Download Radioactivity Reading Assignment Name: Chemistry Date: Hour

Radioactivity Reading Assignment
Chemistry
Name: __________________________________
Date: _____________________ Hour: ________
Before you read this article, please answer these first four questions honestly. There aren’t right or
wrong answers to these first four questions. They are your opinion.
What is the first thing you think when you hear the word radioactivity? ___________________________
Is radioactivity good or bad? Give a reason for your answer. ____________________________________
_____________________________________________________________________________________
Is radioactivity natural or human-caused? Give a reason for your answer. __________________________
_____________________________________________________________________________________
Have you ever studied radioactivity? If so, when? ____________________________________________
________________________________________________________________________________ _____
From Types of Radioactivity (http://www.darvill.clara.net/nucrad/types.htm), Natural Types of
Radioactivity (http://www.launc.tased.edu.au/online/sciences/PhysSci/done/nuclear/typesrad/
Types.htm), and Three Types of Radioactive Decay (http://library.thinkquest.org/3471/radiation_types_
body.html)
Radioactivity: What Is It?
All substance are made of atoms. These have electrons (e–) around the outside and a nucleus in
the middle. The nucleus consists of protons (p+) and neutrons (n0) and is extremely small. (Atoms are
almost entirely made of empty space!) In some types of atoms, the nucleus is unstable and will decay
into a more stable atom. This radioactive decay is completely spontaneous. When an unstable nucleus
decays, there are three ways that it can do so. It may give out an alpha particle (we use the symbol á), a
beta particle (symbol â), or a gamma ray (symbol ã). Many radioactive substances emit á particles and â
particles as well as ã rays. In fact, you won't find a pure ã source; anything that gives off ã rays will also
give off á and/or â, too.
As the atomic number (that’s the number of protons) increases, atoms seem to need more
neutrons. Carbon-12 has 6 protons and 6 neutrons, while uranium-238 has 92 protons and 146 neutrons.
Once we get more than 82 protons (that’s lead), the nuclei are no longer stable, and we’re into
radioactive elements. Elements with more than 92 protons are so unstable that they don’t exist naturally
and have to be made by humans in nuclear reactors. An example is americium-241 (95 protons), which
emits alpha particles and is used in smoke detectors. You probably have some of this in your house!
Alpha particles are made of 2 protons and 2 neutrons. This means that they have a charge of +2
(since protons are positively charged) and a mass number of 4 (measured in atomic mass units, where
each proton and neutron is about 1 amu). We can write them as
, or, because they’re the same as a
helium-4 nucleus,
. Alpha particles are relatively slow and heavy. They have a low penetrating
power–you can stop them with just a sheet of paper, skin, or clothing.
Americium-241 (an á-source used in smoke detectors), which has an atomic number of 95 and a
mass number of 241, will decay to neptunium-237 (which has an atomic number of 93 and a mass
number of 237). The equation would look like this:
Alpha-decay occurs in very heavy elements, for example, in uranium and radium. These heavy
elements have too many protons to be stable. They can become more stable by emitting an alpha particle.
Alpha particles have a large charge (+2), so they easily ionize other atoms that they pass. Ionizing atoms
requires energy, so alpha particles lose energy rapidly as they travel. Thus they have a range of only a
few centimeters in air.
Alpha particles are extremely dangerous inside the body but not very dangerous outside as they
cannot penetrate the skin. The speed of á radiation is about 1/10 of the speed of light.
Fill in this chart about alpha radiation:
Name
Symbol
Made of? Caused by?
Charge (+, -, 0)
Stopped by
Beta particles have a charge of –1 and a mass of about 1/2000th of a proton. This means that
beta particles are the same as an electron. We can write them as â– or, because they’re the same as an
electron, e–. They are fast and light. Beta particles have a medium penetrating power–they are stopped
by a sheet of aluminum or plastic. Beta particles ionize atoms that they pass but not as strongly as alpha
particles do.
