Atomic structure
Definitions
Protons - positively charged particle in nucleus
Neutron - non-charged particle in nucleus
Electron - negatively charged particle orbiting nucleus
Proton/atomic number - the amount of protons in the nucleus Xa
Mass number - the amount of protons and neutrons in the nucleus Xb
Isotopes - atoms which have the same atomic number but different mass numbers (aka, more or less neutrons)
Ions - a charge particle, as more or less electrons than protons.
How was the nucleus discovered?
Rutherford's student geiger fired alpha particles at gold leaf and instead of passing straight through, which is what would would happen with no nucleus, these particles very rarely (1/9000) would hit something, and be scattered. This thing that it bounced off was discovered to be the nucleus.
Radioactive particles
Properties of alpha, beta, gamma radiation
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Type
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What is it?
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Charge
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Ionisation
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Size
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Penetration
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Picture
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|
Alpha
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Helium atom, with two protons and two neutrons
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+2
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Strong
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Large
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Low
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|
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Beta
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Fast moving electron
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-1
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Medium
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Small
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Medium
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|
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Gamma
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High frequency electromagnetic waves (e.g photons)
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0
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None
(Very little)
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None
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High
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Why do particles slow down, when penetrating mass?
They are “pulled on” or “ pushed by” by the charges of other particles in the matter and sometimes even come in contact with these particles. Alpha particles are large enough that it may have a high frequency of collisions while beta particles may have very few, thus the alpha particles have very low penetration in comparison to beta and gamma (no mass and no charge).
Radioactive Decay
Types of decay
Alpha decay:
A large unstable particle has too many protons/neutrons and so it can release some of these by using alpha decay. Two protons and two neutrons are released from the nucleus.
Formula: XZAYZ-2A-4+24
Beta decay:
A large unstable particle splits one of it’s neutrons in order to reduce the number of neutrons/protons. Neutron is split to release a proton and an electron.
Formula: XZAYZ+1A+-10
Gamma rays
An unstable particle releases energy in the form of high frequency electromagnetic wave, it does this after a beta or alpha decay because sometimes there is still to which energy left over.
Geiger counter
State - A device which detects radioactivity.
Describe - When alpha particles enter the tube, they ionize the particles in the outer metal shell of the device, this means they strip them of electrons. The negatively charged outer shell causes the cations to be attracted to it and would stick to it. The newly stripped away electrons enter the metal at the centre and start producing electricity. This current is detected and makes a clicking sound.
Half life
State - How long it takes for a particle to half its mass.
Describe - Particles which are unstable will naturally decay. An example of this is carbon-214, this takes a very long time to decay. When something decays it decreases in mass, but the length of time it takes for a single particle to decay is variable and can’t be measured. Instead we use the length of time it takes for half the particles to decay other wise known as the half life. After one half life of Carbon-214 for example, it will have halved its mass.
Uses of radioactivity
Gamma radiation uses:
Radiotherapy - Gamma rays are used to kill off cancerous cells.
Sterilizing - Medical equipment and food can be bathed in gamma radiation to remove all bacteria. As it is non-ionizing this is not dangerous.
Tracers - Non-ionizing gamma radiation particles are inserted into the body to map out the path of fluid.
Beta radiation uses:
Tracers - Radioactive particles are inserted into the body to map out the path of fluid. Used when it shouldn’t be in the body for very long, or needs more detectability due to higher ionisation
Thickness control - Beta particles can go through thin layers but no through thick so by using beta particles spaced at a preknown distance from the detector, the particles will pass through the correct size of the material but not when it is too thick.
Juice level - Similarly to thickness control, if the juce level is too low the particles will not get stopped by the juice and hit the detector but if the juice level is correct the particles will get stopped by the juice
Alpha radiation uses:
Smoke alarm - In smoke alarm the air particles are constantly getting ionized by the alpha radiation, creating an electric current. However when smoke enters the ionisation of the air is disrupted, so the current drops. This is detected by the smoke alarm.
Use short or long half life?
Short half life
- To not last long, so as to not ionize the body too much.
- When a strong emission of particles is needed
- For medical tracers
When to use long half lifes
- When it needs to last longer (carbon dating)
- For non-medical tracers
Carbon dating
Using half life one can calcualte the age of really old things. After one half life the mass will have halved. So, if one knows the final mass and the starting mass of carbon based matter, one can calculate how many half lives have elapsed, multiply that number by the half life of carbon-214(5700) and calculate how old the carbon is.
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