PHYSICS UNIT 9 - ATOMIC STRUCTURE 
Isotopes
Nuclear Decay Equations for Alpha/Beta Decay
Half-life
Hazards of Radiation
Models of the Atom
Bohr 1912
Key features: electrons contained in shells (orbits) or would be attracted to nucleus & cause collapse
Modern Atomic Structure
Rutherford 1911
2) Nuclear Model
Thomson 1897
Key features: -ve charged electrons embedded within a +vely charged "soup"
1) Plum Pudding Model - Spheres of +ve charge with tiny -ve electrons stuck in them like "plum pudding"
1804 Dalton
Agreed with D but thought each element made of a different type of "atom"
Greek called Democritus in 5th Century BC - all matter made up of identical lumps called "atomos" that couldn't be broken up
Key features : atom has a small +ve nucleus with -ve electrons orbiting around the nucleus
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.
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
Almost all the mass of the atom is in the nucleus
Atom = +vely charged nucleus surrounded by -vely charged electrons
Nucleus = protons & neutrons
Atoms of same element can differ in nuclear mass by having different numbers of neutrons
Same number of protons (atomic number) so same charge on nucleus
Different number of neutrons (mass number)
= Different forms of same element
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
Ionising Radiation
Beta
Gamma
Alpha
Don't penetrate very far & stopped quickly, only travel few cm in air & absorbed by sheet of paper
Strongly Ionise atoms when collide into them because of their size
Alpha particle (2 neutrons & 2 protons - like helium) is emitted from nucleus
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
Moderately ionising & penetrates moderately, range in air of few metres & absorbed by sheet of aluminium (5mm thick)
For every beta particle emitted a neutron in nucleus has turned into a proton
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
Penetrate far into materials & travel long distance through air
Weakly ionising as pass through rather than collide, eventually hit and do damage
Waves of electromagnetic radiation released by nucleus
Absorbed by thick sheets of lead or metres of concrete
Uses - killing cancer cells & sterilising equipment
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
Isotopes can take just few hours or millions of years
Radiation can be measured with a Geiger-Muller tube & counter which records count rate = number of radiation counts reaching it per second
Half-life = time taken for activity (count rate) to halve
Find half-life by dividing total Bq by 2
Irradiation
Contamination
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
= Exposed to radiation
Keep sources in lead-lined boxes, standing behind barriers or different room, using remote controlled arms when working with radioactive sources
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.
= Radioactive particles get onto or into an object e.g.. touching radioactive source without gloves
Inside body alpha most dangerous because most ionising & damage localised area
Outside body beta & gamma are the most dangerous as can penetrate body
Alpha
Beta
Gamma rays don't change charge or mass of nucleus so don't do equations
Atomic number reduces by 2 & mass by 4
Decay decreases charge & mass of nucleus
Written as Helium nucleus
Number of protons in nucleus increased by 1 & increases atomic number, mass stays the same
Beta particle
Neutron in nucleus turns into proton & releases fast-moving electron
Atom before decay --------> atom(s) after decay & radiation emitted (total mass & atomic numbers must be equal on both sides)
Object is still radioactive as source is within e.g.. injected tracer treating cancer
Experimented with cathode ray tubes which showed all atoms contain tiny -vely charged subatomic particles or "electrons"
Experiment - electricity passed through a gas & made to glow & the light analysed using a spectroscope . Bright lines seen in the spectroscope
3) Shells