only about 0.1% more massive than protons. The mass therefore of an atom is concentrated in the nucleus. In fact, about 99.98% of an atom’s mass is in the nucleus and most of the atom is actually empty space. Electrons are very light weight, but, just remember they have the same-sized but opposite charge to protons.
Whenever you see an animation or a diagram of an atom, it’s never drawn to scale, because the nucleus has a diameter of only about 1/100,000th of the diameter of the whole atom.
If the nucleus of an atom had a diameter of 1 cm, then the atom itself would have a diameter of about 1 km, about twice the length of the 500-metre span of the Sydney Harbour Bridge. Protons, neutrons and electrons are tiny compared to the space that the whole atom takes up. However, because of the way that the electrons move around the nucleus, atoms are more or less spherical and they can’t just pass through each other.
Now it’s the number of protons in the nucleus of each atom that defines the type of atom that it is. This number is called the atom’s atomic number and it’s typically displayed on Periodic Tables. An “element” is a substance that is made of only one type of atom. Pure aluminium, for example, is made up entirely of aluminium atoms, and every single one of them has 13 protons in its nucleus.
Atoms have the same number of protons and electrons. All hydrogen atoms (which all have an atomic number of 1) have 1 proton and 1 electron, all helium atoms (which all have an atomic number of 2) have 2 protons and 2 electrons, and so on. All copper atoms have 29 protons and 29 electrons.
The fact that protons have a positive charge and that electrons have a negative charge means that there’s a force of attraction between them. Things that have an opposite electrical charge attract one another. This force is called electromagnetism and it keeps the electrons from flying away from the nucleus. Things that have a like electrical charge, such as two positively charged protons, experience a force of repulsion due to electromagnetism.
This electromagnetic force of repulsion is present in the nucleus, but the reason that the protons don’t all separate is that there’s another force present called the strong nuclear force. The strong nuclear force keeps the protons and the neutrons bound together in the nucleus. This force isn’t obvious in our everyday lives because it only operates at the atomic level.
Part C: Isotopes and Nuclides
Now, the number of neutrons in the atoms of a particular element can vary a little. Underneath the dark layer of corrosion, we can see some lithium, a lightweight silvery metal. All lithium atoms have the same number of protons: 3, by definition, but they don’t all have the same number of neutrons. Most have 4 neutrons, but some have only 3 neutrons.
In fact, about 92.5% of all lithium atoms on Earth have 3 protons and 4 neutrons, while only 7.5% have 3 protons and 3 neutrons. They all have 3 electrons of course. These different versions of lithium atoms are called isotopes of lithium. Lithium has 2 naturally occurring isotopes. Other elements can have a different number of isotopes. The combined number of protons and neutrons (that is nucleons) in an atom’s nucleus is called the atom’s mass number. So, the more common isotope of lithium has a mass number of 7, while the less common isotope of lithium has a mass number of 6. As we’ve seen, the atomic number of lithium is 3, of course. The neutrons don’t play much of a role in the way that atoms chemically react, so all lithium atoms chemically react the same way. Here a small sample of lithium is chemically reacting with water. Both isotopes are reacting the same way.
Carbon has three isotopes that exist naturally on Earth. All three of them have an atomic number of 6, which means that they’ve got 6 protons. 99% of carbon atoms have 6 protons and 6 neutrons, a total of 12 nucleons, so this isotope of carbon has a mass number of 12. 1% of carbon atoms have 6 protons and 7 neutrons, a total of 13 nucleons, so this one has a mass number of 13. Something like 1 in every trillion carbon atoms have 6 protons and 8 neutrons, 14 nucleons all together, giving this isotope a mass number of 14. Now coal and wood are made in large part of carbon atoms. When coal or wood burn, the three isotopes of carbon burn in the chemical reaction in exactly the same way because they’re all carbon atoms and it’s the number of protons and the arrangement of electrons that determine how an element chemically reacts.
