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ChemistryOnline is another engaging and interactive subject site from the ScholarNET Online Education stable of online learning resources. It covers the major Chemistry topics that are fundamental to every senior Chemistry course - Atomic Structure and Bonding, Aqueous Chemistry, Organic Chemistry, Redox Chemistry as well as Thermochemistry (Energy).
ChemistryOnline has been designed with both the teacher and student in mind, providing a wealth of interesting and interactive materials to help make learning Chemistry a more palatable experience.
In this document:
You should already know:
What the subatomic particles in an atom are (Atoms page).
The way of writing elements with both atomic number and mass number included. 
Introduction
A nuclear transformation is when atoms of one element turn into atoms of a different element. Alchemists used to try to turn lead into gold, not realising that certain naturally occuring elements were already spontaneously turning into other elements - unfortunately, they tended to stop once they'd become lead, and go no further. This page looks at the naturally occuring processes involved in nuclear transformation, beginning with explaining what isotopes are.
Isotopes
All atoms of a particular element have exactly the same number of protons. Always. That's the way you know which element you have. But atoms of the same element do not have to have the same number of neutrons. The name to describe atoms of the same element that have different numbers of neutrons is isotope.
For example, carbon-12 and carbon-14 are both isotopes of carbon. They both have 6 protons, but carbon-12 has 6 neutrons, and carbon-14 has 8 neutrons. The little number in the name of each isotope is actually its mass number - 6 protons and 6 neutrons gives a mass number of 12, but 6 protons and 8 neutrons gives a mass number of 14.
Just as isotopes of an atom have the same number of protons, they also have the same number of electrons. This means that isotopes are the same element as each other (same number of protons), they act and react exactly the same chemically as each other (same number of electrons), the only difference is that they have different masses from each other (different number of neutrons). One isotope will be heavier than another.
Radioactive isotopes are isotopes whose nucleus changes over time. The number of particles in their nucleus changes, and one (or more) particles and some energy gets released from the nucleus in the process. The particles and energy that are emitted from a nucleus are referred to as radiation. The release of the radiation alters the make-up of the nucleus so that the isotope turns into another isotope, sometimes into another isotope of the same element and more commonly into an isotope of another element. This process is called radioactive decay.
A new term you might hear is nuclide. A nuclide is the word for a different isotope of an element. What's the difference? Isotopes have to be isotopes of a particular element, like carbon-12 and carbon-14, whereas nuclides are a general term that can be used to describe any element's isotopes. When you say nuclide, you can be talking about a specific isotope, or a group of isotopes from different elements, or just about isotopes in general. For example, if I wanted to talk about a whole bunch of different atoms that undergo alpha decay, I'd use the word nuclide because I'm interested in them as individual radioactive nuclei in their own right. I'm not interested in them as being part of an isotope family, connected to a particular element. Confused? Don't worry, just think that nuclide is a general word to describe isotopes.
Why?
Why do some nuclei lose particles? Because they aren't stable enough as they are. Think of it like you are carrying a few awkward parcels. You start off balanced okay, but eventually one of them starts to slip out of your grasp and you drop it. These nuclei are a little like that. The combination of protons and neutrons starts off seeming okay, but eventually it's just not stable enough to stay as it is, and a particle is lost out of the nucleus.
For some atoms, just losing one particle is enough to stabilise things right up and no further radioactive decay happens - no further changes. For other atoms, the new combination of protons and neutrons still isn't stable enough and eventually they "drop another parcel" or lose another particle out of the nucleus. Sometimes it takes hundreds of years for the "parcel to drop", and sometimes the nucleus loses a particle after less than a second. It all depends on which element, and which isotope of that element you are dealing with.
Note: The original nuclide is called the parent nuclide (or parent isotope or parent nucleus). The new nuclide it has decayed into is called the daughter nuclide (or daughter isotope or daughter nucleus). Here, radium is the parent nuclide, and its decay produces the daughter nuclide, radon. Then radon is in turn, the parent nuclide to polonium, its daughter isotope.
