Download - Nuclear Changes
7.1 What is Radioactivity?
• Large atoms are unstable.
• When the nucleus is crowded with protons and neutrons, it’s just ”too much.”
• The nucleus begins to emit (shoot out) particles and/or energy.
RadioactivityRadioactivity
Marie (1867-1934) and Pierre Curie (1859-1906)
• isolated polonium and radium from pitchblende
• both elements more radioactive than pure uranium
• discovered that the source of energy (radiation) were the atoms themselves
• nature of radioactivity was still unknown
RadioactivityRadioactivity
Ernest Rutherford (1871-1937)
• studied absorption of 'rays' emitted by uranium-containing minerals
• two types of rays: - and-rays
• -rays are more penetrating than -rays
• - and-rays are not rays at all (like X-rays or light) but streams of particles
RadioactivityRadioactivity
• - and-rays are streams of charged particles:
How can you test if a particle is positively or negatively charged?
RadioactivityRadioactivity
• - and-rays are streams of charged particles:
How about their mass?
• light particles are easier to deflect than heavy ones (pushing a freight train versus a bicycle!)
RadioactivityRadioactivity
Ernest Rutherford (1871-1937)
• -particles behave like electrons, (1 negative charge) - move very fast
• -particles and have 4 times the mass of a hydrogen nucleus and twice the charge (2 positive charges)
-particle = Helium nucleus
(2 protons, 2 neutrons)
RadioactivityRadioactivity
• - and -radiation are made up of particles, -radiation is not!
• -radiation is electromagnetic radiation (just like light and X-rays):
no mass, no charge
RadioactivityRadioactivity
Radioactive decay:
-decay
U 92238
the atomic number counts the number of protons
the mass number counts protons and neutrons
RadioactivityRadioactivity
Radioactive decay:
-decay
U 92238
24
+ Th 90234
• the atomic number decreases by 2 (loss of 2 protons)
•the mass number drops by 4 (loss of a total of 2 protons and 2
neutrons)
RadioactivityRadioactivity
Radioactive decay:
-decay
Ra 88226
24
+ Rn 86222
Rn 86222
24
+ Po 84218
Cm 96245
24
+ Pu 94241
RadioactivityRadioactivity
Radioactive decay:
-decayProton
Neutron
a Neutron may split into a Proton plus an Electron
Electron
RadioactivityRadioactivity
Radioactive decay:
-decayProton
Neutron
Electronthe electron is ejected from the nucleus as -radiation...
...leaving behind a nucleus with an
extra proton
RadioactivityRadioactivity
Radioactive decay:
-decay
Bi 83210
1-0
+ Po 84210
• the atomic number increases by 1 amu (1 more proton)
•the mass number is unchanged(the electron mass in negligible)
RadioactivityRadioactivity
Radioactive decay:
-decay
C 614
1-0
+ N 714
H13
1-0
+ He23
Pb 82214
1-0
+ Bi 83214
Nuclear vs Chemical ReactionNuclear vs Chemical Reaction
Na
NaOH + HCl H2O + NaCl
OH H Cl NaO
HH Cl
*** Not a true representation of this reaction in solution
Chemical Reaction
Nuclear Reaction
212Po 4 + 82Pb2
208
84
*** Not a true representation of the nuclei
The Half-Life (tThe Half-Life (t1/21/2) of a ) of a NuclearNuclear Reaction ReactionHalf-life (t1/2): The time it takes for half of the radioactive nuclei in a sample to decay.
48 radioactiveparticles at t=0
24 radioactiveparticles at t=1(1 half life)
12 radioactiveparticles at t=1(2 half life)
6 radioactiveparticles at t=1(3 half life)
# of radioactivenuclei
The Half-Life (tThe Half-Life (t1/21/2) of a ) of a NuclearNuclear Reaction ReactionHalf-life (t1/2): The time it takes for half of the radioactive nuclei in a sample to decay.
48 radioactiveparticles at t=0
24 radioactiveparticles at t=1(1 half life)
12 radioactiveparticles at t=2(2 half lifes)
6 radioactiveparticles at t=3(3 half lifes)
# of radioactivenuclei
Fraction ofnuclei
48/48 = 1
@ t1/2 = 1
24 = 148 2
@ t1/2 = 2
12 = 1 * 1 = 1 48 2 2 4
@ t1/2 = 3
6 = 1 * 1 * 1 = 1 48 2 2 2 8
The Half-Life (tThe Half-Life (t1/21/2) of a ) of a NuclearNuclear Reaction ReactionHalf-life (t1/2): The time it takes for half of the radioactive nuclei in a sample to decay.
48 radioactiveparticles at t=0
24 radioactiveparticles at t=1(1 half life)
12 radioactiveparticles at t=2(2 half lifes)
6 radioactiveparticles at t=3(3 half lifes)
# of radioactivenuclei
Fraction ofnuclei
48/48 = 1
@ t1/2 = 1
24 = 148 2
@ t1/2 = 2
12 = 1 * 1 = 1 48 2 2 4
@ t1/2 = 3
6 = 1 * 1 * 1 = 1 48 2 2 2 8
General FormulaFraction remaining = 1 2n
where n is the # of half lifes
Radioactive Decay
0
0.2
0.4
0.6
0.8
1
0 2 4 6
# of half lives
Fra
ctio
n r
emai
nin
g
Phenomenon of Radioactivity
Some elements, such as uranium (U) and thorium (Th), are unstable:
They decay spontaneously.
Uranium Nucleus
spontaneously emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).
Uranium - Thorium Decay
U He + Th238
92
4
2
234
90
spontaneous
decay
“parent” “daughter product”
alpha particle =2 protons + 2 neutrons = positively charged ion of Helium
Thorium:90 protons + 144 neutrons
Th + Pa234
90
234
91
Thorium - Protactinium Decay
beta particle
beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron
Protactinium:91 protons + 143 neutrons
Title
beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron
U PbSeries
This process is called radioactive decay, and eventually uranium (parent) decays to lead (daughter product).