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• Created by: Lizzy Day
• Created on: 25-06-16 14:53
What was the model of the atom at the start of the 20th century?
The plum pudding model. A positively charged globule with negatively charged electrons sprinkled in it.
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What did Rutherford study?
They studied the scattering of alpha particles by thin metal foils.
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How did the Rutherford experiment take place?
A stream of radioactive a particles from a source are fired at thin gold foil. The a particles strike a fluorescent screen and tiny flashes of visible light are produced. Geiger and Marsden recorded the number of a particles scattered.
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What was observed from Rutherford's experiment?
Some of the a particles were scattered at angles greater 90 degrees. This can only be possible if they are striking something more massive than themselves.
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What is discovered from Rutherford's experiment?
Small, positive nucleus. Positive - repels the positive alpha particles. Small - very few a particles are deflected. Dense - a particles deflected by large angles.
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What do we know about alpha particles that have been deflected through 180 degrees?
Such a particles will have stopped a short distance from the nucleus. It does this at the point where its electrical potential energy equals its initial kinetic energy. So Ek = Eelec = Qnucleusqalpha/Ar
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What is electron diffraction?
As electrons show wave-particle duality, electron beams can be diffracted. Electrons don't interact with the strong nuclear force. So, if an electron beam is directed to a thin film, a diffraction pattern is produced.
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What is a de Broglie wavelength?
/\ = hc/E
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Where does the first minimum of electron diffraction appear?
The first minimum appears where sin0 = 1.22/\ / d
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What is the approximate size of the atom? How do we know this?
By probing atoms using scattering and diffraction methods, we know that the radius of an atom is about 0.05nm, and the radius of the nucleus is about 1fm.
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What is a molecule?
Molecules are a number of atoms joined together. The size of a molecule equals the no. of atoms in it multiplied by the size of one atom
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What are nucleons?
Nucleons are the particles that make up the nucleus. (protons and neutrons).
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What is nucleon number?
The number of nucleons in an atom is called the nucleon (or mass) number, A. As more nucleons are added to the nucleus, it gets bigger.
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What is the relationship between the radius of the nucleus and the nucleon number?
r = r0 A^1/3
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What is noticeable about nuclear density?
The volume that each nucleon takes up in a nucleus is about the same. As protons and neutrons have nearly the same mass, it means that all nuclei have similar density.
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What is the relationship between nuclear density and atomic density?
Nuclear density is significantly larger than atomic density.
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What does nuclear density suggest about the atom?
Most of an atom's mass is in its nucleus. The nucleus is small compared to the atom. An atom must contain a lot of empty space.
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If an atom is unstable, it will break down to become more stable. The atom decays by releasing energy and/or particles until it reaches a stable form.
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What are the four types of nuclear radiation?
alpha, beta-minus, beta-plus and gamma.
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alpha (He nucleus), beta minus (electron), beta plus (positron), gamma (short /\ high f em wave).
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What are the relative charges and masses of these?
alpha (c.+2. m.4), beta minus (c.-1. m.negligible), beta plus (c.+1. m.negligible), gamma (c.0.m.0)
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What are the penetrating powers of these?
alpha (paper or cm of air), beta minus (3mm of aluminium), beta plus (zero range, annihilates immediately), gamma (cm of lead or m of concrete)
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How can you identify radiation with magnetic fields?
Charged particles moving perpendicular to a uniform magnetic field are deflected into a circular path.
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What are their ionising powers?
alpha (strong), beta minus (weak), beta plus (zero, it is annihilated), gamma (very weak).
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What are the speeds of the types of radiation?
alpha (slow), beta (fast), gamma (speed of light)
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When a radioactive particle hits an atom it can remove electrons, creating an ion - so radioactive emissions are also known as ionising radiation.
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What are the ionising properties of alpha radiation?
a particles are strongly positive, this means that they can easily pull electrons off atoms, Some of its energy is also transferred to the atom. The a particle quickly loses all its energy, as it rapidly ionises.
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What is alpha radiation used for?
Alpha sources are suitable for use in smoke alarms because they allow current to flow, but won't travel very far.
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What are the ionising properties of beta radiation?
B- particles have lower mass and charge than the a particle, but travels at a higher speed. This means it can still knock electrons off atoms. The B- particle also loses energy at each interaction. B- radiation causes much less damage to body tissue.