Some radioactive nuclides are formed from the radioactive decay of a parent nuclide, like radon-222 being formed when its parent nuclide, radium-226, undergoes alpha decay. Other radioactive nuclides, however, are created out of stable nuclides when a high energy particle bangs into the nucleus. The impact of this high energy particle can force one or more of the particles inside to either change or be knocked out of the nucleus, and create a less stable situation in there. Carbon-14 is formed like this, from a stable nitrogen atom that has been hit by a high energy neutron and had one of its protons knocked out.
α-decay
α is the Greek letter called alpha. Alpha decay (α-decay) is when an atom loses an alpha particle from its nucleus. An alpha particle is made up of two protons and two neutrons. It has no electrons associated with it, so in essence an alpha particle is a helium ion, He2+. The alpha particle is released from the nucleus and emitted as radiation.
What is the effect? The mass number decreases by 4 and the atomic number decreases by 2. The nucleus becomes lighter, and becomes a nuclide of a different element, 2 over from the element it was to start with.
β-decay
β is the Greek letter called beta. Beta decay (β-decay) is when one of the neutrons in the nucleus splits into a proton and a high energy electron. The proton stays put in the nucleus, but the high energy electron is released from the nucleus and emitted as radiation.
What is the effect? The mass number doesn't change because the only thing that went elsewhere was an electron, and an electron's mass is too small to notice. The atomic number changes, the appearance of a new proton increases the atomic number by one. The nuclide stays the same mass, but becomes a different element, the next one along on the periodic table.
γ-decay
γ is the Greek letter called gamma. Gamma decay (γ-decay) usually occurs after one of the other types of decay. The change in the nucleus usually ends up with the nucleus being overly energetic, so to settle everything down again, the excess energy gets released. This excess energy released by the nucleus is called gamma radiation or gamma rays, and takes the form of a photon of energy.
What is the effect? Gamma rays are not solid particles, they have no mass and no charge and so gamma decay makes no change to either the mass number or the atomic number of the nuclide. The nuclide just gets rid of its extra energy and gets back to normal.
Comparing the energy of the emissions
Because each different type of radioactive emission is made of different stuff, it shouldn't surprise you to hear that they have different levels of penetration power. In other words, each different type of radiation is able to pass through different substances.
Alpha particles are made of two protons and two neutrons. This makes an alpha particle much bigger (comparatively) to a beta particle, which is only an electron. A beta particle, however, is still larger than a photon of light, which is what a gamma wave is made from. So it shouldn't sound strange that substances that can stop a big alpha particle, can still let a beta particle and gamma ray through, and then some of those substances that stop a beta particle can still let a gamma wave through.
- Alpha particles are stopped by anything not too porous (without particle-sized holes for the alpha particles to get through). Paper will stop them, skin will stop them, but woven cloth would look like a giant rope web, with lots of big gaps to pass through.
- Beta particles will go through skin, paper etc. They are stopped by a sheet of metal foil, like aluminium, as well as dense objects like thick walls.
- Gamma rays go through aluminium foil, no problem. It would take a dense metal like lead to stop gamma radiation, or again a thick wall will do the trick.
- High energy neutrons are a fourth type of radioactivity, given off by fission or fusion reactions. High energy neutrons have so much energy that they rip through most substances, and only thick, dense substances like a thick layer of concrete will be able to slow the neutron down and wear it out (make it lose its energy).
Detecting radioactivity
There are several ways to detect if radiation is being emitted by a substance, but a common detector is the Geiger counter. Its probe end is usually a cylinder containing a gas and two electrodes, with a window made of mica at one end (the mica won't block any alpha or beta particles). Each particle or photon of radiation that gets through the mica window, ionises the gas particles, which creates a tiny bit of current between the electrodes. Each tiny current surge is turned into a signal, which can then be counted, graphed, or turned into a loud click, depending on how you want the information. These surges are often recorded as counts per minute (CPM). The more radiation reaching the Geiger counter, the more counts are recorded per minute. For a Geiger counter that gives off sound, the clicks get more and more frequent as the amount of radiation increases.
It is usually difficult to tell which type of radiation is present, alpha, beta or gamma, because all three types have the same effect on their detection device. So in general, radiation detectors can tell you the amount of radiation present, but not how harmful it is - whether it can be deflected by thin surfaces like paper, or needs a dense metal like lead to stop it.
