- Created by: Benny52
- Created on: 03-03-19 14:23
Developing the Model of the Atom
- John Dalton - matter made up of tiny spheres that couldn't be broken up & each element made of different type.
- J J Thompson - electrons - could be removed from atoms. Atoms - spheres of positive charge with electrons in them - plum pudding model.
- Ernest Rutherford - alpha scattering experiment - fired beam of alpha particles at thin gold foil. Expected particles to pass straight through or slightly deflect, due to plum pudding model. But some deflected more than usual and some deflected back - most of mass of atom concentrated at centre in nucleus - positive charge - repelled positive alpha particles. Most particles passed straight through - most of atom empty space. First nuclear model of atom - positively charged nucleus surrounded by cloud of negative electrons.
- Neils Bohr - electrons orbit nucleus in energy levels.
- Nucleus consists of protons with same positive charge adding up to total nucleus charge.
- James Chadwick - neutrons - explained imbalance between atomic & mass no.
- If electrons gain energy by absorbing EM radiation, they move to higher energy level, further from nucleus. Release EM radiation - lower energy level. 1 or more outer electrons leave atom - becomes positively charged ion.
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Isotopes & Nuclear Radiation
- All elements - different isotopes. Only usually 1 or 2 stable ones. Unstable ones decay into other elements & emit radiation trying to become more stable - try to balance no. of protons & neutrons in nucleus or get rid of excess energy - process is radioactive decay.
- Radioactive substances spit out 1 or more types of ionising radioation from nucleus - alpha, beta & gamma radiation. Can also release neutrons when decaying to rebalance no. of protons & neutrons.
- Ionising radiation - knocks electrons off atoms - positive ions formed. Ionising power of radiation source is how easily this is done.
- Alpha particles - 2 neutrons & 2 protons. Don't penetrate far into materials - stopped quickly - can only travel few cm in air, absorbed by sheet of paper. Strongly ionising due to size.
- Beta particles - fast moving electrons - virtually no mass & -1 charge. Moderately ionising - penetrate moderately far into materials before colliding & have range of few m in air. Absorbed by sheet of aluminium. For every particle emitted, neutron turns into proton.
- Gamma rays - waves of EM radiation released by nucleus. Penetrate far into materials & travels long distance in air. Weakly ionising - pass through rather than collide with atoms. Will eventually hit something & damage. Absorbed by thick lead sheet or metres of concrete.
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- Written in form: atom before decay --> atom after decay + radiation emitted.
- Total mass & atomic no. must be equal on both sides.
- When atom emits alpha particle, its atomic no. reduces by 2 and mass no. by 4. Proton is positively charged & neutron is neutral, so charge of nucleus decreases.
- Beta decay - no. of protons in nucleus increase by 1 - increases charge. Nucleus has lost proton & gained proton so mass of nucleus doesn't change.
- Gamma rays - way of getting rid of excess energy from nucleus - no change to atomic mass or number.
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- Radioactive substances emit radiation from nucleus no matter what. Can be measured with a Geiger-Muller tube & container - records the count-rate - no. of radiation counts reaching it per second.
- Radioactive decay - random. Can find out time it takes for amount of radiation emitted by source to halve - half-life. Used to make predictions about radioactive sources. Used to find rate at which a source decays - its activity - measured in Bq - 1 Bq = 1 decay per second.
- Each time radioactive nucleus decays to become stable nucleus, activity decreases, but never reaches 0.
- Half-life = time taken for the no. of radioactive nuclei in an isotope to halve.
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Irradiation & Contamination
- Ionising radiation can enter living cells & ionise atoms in them - damages cells, causing cancer, etc. or kills them.
- Objects near radioactive source irradiated (exposed) to it. Doesn't make object radioactive. Keeping sources in lead-lined boxes & standing behind barriers when using sources reduces effects. Source can be in different room & remote-controlled arms used to handle it.
- Radioactive atoms get onto or into object - contaminated. Atoms may decay, releasing radiation - harmful. Radioactive particles can get inside body - dangerous. Gloves & tongs used when handling sources to avoid particles getting stuck to skin or under nails. Workers - protective suits to stop them breathing in particles.
- Outside body - beta & gamma sources most dangerous - can penetrate body & get to delicate organs. Alpha less dangerous - can't penetrate skin & easily blocked by small air gap. High irradiation levels from all sources dangerous, but mostly from ones emitting beta & gamma.
- Inside body - alpha most dangerous - do all damage in localised area - contamination concern with alpha sources. Beta less damaging inside body - radiation absorbed over wider area & some passes out of body. Gamma least dangerous - mostly pass straight out - lowest ionising power.
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