Unit 2 Notes – Radioactivity Mr Nelson Facts about the nucleus Radius = 10 -13 cm (0.0000000000001...
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Transcript of Unit 2 Notes – Radioactivity Mr Nelson Facts about the nucleus Radius = 10 -13 cm (0.0000000000001...
Unit 2 Notes – Radioactivity
Mr Nelson
Facts about the nucleus
Radius = 10-13cm (0.0000000000001 cm) Radius = 1/100000th of an atom
Density = 1.6x1014g/cm3
That’s 160000000000000 g/cm3
A sphere the size of a ping pong ball = 2.5 billion tons
Why does the nucleus stay together?
4 basic forces
Gravity
Weak Nuclear
Electromagnetic
Strong Nuclear
What holds nucleus together?
The answer was proposed by Hideki Yukawa in 1935, he called it a STRONG NUCLEAR FORCE. It is exerted by the p+ and n0 on each other SNF overwhelms the electrostatic charge (++)
SNF
The SNF acts over very short distance, so larger nuclei the force does not reach far enough
If the SNF is not enough, the electrostatic forces will dominate and the nucleus will break apart. Instability occurs when nucleus is too big
Nuclear symbols
Chapter 19 requires that you understand nuclear symbols and how to manipulate them.
Nuclear vs Chemical Reactions
In a chemical reaction: Atoms are rearranged to form new substances
In nuclear reactions: We are creating new elements!
ALCHEMY!
Radioactive Decay
Radioactive – something is radioactive if the nucleus spontaneously decomposes When it does this a different nucleus is formed Extra particles are usually formed also
Example: eNC 01
147
146
The symbol represents a beta () particlee01
particle production
The mass of a beta particle is 0, and the atomic number is –1:
So, beta particles change neutrons to protons.
e01
ePaTh 01
23491
23490
eI 01
13153 ?
Where did the e- come from?
A decaying neutron!!!
eHn 01
11
10
Nuclear equations & Laws
Notice that in the equation, atomic mass (A) and atomic # (Z) are the same on both sides of the arrow. Law of conservation of Mass!
eNC 01
147
146
Types of Radioactive Decay
Alpha () particle: An alpha particle is a Helium Nucleus.
The nuclear symbol is: He42
RnHeRa 21886
42
22288
?42
23090 HeTh
Gamma Rays ()
A gamma ray is a high energy photon of light.
Released along with other types of radiation
00
00
42
23490
23892 HeThU
Positrons
A particle with the same mass as an e- but a positive charge:
Positrons are anitmatter!
Example:
e01
NeeNa 2210
01
2211
Electron capture
A nucleus CAPTURES an inner electron Gamma rays are always produced.
e01
00
20179
01
20180 AueHg
NOTE that in electron capture the beta particle is always on the LEFT of the arrow!!
nep 10
01
11 Explanation:
Decay Series
An unstable nucleus undergoes a series of changes until it becomes a stable nucleus
Band of Stability
Predicts whether or not an atom’s nucleus will be stable.
For atoms less than 20 p+, n0:p+ are 1:1
n0/p+ ratio greater than 1.5/1 will be unstable
Binding Energy & Mass Defect
Binding Energy – energy that holds nucleus together
Mass defect – the ‘missing mass’ when the mass of nucleons are added together.
Example: Iron – 56
E = mc2
Einstein’s idea
E = energy m = mass in kilograms c = speed of light 3x108 m/s
Reconstructed Collision of e+e-
e+ e -
e+
e-
q
q
Hadronization(hadrons produced)
Half Life
½ life: the time for ½ of a sample of nuclei to decay For U-238, it is 4.5 billion years Pa-234 has a ½ life of 1.2 minutes!!
Transuranium elements & Radioactivity
Transuranium elements are just elements #93-11? (anything after Uranium!)
All elements after Bismuth (Z = 83) are radioactive
Fission and Fusion
Combining two light nuclei to make a heavier nucleus is called fusion
Splitting a heavy nucleus to form two nuclei with smaller masses is called fission.
Fission
Releases a lot of energy!
26 million times that of burning methane
Can cause chain rxns
Chain Rxns
At least one neutron must go on to split another nucleus
Nuclear Reactors
Nuclear Fusion
Produces more energy than fission! Takes very high temperatures to get protons
close enough to “stick” to each other Why is “cold fusion” a hollywood fad?
energyHeHH
energyeHHH
32
21
11
01
21
11
11
Effects of Radiation
Different particles penetrate in different ways energy mass How easily they make ions Chemical properties
These things make radiation more or less harmful
Detecting Radiation
High energy particles from radioactive decay make ions when they travel through matter. The fast particles “knock” electrons off the atoms
they hit.
A geiger counter can detect these ions through the flow of electricity
A geiger counter
Carbon Dating (Radioactive Dating)
Based on the radioactivity of carbon-14, which decays through -particle production.
eNC 01
147
146
Carbon-14 is continuously made in the atmosphere when neutrons from space hit the nitrogen in the air.
HCnN 11
146
10
147
More on carbon dating:
Living things always have the same amount of C-14 because they take it in through the air.
As soon as a living thing dies it stops taking in carbon from the atmosphere. The C-14 decays. The ½ life of C-14 is 5730 years. If a piece of wood has ½ the C as a living tree, it is
5730 years old. What are some of the problems with this method?
Making new elements
Scientists make new elements by bombarding existing nuclei to create elements with larger and larger atomic numbers.
eNpUnU 01
23993
23992
10
23892
Changing one element into another
Radioactive decay is when a nucleus decomposes and turns in to another nucleus
Bombarding nuclei with particles can form new nuclei also.
HOHeN 11
178
42
147
nPHeAl 10
3015
42
2713