However, though the number of neutrons a particular isotope of an element has doesn’t really affect the way that it chemically reacts, the number of neutrons it has can change how stable the nucleus is. Too many or too few neutrons can make a nucleus unstable, so that isotope will be radioactive. We’ll be getting into that throughout this series.
Now to express which isotope of an element we’re talking about, we use what’s called atomic notation. The A is the symbol for mass number, which, as I’ve said, is (let me put it in writing again) the combined number of protons and neutrons (that is, nucleons) in an atom’s nucleus. The letter A comes from the German word Atomgewicht, which means atomic weight. The Z (zed or zee if you prefer) is the symbol for the atomic number, which is, again, the number of protons in an atom’s nucleus. Z comes from the German word “zahl” which means number. The X just represents the atomic symbol of the element.
So, the more common isotope of lithium is written as 7
3Li
in atomic notation (and is pronounced as “7 3 Li”). There are 3 protons so the atomic number is 3, and there are a total of 7 nucleons, so the mass number is 7. Obviously 7 – 3 = 4 which is the number of neutrons. Some students, if they’re doing calculations, sometimes forget that the top number in atomic notation is the mass number and they treat the number as the number of neutrons. Try to avoid that mistake. Now the other isotope of lithium is written as 6
3Li
(6 3 Li). Three protons and a total of 6 protons and neutrons. Easy. Now since all lithium atoms have 3 protons, we can also refer to the isotopes as lithium-7 and lithium-6. The fact that it’s lithium, tells you automatically that there are 3 protons involved, but this one has 7 nucleons and this one has 6 nucleons. You can also write Li-7 and Li-6.
Atomic notation in this form is really useful when you learn to write atomic equations in the next episode, but if you just want to tell someone about a particular isotope, it’s easier just to say lithium-7 or lithium-6 or whatever.
The isotopes of carbon that we saw earlier, can be expressed as 12
6C
, 13
6C
, and 14
6C
(12 6 C, 13 6 C and 14 6 C), or as carbon-12, carbon-13, and carbon-14. So, let’s put together the five distinct atoms that we’ve already looked at. The two isotopes of lithium are stable as are the two lighter isotopes of carbon. However, the carbon-14 nucleus is unstable and therefore radioactive. Over time, all carbon-14 atoms emit nuclear radiation and turn into nitrogen atoms. We’ll start getting into the details of nuclear radiation in our next episode. Since carbon-14 is a radioactive isotope of carbon, it’s called a radioisotope. A radioisotope of an element is an isotope of that element that is radioactive.
So here we see three isotopes of carbon and two isotopes of lithium. If we want to talk about all of them, collectively or individually, the word “nuclide” is used. A nuclide is any atom with a unique combination of protons and neutrons. All together, we’re looking at five different nuclides. Carbon-14 is a radionuclide because it’s radioactive.
As an analogy, here we see the two Wright brothers who invented and built the first aeroplane in 1903, and the two Montgolfier brothers who built the first hot-air balloons in the 1780’s. Two brothers and two brothers, but if I said that we’re looking at four brothers, that would be confusing because the two sets of brothers have different mums and dads of course. We can, however, call them inventors or humans or whatever.
Likewise, here we’re not looking at five different isotopes, because if I said that I’d be implying that they’re all isotopes of the same element. Instead, I can say that there are five different nuclides shown here, three of them isotopes of carbon and two of them isotopes of lithium. I can also say, for example, that this nuclide has twice as many nucleons as this nuclide, 12 vs 6. I’ll be using the words nuclide and isotope a lot throughout the rest of the series.
I’ll also be using the words radioisotope and radionuclide of course. These two words are used pretty-much interchangeably by people who work in any industry that involves nuclear radiation.
You can refer to carbon-14 for example, which emits nuclear radiation, either as a radioisotope or as a radionuclide and everybody knows what you’re talking about.
Now many periodic tables don’t just show the atomic number of each element, they also often show what’s called the standard atomic weight of each element, which I’ve shown here in blue. The number expresses how heavy, on average, the atoms of a given element are. Let me give you a very basic rundown.
