Download Radioactivity - Science 9

Name: ______________________________ Period: __________ Date: ______________
Radioactivity
RADIOACTIVE:
 An element that gives off nuclear radiation by releasing particles and rays is said to be radioactive.
 Radioactivity cannot be seen.
 However, it can be detected because it:
can alter photographic film
produces an electric charge in the surrounding air
can produce fluorescence (glowing)
 Nuclear radiation damages most cells.
 The source of radiation is the nucleus.
NUCLEAR STABILITY:
 The nucleus contains protons and neutrons. This poses a problem as the positive protons try to repel
each other.
 The nucleus therefore needs a strong force to keep them together and keep itself stable.
 In some atoms, this force is not great enough and the nucleus becomes unstable and will come apart.
 Some atoms are always unstable (polonium, radon, uranium) and other stable atoms have isotope forms
that are unstable.
ISOTOPE: An atom with a different number of neutrons and therefore a different mass. (Number of protons
must remain the same)
Symbols:
Example:
Atom A
Atom B
15
10
# Protons
15
10
# Neutrons
15
15
# Electrons
Mass
a) Are atoms A and B isotopes of the same element? Explain.
Atom C
15
10
15
Atom D
15
15
10
b) Are atoms A and C isotopes of the same element? Explain.
c) Are atoms A and D isotopes of the same element? Explain.
AVERAGE ATOMIC MASS: The weighted average of all naturally occurring forms of the atom and its
isotopes. The number that the mass is closest to therefore is the most abundant form.
Example: A particular atom X, naturally occurs with 4 different masses. 65% of all atom X weigh 10 grams,
10% weigh 9 g, 15% weigh 8 g and 10% weigh 11 g. What is the average atomic mass of atom X that would be
found on the periodic table?
Name: ______________________________ Period: __________ Date: ______________
Example: Copper has two isotopes: Cu-63 and Cu-65. Given the atomic mass of copper from the periodic table
is 63.546 amu, which of the two isotopes is the most abundant (common)? Explain.
RADIOACTIVE DECAY:
 An unstable nucleus will change to become stable. (Do not confuse this with an unstable electron
configuration, which will cause the atom to bond with another, not change its entire nucleus).
 In order to become stable, the nucleus will emit particles or rays and eventually will become lighter and
stable. The particles that it emits include protons.
 When these particles are emitted, the proton number decreases and therefore the atomic number
decreases. With a new atomic number, the type of element changes as well.
 A sample does not decay all at once and there is no way of telling which ones will decay at any given
time.
 Instead, the nuclei decay over time at a fixed rate (for each type of atom), which we call the half-life.
HALF – LIFE:
 The amount of time it takes for one half of the atoms in a sample to become stable.
Example: Draw a bar graph representing the decay of barium-139 if you start with a 20 g sample and it must
decay to less than 2 g. The half life of barium-139 is 80 minutes.
HARNESSING THE NUCLEUS:
 The nucleus contains a huge amount of energy that holds it all together. When this energy is released, a
large amount can be gathered from the tiny mass.
 NUCLEAR FISSION: The splitting of an atomic nucleus into two smaller nuclei of approximately
equal mass. This does not happen spontaneously, but usually happens in a chain reaction where the
splitting of one nucleus triggers several others around it to do the same. Although the energy from one
split is not great, billions of fission reactions can take place each second, which releases huge quantities
of energy. This is how nuclear power plants receive their power.
 NUCLEAR FUSION: The joining of two atomic nuclei of smaller masses to form a single nucleus of a
larger mass. For this to happen, temperatures well over a million degrees Celsius must be reached. This
is how the sun and the stars get their energy. The energy released is much greater than by fission and
there is less radioactive waste, however it is almost impossible to artificially create the extreme
temperatures needed for the reaction to take place.
USES OF RADIOACTIVIY:
 Carbon dating to learn how old something is
 To find leaks or weak spots in metal pipes
 Nuclear medicine: radiotracers to help doctors diagnose health conditions, X-Rays, MRIs, cancer
treatments to kill the cancer causing cells
 Nuclear power
 Nuclear bombs
 Microwaves
Name: ______________________________ Period: __________ Date: ______________
 Radioactive Element: an element that gives off ___________________ radiation
 Radioactive elements have an __________________ ____________________
 A _________________ that is unstable can become stable by undergoing a ____________ reaction or
change which we call ___________________ _____________
 Radioactive Decay: when an atom spontaneously emits _____________________ and
____________________ to become __________________. Since it is emitting protons, the atom loses
protons and therefore changes its ___________ ___________ and becomes an entirely new
________________ in its new stable form.
 The decay occurs over a period of time at a _____________ ________________
 Half Life: the amount of time needed for _________of the atoms in a sample to ____________ to a
____________ form
 Example: 100 grams of sodium-24 with a half life of 15 hours
100g
50g
25g
0 hrs
12.5g
6.25g
15 hrs 30 hrs 45 hrs 60 hrs
 Nuclear Fusion: joining
 The ____________________ of two atomic nuclei of _________________ masses to form a
__________________ nucleus of a larger mass
 Requires extremely high ____________________ (well over a million degrees Celsius)
 Nuclear fusion produces the energy of the ________ and _____________
 The energy released is ______________ than the energy released during a fission reaction and there is
less ___________________ waste
 However, these reactions are more _______________________ to work with
 Nuclear Fission: splitting
 The _______________ of an atomic nucleus into two ________________ nuclei of approximately
________________ mass
 Does not occur spontaneously
 Huge quantities of ________________ are released
 An uncontrolled chain reaction of nuclear fission can create a nuclear _______________ like the
_______________________
Name: ______________________________ Period: __________ Date: ______________
QUESTIONS
1. What is a radioactive atom? What is the source of radioactivity?
2.
Why is a radioactive nucleus unstable?
3.
What is an isotope?
4.
What is a half-life?
5.
Pictorially represent nuclear fission and fusion.
6.
List 5 uses of radioactivity.
7.
What does the 24 represent in sodium-24. Is this an ion, isotope or atom? Explain.
8.
A particular atom Y, naturally occurs with 5 different masses. 55% of all atom Y weigh 5 grams, 20% weigh 6
grams. 15% weigh 4 grams, 7% weigh 7 grams and 3% weigh 8 grams. What is the average atomic mass of atom Y
that would be found on the periodic table?
9.
Draw a bar graph representing the decay of sodium-24 if you begin with a 100-gram sample and it must decay to less
than 5 grams. The half-life of sodium-24 is 15 hours.
10. For an atom of radioactive strontium-90, give the mass, number of protons, neutrons and electrons.
11. There are three isotopes of silicon with the mass numbers 28, 29 and 30 amu. The atomic mass of silicon from the
periodic table is 28.086. Which of the three forms is most abundant? Explain.
12. Complete the mass for each atom in the table below and answer the following questions.
Atom A
Atom B
Atom C
10
10
10
# Protons
11
10
11
# Neutrons
10
10
11
# Electrons
Mass
a) Are atoms A and B isotopes of the same element? Explain.
b) Are atoms A and C isotopes of the same element? Explain.
c) Are atoms A and D isotopes of the same element? Explain.
Atom D
11
10
10