What’s in an atom?
Particle
Where it’s
found
What’s its
charge?
What’s its
mass?
Proton
In nucleus
+1
~ 1 amu
Neutron
In nucleus
0
~ 1 amu
Electron
Flying around
nucleus
-1
~ 1/1840
amu
Basic Terminology
Recall:
Z = atomic number = # of protons
N = number of neutrons
A = mass number = Z + N
Since each proton and neutron weighs roughly 1 amu,
the mass of a nucleus (in amu) is roughly equal to its
mass number.
In the Nucleus
Most of the mass of the atom
VERY small volume
radius ~ 10-15 m
Number of protons determines identity
Number of neutrons in an element can vary
Isotopes: atoms with = protons but different numbers of
neutrons
Average atomic mass = weighted average of the masses of all
naturally-occurring isotopes.
In the Nucleus
Electrostatic forces: protons repel each other
“Strong” nuclear forces: attract protons to
neutrons
The balance between these two forces determines
whether or not a given nucleus will be stable.
Strong force
explanation
More on
strong and
weak forces
Unstable Nuclei
wrong Z/N balance, radioactive decay
i.e. the atom will emit radiation.
Radioactive nuclei spontaneously decay
.
The degree of unbalance determines the type of decay
Radioactive nuclei (parent nuclei) decay into daughter
nuclei.
Types of Radiation
Alpha Radiation
a positive charged particle is released
a helium nucleus
Beta Radiation
a negatively charged particle is released
an electron
Gamma Radiation
no particle is released
high energy waves are released
Alpha ()Decay
The nucleus is too large for the strong force to hold it
together,
an -particle (24He nucleus) is emitted
Daughter nucleus has 2 fewer protons and 2 fewer
neutrons than parent.
Alpha radiation is too weak to penetrate paper or skin
Nuclear equation: 92235U 24He + 90231Th
Beta ()Decay
When a nucleus has too many neutrons,
a -particle ( -10e) is emitted, a neutron in the
nucleus splits into a p+ and an e
The p+ stays in the nucleus.
The e- is ejected and called a -particle.
Daughter nucleus has 1 more proton and 1 fewer
neutron than parent.
Nuclear equation:
614C
--> 714N + -10e
Beta radiation is unable
to penetrate aluminum foil
or wood
Gamma () Radiation
When a nucleus has too much energy, it can give off
very high energy waves of light.
the nucleus is unchanged - it still has the same # of p+ and #
of n0.
Nucleus goes from “excited state” to “ground state,” losing
excess energy.
Gamma rays are often given off
with other types of radioactivity.
Gamma radiation can pass
through people
Radiation Shielding
Electron Capture
When a nucleus has too few neutrons,
An electron falls into the nucleus
unites with a proton to form a neutron.
Electrons cascade in to fill in for the missing electron
Daughter nucleus has 1 more neutron and 1 fewer
proton than parent.
Nuclear equation:
69C
+ -10e --> 59B
Chapter 18
Section 2 Nuclear Change
Gamma Rays Are Also Emitted in Positron Emission
• Some nuclei that have too many protons can become stable
by emitting positrons.
• Positrons are the antiparticles of electrons.
• The process is similar to electron capture in that a proton is
changed into a neutron.
• However, in positron emission, a proton emits a positron.
• When a proton changes into a neutron by emitting a
positron, the mass number stays the same, but the atomic
number decreases by one.
• The positron is the opposite of an electron.
• Unlike a beta particle, a positron seldom makes it into
the surroundings.
• Instead, the positron usually collides with an electron,
its antiparticle.
• Any time a particle collides with its antiparticle, all of the
masses of the two particles are converted entirely into
electromagnetic energy or gamma rays.
• This process is called annihilation of matter.
• The gamma rays from electron-positron annihilation have a
characteristic wavelength.
• Therefore, these rays can be used to identify nuclei that
decay by positron emission.
Chapter 18
Section 2 Nuclear Change
Radioactive Decay, continued
Stabilizing Nuclei by Converting Neutrons into Protons
Radiation - Summary
Problem
Decay
Type
Symbol
Nucleus is
too large
alpha
or 2
Resulting Daughter
4He
Atomic number -2
Mass number -4
Too many
in nucleus
beta
- or -10e
Atomic number +1
Mass number stays
same
Too much
energy
gamma
emission
Same nucleus, just
lower energy
n0
Radiation - Summary
Problem
Decay
Type
Too many
Protons
Positron
emission
Symbol
Resulting Daughter
+ or +10e
Atomic number -1
Mass number stays
same
Stable Nuclei
For small isotopes N ≈ Z
e.g.: 16O is most stable
isotope of O:
Z = N = 8 or N/Z=1
For larger isotopes,
N/Z is between 1 and 1.5
e.g.: 208Pb is most stable
isotope of Pb:
Z = 82; N = 126 or
N/Z=1.5
Practice
Nuclear Equations
Half-Life
amount of time it takes
for half of a given
sample to decay.