The mass of a single proton is 1.673 × 10-27 kg while the mass of a single neutron is 1.675 × 10-27 kg. These numbers are so small, scientists decided it would be much easier to express the weight of different atoms on a scale that more closely resembled the numbers of nucleons in those atoms.
Over the years, the system of units for the weights of atoms varied a little, but basically, carbon-12 atoms were given an atomic weight of exactly 12 units, and the weights of all other nuclides on Earth were then expressed in relation to the weight of carbon-12 atoms. (Carbon-13 atoms, I’ll point out, do not have a weight of exactly 13 units. Protons and neutrons have a slightly different mass, so while the addition of a neutron means that carbon-13 atoms have a weight that is very close to 13 units, they do not weigh exactly 13 units.)
The standard atomic weight for the element carbon is 12.01 because it’s an average of the atomic weights of all the carbon atoms on Earth.
As I’ve said, most carbon atoms (99%) have 12 nucleons, but the small number of carbon-13 atoms brings the average weight of all the carbon atoms on Earth up to about 12.01 units which is what we see on the Periodic Table. As for the carbon-14 atoms, they’re so rare, that they don’t really affect the average at all… unless you go out to about 12 decimal places I suppose, which we won’t.
Germanium has 5 naturally occurring isotopes on Earth, each with a different abundance, and if you average out the weights of all the germanium atoms on Earth, you get a standard atomic weight of 72.63, which is somewhere in here.
The standard atomic weight is really important in chemistry and you’ll use it a lot if you study senior chemistry, but it’s a lot less important in nuclear physics, because in nuclear physics, we’re usually interested in the mass and behaviour of individual nuclides.
Now most nuclides have more neutrons that protons. This graph shows the number of neutrons and the number of protons of every stable nuclide.
The red line shows a neutron to proton ratio of 1:1, while the green line shows a neutron to proton ratio of 1.5:1. So, for example, calcium-40 atoms (40
20Ca
), which make up about 97% of all calcium atoms on earth, have the same number of protons and neutrons, 20 of each, for a total of 40 nucleons. They’re the heaviest stable atoms with an equal number of protons and neutrons. All stable atoms heavier than calcium-40 atoms have more neutrons than protons, including the other isotopes of calcium (calcium-42, calcium-43, calcium-44, and calcium-46). The atoms of gold-197 (197
79Au
), the only stable isotope of gold in fact, have 79 protons and 118 neutrons, so there are about 1½ times as many neutrons as there are protons. The neutron to proton ratio in gold atoms is about 1.5:1.
Now this chart shows only the stable isotopes of each element. There are about 250 stable nuclides all together. In addition to these, there are about 80 naturally occurring radionuclides on Earth, and about three thousand artificially produced radionuclides.
If a nucleus has too many or too few neutrons for the number of protons that it has, or if it just has too many nucleons all together, it will be unstable and at some point, it will emit nuclear radiation, which usually involves the atom turning into a different atom.
For example, this table shows the 6 naturally occurring isotopes of platinum that can be found on Earth. With about 34% of all platinum atoms being platinum-195 atoms, this isotope of platinum is the most abundant, while platinum-190 atoms make up only a tiny percentage of all platinum atoms. The 6 isotopes of platinum have the same number of protons, 78, but they have a different number of neutrons. Now all of these ones are stable, but platinum-190 is unstable. Over time, all platinum-190 atoms emit what are called alpha particles, which are a type of nuclear radiation, and they change into osmium atoms.
Uranium has three naturally occurring isotopes on Earth, and none are stable. They all emit alpha particles. So, what is nuclear radiation and what are alpha particles? Well, that what we’re going to look at in our next episode. See you then.
CREDITS:
Written and directed by Spiro Liacos
The IAEA and Health: Nuclear Medicine – Sri Lanka by IAEAvideo. https://youtu.be/UvIhXa0_4N4 Creative Commons License.
Special thanks to the University of Physics School of Physics