Writing equations for nuclear transformations
When you are balancing standard chemical equations, you need to make sure that everything on the left is accounted for on the right (and vice versa). It is exactly the same for nuclear transformation equations, the only difference is that there are more things to balance, because you are taking into account the subatomic particles too. Don't worry, this only means that you need the mass numbers and atomic numbers to balance too.
Notice how this equation has been divided up into 4 sections? That is to show you the different areas that need to be balanced for this equation to be correct. The top two sections include the mass numbers for the reactant side ( ) of the equation and the mass numbers for the product side ( ). The bottom half shows the atomic numbers for the reactant side ( ) of the equation and the atomic numbers for the product side ( ). As long as the total of the mass numbers on the left side of the arrow equals the total of the mass numbers on the right, and the atomic numbers on the left equals the atomic numbers on the right (and you have the right elements!), then your equation should be correct.
Nuclear instability
There are some generalisations that can be made about what type of decay an unstable nuclide might naturally go through. Remember these are generalisations, and so you will find examples that don't match - particularly when radioactive decay is triggered by the impact of another high energy particle.
- All elements bigger than bismuth are unstable, and all their isotopes will naturally decay into smaller elements (some quickly, some very slowly). This continues on a stepwise path called a radioactive series until the nuclides eventually become small enough to be stable (usually when they become an isotope of lead).
- Big elements (bigger than neodymium) tend to give off alpha particles.
- Smaller elements are more likely to undergo beta decay.
- Uranium, polonium, astatine, radon, francium, radium, actinium, thorium and protactinium are all naturally occuring radioactive elements that are bigger than bismuth. The other elements above bismuth not in this group are man-made elements (and are obviously unstable elements as well).
Checkpoint quiz
Summary of this page
- Nuclear transformation is the name for atoms of one element changing into atoms of a different element.
- Isotopes of an element have the same number of protons (and electrons) but a different number of neutrons, eg carbon-12 and carbon-14.
- A nuclide is a general term used for isotopes regardless of what element they are eg carbon-14, cobalt-60 and uranium-238.
- Radioactive isotopes are unstable and their nuclei will eventually undergo radioactive decay - give off a radioactive particle and change into a different element. If the new isotope is also unstable, then the nuclei undergo radioactive decay again, until they become a stable nuclide (called a radioactive series).
- The nuclide that undergoes decay is called the parent nuclide, and the new nuclide it becomes because of the decay, is called the daughter nuclide.
- Radioactive nuclides can be formed from a stable atom if its nucleus is hit by a high energy particle (like a neutron) and this causes changes in the nucleus.
- Alpha decay is when an alpha particle is emitted by radioactive decay. An alpha particle is made from 2 protons and 2 neutrons. The new nuclide is lighter by 4 atomic units and 2 atomic numbers smaller than its parent nuclide.
- Beta decay is when a beta particle is emitted by radioactive decay. A beta particle is a high energy electron. It is made when a neutron breaks down into a proton and an electron (the beta particle). The new nuclide has the same mass as before but is 1 atomic unit bigger than its parent nuclide.
- Gamma decay is when a gamma ray is emitted by radioactive decay. A gamma ray is not a solid particle, but a photon of light energy. The nuclide's atomic number and mass number are unchanged, this is just the way a nuclide can release excess energy from the nucleus (often caused by previous alpha or beta decay).
- Alpha particles are stopped by paper or skin (and anything denser). Beta particles pass through these, but are stopped by metal foil eg aluminium (and anything denser). Gamma rays pass through this but are stopped by lead (and anything denser like a thick concrete wall).
- Radiation can be detected by items like a Geiger counter. Geiger counters can only detect the amount of radiation being emitted, not what type of radiation it is (alpha, beta or gamma).
- Equations showing nuclear transformations (radioactive decay) show the mass number and atomic number of all the nuclides and particles involved. The total of all the mass numbers on one side of the arrow must equal the total of all the mass numbers on the other side of the arrow. The same goes for the atomic numbers - each side of the arrow must equal the same number.
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