But wait a minute! If a nucleus contains protons and neutrons, what’s an electron doing coming
out of a nucleus? To answer this, we need to know more about protons and neutrons. Protons and
neutrons are made of combinations of even smaller particles called quarks. Under certain conditions, a
neutron can decay to produce a proton plus an electron. The proton stays in the nucleus, while the
electron flies off at high speed. This means that when a nucleus emits a â-particle, the mass number is
unchanged, but the atomic number increases by 1. This is because a neutron has changed into a proton
(with almost the same mass–we can ignore the tiny mass of the electron) and the number of protons has
gone up. For example, strontium-90 undergoes decay and forms yttrium-90.
Beta decay occurs in very neutron-rich elements, for example, strontium-90 and iodine-130.
These elements are typically created in nuclear reactors. These elements have too few protons and too
many neutrons to be stable. They can thus become more stable by emitting a beta particle. Beta particles
have a charge of -1 and weigh only a tiny fraction of a neutron or proton. As a result, particles interact
less readily with other atoms than alpha particles. Thus beta particles cause less ionization than alphas
and have a longer range, typically a few meters in air. These are moving fast, about 9/10 of the speed of
light, so they can get through skin and have a reasonably long range in air. â radiation is dangerous if
ingested.
Fill in this chart about beta radiation:
Name
Symbol
Made of? Caused by?
Charge (+, -, 0)
Stopped by
Gamma rays are waves, not particles. This means that they have no mass and no charge. So we
sometimes write
. Gamma rays have a high penetrating power–it takes a thick sheet of metal such as
lead or concrete to reduce them significantly. Gamma rays do not directly ionize other atoms, although
they may cause atoms to emit other particles which will then cause ionization.
We don't find pure gamma sources–gamma rays are emitted alongside alpha or beta particles. If
we want a source of pure gamma rays, we can get it by using a substance that emits both beta and
gamma, and simply keep it in an aluminum container that stops the beta particles. Strictly speaking,
gamma emission isn’t radioactive decay because it doesn’t change the state of the nucleus; it just carries
away some energy. Gamma rays (ã) are electromagnetic waves, rather like X-rays and radio waves.
Thus gamma rays have no mass and no charge.
á-particles and â-particles pull electrons off atoms as they pass (we say they ionize the atoms),
but ã rays don’t. This means that they do not lose much energy as they travel, as they do not interact as
much with the matter they pass. Therefore, gamma rays have a high penetrating power and a very long
range.
Useful gamma sources include technetium-99 which is used as a tracer in medicine. This is a
combined beta and gamma source and is chosen because betas are less harmful to the patient than alphas
(less ionization) and because technetium has a short half-life (just over 6 hours), so it decays away
quickly and reduces the dose to the patient.
Compared to light, each bit (photon) of gamma radiation has 1 million times as much energy (or
1000 time more energy than an X-ray photon). Gamma rays travel at the speed of light. They happily
travel through centimeters of lead and travel easily through air. They are a danger to the human body
even when not ingested due to this penetrative ability.
Fill in this chart about gamma radiation:
Name
Symbol
Made of? Caused by?
Charge (+, -, 0)
Stopped by
What is the most interesting or surprising thing you have read in this article? _______________________
__________________________________________________ ___________________________________
____________________________________________________ _________________________________
Radioactivity Worksheet
1.
Name: ________________________
(a) What is an isotope?
(b) How can we tell if an isotope is radioactive?
2.
(a) What is an alpha particle? (symbol and what it is made of)
(b) What is a beta particle? (symbol and what it is made of)
(c) What is a gamma ray? (symbol and what it is made of)
3.
During radioactive decay, which is the parent atom? Which is the daughter atom?
4.
Rate the three major types of radiation (alpha, beta, and gamma) in terms of their
penetrating power (i.e. tell me which penetrates more than which).
5.
What type of shielding is necessary to block each of the three types of radiation? Explain
why we need thicker/more dense shielding for some type of radiation than we do for
other types.
6.
Using the table below, tell me if an isotope with an atomic number of 80 and an atomic
mass of 175 would be stable or unstable. Would this isotope be radioactive?