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PHYSICS UNIT 9 - ATOMIC STRUCTURE (Half-life graph (Half-life used to find…
PHYSICS UNIT 9 - ATOMIC STRUCTURE 
Isotopes
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Unstable isotopes tend to decay into other elements & give out radiation as try to become more stable = radioactive decay e.g.. alpha, beta & gamma radiation
Nuclear Decay Equations for Alpha/Beta Decay
Alpha
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Written as Helium nucleus
Beta
Number of protons in nucleus increased by 1 & increases atomic number, mass stays the same
Beta particle
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Atom before decay --------> atom(s) after decay & radiation emitted (total mass & atomic numbers must be equal on both sides)
Half-life
Half-life used to find rate at which source decays, its ACTIVITY (measured in becquerels, Bq) 1Bq = 1 decay per second
Each time a radioactive nucleus decays to become stable the activity as a whole decreases - older sources emit less radiation
Radioactive decay is entirely random so can only find out time it takes for amount of radiation emitted by a source to halve = half-life
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Radiation can be measured with a Geiger-Muller tube & counter which records count rate = number of radiation counts reaching it per second
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Hazards of Radiation
Irradiation
We're always being irradiated by background radiation e.g.. natural sources = cosmic rays from sun, rocks & soil, artificial = X-rays & nuclear power stations
Irradiating something does not make it radioactive - 0bject is no longer radioactive when source removed e.g.. cancer treatment
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Keep sources in lead-lined boxes, standing behind barriers or different room, using remote controlled arms when working with radioactive sources
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Contamination
Contaminating atoms may then decay releasing radiation causing you harm & radioactive particles could get inside body
Gloves & tongs should be used to avoid particles getting stuck to skin or under nails. Industrial workers may use protective suits.
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Models of the Atom
Bohr 1912
Key features: electrons contained in shells (orbits) or would be attracted to nucleus & cause collapse
Experiment - electricity passed through a gas & made to glow & the light analysed using a spectroscope . Bright lines seen in the spectroscope
3) Shells
Modern Atomic Structure
Molecule bigger as made up of atoms, atom significantly bigger than nucleus which is like a full stop if the atom were magnified to size of school hall
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Rutherford 1911
2) Nuclear Model
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Experiment - fired alpha particles at thin gold foil. Plum pudding predicted they would travel straight through but some reflected back. Showed must be a small +ve nucleus & most of atom empty space.
Thomson 1897
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1) Plum Pudding Model - Spheres of +ve charge with tiny -ve electrons stuck in them like "plum pudding"
Experimented with cathode ray tubes which showed all atoms contain tiny -vely charged subatomic particles or "electrons"
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Greek called Democritus in 5th Century BC - all matter made up of identical lumps called "atomos" that couldn't be broken up
Ionising Radiation
Beta
Moderately ionising & penetrates moderately, range in air of few metres & absorbed by sheet of aluminium (5mm thick)
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Fast moving electron released by nucleus, virtually no mass & a charge of -1
Use - test thickness of sheets of metal as slight variations in thickness affect amount of radiation passing through
Gamma
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Weakly ionising as pass through rather than collide, eventually hit and do damage
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Alpha
Don't penetrate very far & stopped quickly, only travel few cm in air & absorbed by sheet of paper
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Use - smoke detectors - contains small amount of radioactive material & receiver counts Alpha made, if smoke present it blocks Alpha particles to receiver & alarm goes off