Radioactivity

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  • Created by: LZ95
  • Created on: 28-04-14 19:18
What was the initial structure of the atom thought to be like?
Atom has a plum pudding like structure; it was thought to be a positively charged piece of matter with smaller electrons floating freely
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Initially, what did the scientist think about the charge of the atom.
They thought it had a neutral charge as they believed the total charge of the electrons was equal to the total positive charge
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When Rutherford fired alpha particles at a thin metal sheet, what did he expect and why?
He expected all of the alpha particles to pass through as the atom was thought to be mostly empty space.
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What was put around the gold sheet in Rutherford's experiment that made alpha particles appear upon contact?
A screen made from zinc sulfide
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How was the contact observed?
With a microscope that was directly above the screen.
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Why was the experiment conduced in complete darkness?
So the light of the alpha particles could be seen.
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What happened in the experiment that Rutherford did NOT expect?
Small amount of particles were deflected by a few degrees; 1 in every 10,000 were deflected at an obtuse angle so they reversed direction
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What are 3 conclusions that Rutherford made about the experiment?
1. Atom must have a positively charged centre that contains most of its mass. 2. Part of the atom is positively charged + heavy. 3. Positively charged part of the atom was small compared to the rest of the atom
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What are the limitations of using the formula for electric potential when working out the radius of a nucleus?
It requires knowledge of the electric potential and number of protons in the nucleas, + the distance calculated may be too large
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State the formula for working out the nuclear radius using the probability 1/10,000
d^2=D^2/10,000n where d=diameter of nucleus, D= diameter of atom and n= number of layers of atoms
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Why are electrons diffracted when they are directed at a thin solid sample of an element?
They have a de Broglie wavelength of the order 10^-15m which is around the diameter of the nucleus
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What happens to the number of electrons diffracted per second as the angle of diffraction is increased.
Decreases until the minimum angle after which it increases then decreases again
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Why are the electrons scattering?
They are negatively charged so are attracted to the oppositely charged nucleus.
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What happens to intensity as the angle increases?
Decreases.
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What is measured and used to calculate the diameter of the nucleus?
Angle of the first minimum from the centre.
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What needs to be known in order for the diameter of the nucleus to be calculated in this way?
Wavelength of the incident electrons.
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What is an alpha particle identical to?
A helium nucleus
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What are some properties of alpha radiation?
Least penetrating radiation, very ionising, very short range in air (few cms), absorbed by thin sheets of material, particles made up of 2 protons + 2 neutrons
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What are beta particles?
Fast moving electrons emitted from the nucleus of some radioisotopes.
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What are some properties of beta radiation?
Longer range in air (few metres), more penetrating than alpha but less ionising, deflected by electric + magnetic fields
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What is gamma radiation?
Very high frequency electromagnetic radiation
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Name some properties of gamma radiation
It is uncharged so not deflected by electric + magnetic fields, least ionising radiation, very penetrating, unlimited range in air
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Name 5 sources of background radiation
Air, Rocks + buildings, cosmic rays, food + drink, medical procedures e.g. X-rays
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When does background radiation have to be taken into account?
When radioactivity measurements are performed in a lab.
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Define the activity of a radioactive source
Number of decays that occur, on average, every second.
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What is activity measured in?
Becquerel (Bq) which is equal to one disintegration per second.
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How is it measured?
Using a Geiger counter
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What sort of event is radioactive decay?
Random
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What does the decay constant represent?
Probability that a nucleus will decay in a given time
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What is the activity of a source proportional to?
Number of nuclei present.
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What is "half life"?
Time taken for the number of active nuclei in a sample to drop to half of the original value.
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How is half life and the decay constant related?
An isotope with a low decay constant has a lower probability of decay so has a longer half life
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Why do fission products need to be stored underwater?
They are highly radioactive as they have a high decay constant so a short half-life
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How can radiocarbon dating be used to estimate the age of an artefact?
Carbon-14 found in plants and when plants die the activity of C-14 drops and since the half life of C-14 is known it is possible to estimate the age of the artefact
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What do stable isotopes with low atomic numbers have?
Similar number of neutrons + protons in their nuclei
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Why are these nuclei stable?
Attraction of strong nuclear force is strong enough to balance the repulsive electrostatic force.
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Why are isotopes with higher atomic numbers more unstable?
They have more protons so the electrostatic force is stronger
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Name 5 modes by which an unstable nucleus can decay
Alpha decay, beta(-) decay, beta(+) decay, electron capture and nucleon emission
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What are alpha emitters?
Proton heavy isotopes
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What happens during beta(-) decay?
A neutron transforms into a proton, emitting a high speed electron + antineutrino
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What are beta(-) emitters?
Neutron rich isotopes which lie above the curve of decay
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What happens in beta(+) decay?
Proton transforms into a neutron, emitting a high speed positron + neutrino
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Where do beta (+) emitters lie on the curve of decay?
Below the curve
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What happens in electron capture?
Nucleus of an atom captures on of the atom's orbiting electrons turning a proton into a neutron.
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What is nucleon emission?
A rare occurrence where an unstable isotope can emit a nucleon.
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What does alpha emission do to the daughter nucleus?
Leaves it in an excited state
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How will the excited nucleus decay?
By the emission of a gamma ray
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Other cards in this set

Card 2

Front

Initially, what did the scientist think about the charge of the atom.

Back

They thought it had a neutral charge as they believed the total charge of the electrons was equal to the total positive charge

Card 3

Front

When Rutherford fired alpha particles at a thin metal sheet, what did he expect and why?

Back

Preview of the front of card 3

Card 4

Front

What was put around the gold sheet in Rutherford's experiment that made alpha particles appear upon contact?

Back

Preview of the front of card 4

Card 5

Front

How was the contact observed?

Back

Preview of the front of card 5
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