In 1909, Ernest Rutherford wanted to carry out an experiment to test Thomson’s Model.
We can’t see inside an atom with our eyes, but he wanted to “see” inside.
“ Seeing” inside things……
“ Seeing” inside things……
“ Seeing” inside things……
He managed to “look inside” the atom by firing tiny particles called alpha particles at a thin gold foil to see how the alpha particles were deflected by the atoms of gold.
Alpha particles are the same as Helium nucleii, they are emitted from some radioactive atoms.
detector Rutherford’s Gold Foil Experiment
What they expected to happen: GOLD FOIL
What did happen: GOLD FOIL
This is what they expected to happen
They expected to see small or no deflection of the alpha particles
This is what did happen
Expected Results Actual Results
Rutherford’s Experiment (1911)
Most particles go straight through.
Some particles are deflected from straight path. A few even go backwards
Most of the atom is empty space.
nucleus must be positive , very dense , more massive than alpha particles.
Negative electrons orbit the nucleus, but are much lighter
Scale of the atom.
If an atom was enlarged to the size of a stadium, the nucleus would be the size of a plum .
The electrons would be the size of match heads wizzing around the stands.
The rest is empty space
Size of the Atom
An atom is roughly 10 -10 m in diameter
This means a full stop is roughly ten million atoms across.
A small nucleus is roughly 10 -15 m in diameter
This is 1/100,000 ths the diameter of the atom
What are the only two Fundamental Forces you are familiar with?
How Do You Make An Atom?
Why don’t the electrons fall into the nucleus?
Physicists realised the nucleus was made of two types of nucleons . Protons and Neutrons. What stops them flying apart? The strong nuclear force!!! extension… Now we’ll look closer at the nucleus
A nucleus can be described by two numbers:
Atomic or charge number (number of protons) ( Z )
Determines the nature of the atom and the element
Mass or nucleon number (Number of protons plus neutrons) ( A )
Isotopes : Atoms of an element with different number of neutrons e.g.
Nuclei that have too much energy are unstable.
They become more stable by firing out some nuclear radiation
There are three types of radioactive decay……
Write the equation for the alpha decay of Radium to Radon Alpha Decay
Write the equation for the beta decay of radium into actinium
This is what causes beta decay beta decay is when a nucleus fires out an electron…
Write the equation for the gamma decay of……
A gamma ray is a high energy photon emitted from a nucleus
When alpha particles collide with atoms, they can knock electrons off.
This will produce a positive ion and a free electron.
Sorting by Absorption Paper 1mm Lead
Sorting with a Magnetic Field
Identify each type of radiation
The half life of an isotope is the time taken for half of a sample to decay into another isotope.
OR, the time taken for the activity of a sample to halve.
The shorter the half life, the less stable it is.
e.g. Uranium 238: 4,500 MY
Radon 218 0.04 s
link to half life 1
Radioactivity and Probability
Radioactivity is all about chance.
You can’t say when a certain nucleus will decay, but it might have a 1 in 10 chance of decaying in the next 5 seconds.
For the example above:
1000 nuclei 100 decays in 5s
100 nuclei 10 decays in 5s
This means there are more decays when there are more nuclei.
A shorter half life means a greater probability of a decay occurring
e.g Beryllium 11 decays to Boron 11 with a half life of 14 s
So If you have 16g of
Beryllium 11 now ,
after 14 s you will
have 8g etc.
link to half life 2
8g 4g 2g 1g 56 42 28 14 16g 0 Amount of B Time(s)
Berillium amount Time 16 g Now 8 g after one half life 4 g after two half lifes 1 HL 2 HL 3 HL 4 HL Boron amount
Note that the total mass of the sample is about the same , because as the Beryllium decays, it doesn’t disappear, it changes into Boron. The Boron is still there.
A radioactive isotope has a half life of 3 years.
A 5 g sample of the isotope produces 30 decays per sec.
What will the decay rate of a 1 g sample be in 9 years time?
This image shows the Shroud of Turin.
It was supposedly the cloth that Christ was buried in. Is it real or a medieval fake???
In the Atmosphere, cosmic rays hit Nitrogen 14 changing it to Carbon 14.
The Carbon 14 decays with a half life of 6300 years.
So a small fraction of CO 2 molecules contain Carbon 14. This is taken in by plants, and hence animals.
When the organism dies, The Carbon 12 stays the same, the Carbon 14 decays.
By measuring the ratio of C14 to C12, the time since it was alive can be calculated
Back to the shroud. The ratio of C14 to C12 showed it was about 800 years old!!!
Final question. A wooden axe handle has a ratio of C14 to C12 that is 1/8 times the ratio for new wood.
How old is it?
NCEA type question
Describe the Dalton model of the atom
Explain the evidence for the Thomson model
Explain the evidence for the Rutherford model
(a) Cobalt-60 undergoes radioactive decay..
Show how the decay of cobalt-60 ( ) results in nickel-60 ( ).
(a) A smoke detector contains radioactive americium 241 which emits radiation. Complete the following equation to identify the radiation emitted.
Explain why the radiation given out by the americium is unlikely to do any harm to the people living inside the house.
The alpha particles ionise atoms in the air. Explain what this means.
Estimate the half-life of americium-241.
(b) Radon-212 ( ) is a radioactive gas. Show that when radon-212 undergoes alpha decay, polonium is formed.
Radon-212 decays with a half-life of 24 minutes.
If you start with 96 mg of radon-212, find the approximate mass of polonium-208 two hours later.
Why is the actual mass less than your calculation?
State what an alpha particle is. 12 years beta particle hydrogen-3 24 minutes beta particle uranium-239 less than 1 second alpha particle polonium-213 138 days alpha particle polonium-210 74 days gamma ray iridium-192 6 days gamma ray technetium-99 Half-life Type of radiation emitted Isotope
Two isotopes of polonium are given in the table. How do the nuclei of these two isotopes differ? 12 years beta particle hydrogen-3 24 minutes beta particle uranium-239 less than 1 second alpha particle polonium-213 138 days alpha particle polonium-210 74 days gamma ray iridium-192 6 days gamma ray technetium-99 Half-life Type of radiation emitted Isotope
A doctor needs to monitor the blood flow through a patient’s heart. She injects a radioactive isotope into the patient’s bloodstream. Explain why she would choose technetium-99 instead of the other isotopes listed in the table above. 12 years beta particle hydrogen-3 24 minutes beta particle uranium-239 less than 1 second alpha particle polonium-213 5 seconds gamma ray Strontium 91 74 days gamma ray iridium-192 6 days gamma ray technetium-99 Half-life Type of radiation emitted Isotope