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What is beta radiation used for?
It is used to treat cancer by destroying the cells.
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What are the ionising properties of gamma radiation?
Gamma radiation is even weaker than beta radiation so will do even less damage to body tissue before losing all of its energy in interactions.
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What is gamma radiation used for?
As gamma is the most penetrating it is useful for diagnostic techniques in medicine, e.g. PET scanners.
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How does ionising radiation affect living cells?
Ionising radiation can destroy cell membranes which causes cells to die. It can also damage vital molecules such as DNA. This can happen both directly or indirectly by creating 'free radical' ions which react with vital molecules.
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How are precautions taken with people who work with ionising sources?
They wear a film badge to monitor exposure. It contains a ***** of photographic film in a light=proof wrapper. Different areas of the film are covered by absorbers of different materials. The amount of exposure is estimated from the film blackness.
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What is a cloud chamber?
A cloud chamber contains air saturated with a vapour at a very low temp. Due to ionisation of the air, an a or B- particle passing through the cloud chamber leaves a visible pattern.
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Why is a pattern left in a cloud chamber?
A visible track of minute condensed vapour droplets is left. This is because the air space is super saturated. When an ionising particle passes through the supersaturated vapour, the ions produced trigger the formation of droplets.
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What proportionality rule does intensity follow?
The inverse square law. The intensity of the radiation is the amount of radiation per unit area.
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What is the equation for intensity?
I = k/x^2. I = Intensity (Wm-2), k = constant of proportionality (W), x = distance from source (m)
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What are some sources of background radiation?
The air (radioactive radon gas released from rocks), The ground + buildings (nearly all rock contains radioactive materials), Cosmic radiation (cosmic rays), Living things (all plants and animals contain carbon-14), Man -made radiation (industrial)
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The materials should be in lead-lined containers, thick enough to reduce gamma radiation to background level. Containers must be kept under lock and key, a record of the sources must be kept.
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What are isotopes?
Isotopes of an element have the same number of protons, but different numbers of neutrons in their nuclei.
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What is half-life?
The half-life T1/2, of a radioactive isotope, is the time taken for the mass of the isotope to decrease to half of the intial mass.
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What is the equation for the no. of nuclei remaining?
N = N e
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What pattern does the number of unstable nuclei show?
The number of unstable nuclei decreases exponentially with time.
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What is activity?
The activity, A, of a sample is the number of atoms that decay each second. It is proportional to the number of unstable nuclei in the sample.
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What is the decay constant?
The decay constant is the constant of proportionality. It is the probability of a specific atom decaying per unit time. It is a measure of how quickly an isotope will decay.
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What is the equation for activity of an isotope?
A = /\N
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What is the equation for the rate of change of number of unstable nuclei?
N/t = /\N
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What is the equation for power of a radioactive source?
energy transfer per second = AE
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What is the equation for half life?
T1/2 = ln2//\
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What is the equation for a changing activity?
A = A e
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Carbon-14 ( radioactive isotope) is used in radiocarbon dating. Living plants take in carbon dioxide from the atmosphere as a part of photosynthesis, including Carbon-14. When they die, the activity of Carbon-14 in a plant falls. (T1/2 = 5730)
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What are the difficulties with radiocarbon dating?
Can only find age of material not the object itself. The object may be contaminated by other sources. May be a high background count that obscures object's count. Uncertainty in the amount of Carbon-14 that existed thousands of years ago.
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Technetium-99m is widely used in medical tracers to show tissue or organ function. The tracer is injected or swallowed and then it moves through the body to the region of interest. The radiation emitted is recorded and an image produced.
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Nuclear fission reactors use Uranium-235. This decays into several different radioactive isotopes with different half lives, emitting different radiation. They must be stored carefully for hundreds of years until their activity falls to a safe level.
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What did Rutherford study?

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How did the Rutherford experiment take place?

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What was observed from Rutherford's experiment?

### Card 5

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What is discovered from Rutherford's experiment?