Each half-life, half of
the sample decays and
half remains.
Half lives vary from
billionths of a second to
billions of years.
Half-Life: Equation Form
How do we put this into equation form?
Let t1/2 = the half-life of the isotope
Let A0 = the amount you start with
Let A(t) = the amount remaining at time t
Then t/t1/2 = the number of half-lives that have elapsed.
A(t) = A0 (1/2)t/t1/2
Nuclear Reactions - Fission
If nucleus too big - nuclear fission.
“fissions” (breaks up) into several smaller nuclei (and usually
some extra neutrons as well).
not easily predicted.
Often initiated by absorbing a neutron.
Example: 92235U + 01n --> 56141Ba + 3692Kr + 3 01n
Nuclear Energy - Fission
Energy can be harnessed.
235U + 1n --> 141Ba + 92Kr + 3 1n
92
0
56
36
0
The neutrons collide with other atoms of 235U,
split, producing more neutrons . . .
causing a chain reaction.
The energy given off can be harnessed to
produce electricity (or, unfortunately, for more
destructive purposes).
Nuclear Energy - Fission
Nuclear Weapons
Nuclear Waste
This pool at the Areva Nuclear Plant near Cherbourg, France, cools spent nuclear
fuel rods before they are moved underground.
Francois Mori / AP
Nuclear Fusion
The opposite of nuclear fission is fusion, when smaller
nuclei come together to form larger nuclei.
Example: 11H + 13H --> 24He
The fusion of hydrogen to form helium is the source of
energy for the sun and many other stars.
Nuclear Energy - Fusion
Release even more energy
than fission.
emitted by stars (mostly
hydrogen fusing to form helium).
no safe way yet to harness
fusion
takes too much energy to
get started
Energy in Radioactive Decay
Radioactive Decay generally gives off large amounts of
energy.
Where does it come from?
The answer lies in something called “mass defect.”
Mass Defect
Law of Conservation of Mass
Mass cannot be created or destroyed.
Law of Conservation of Energy
Energy cannot be created or destroyed.
But, during nuclear reactions:
Mass can be converted into energy and energy
can be converted into mass.
Mass Defect
During nuclear reaction, some mass is either lost or
gained. This change in mass is called the mass
defect (Dm).
Dm = mass of products - mass of reactants
The relationship between the mass defect and the
amount of energy given off or absorbed (DE) is
DE = Dmc2
where c = the speed of light = 3.0 x 108 m/s.
Mass Defect
DE = Dmc2
Example: If 0.01g of mass is converted into energy, 9 x 1011 J
of energy is given off. That’s enough to heat 2.7 million
liters of water from room temperature to boiling!
By comparison: you would need to burn 16 million grams
of methane to give off the same amount of heat.
Ten most common elements
in the Milky Way
Z
Element
%
Composition
1
Hydrogen
74%
2
Helium
24%
8
Oxygen
1%
6
Carbon
0.4%
10
Neon
0.1%
26 Iron
0.1%
7
Nitrogen
0.09%
14
Silicon
0.06%
12
Magnesiu
m
0.05%
16
Sulfur
0.04%
Stellar Nucleosynthesis – the Stellar Metals
H, He, Li
H is exhausted,
He is burned, fission creates
Li, B, Be, C
carbon burning white dwarfs
more massive stars use nuclear fusion to create
N, O, F, Ar, Na and Mg
most massive stars use nuclear fusion to create
Al, Si, P, S, Cl, Ar, K, Ca, Sc, Ti, V, Cr, Mn, Fe
Stellar Nucleosynthesis – beyond Z=26 . . .
• Most massive stars (12x our sun)
• burn through Mg, Si etc,
• only Fe core remains
• implode from its own gravity
• matter is crushed
• p’s and e’s form n’s
• explode during which
• Fe nuclei gain n’s then decay into protons
• new elements (thru 92) are formed
Common Uses of Radioactivity
Food Irradiation
Archaeological Dating
Medical Detection
Medical Treatment
Daily Exposure to Radiation
Cosmic rays
Radon gas
Smoke detectors
Detecting Radiation
Geiger Counters
(beta)
Scintillation Counters
(alpha, beta and gamma)
Film
(beta, gamma)
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