Atomic Structure- Physics

History of Atom

Democritus —> John Dalton —> JJ Thomson —> Rutherford—> Niels Bohr —> James Chadwick

Plum Pudding Model

• JJ Thompson

• sphere of positive charge and negative charged discrete electrons inside of it

Alpha particle scattering experiment

• alpha particle experiment where alpha particles are directly at gold foil

• expected positively charged alpha particles would go straight through or be slightly deflected by electrons if pp model was correct

• most particle went straight through but some were deflected more then expected and some came back at them

• so mass is concentrated at centre of the atom

• nucleus is positvely charged

Detailed Discovery Alpha Particles

• Most of the atom must be empty space as most passed straight through the foil

• Nucleus must have large positive charge as some positively charged alpha particles were repelled and deflected by a big angle

• nucleus must be small as very few alpha particles were deflected back

Atomic and Mass Number

Atomic number - protons is the nucleus

Mass Number - Protons + Neutrons in the nucleus

Ion

• charged particle with a different number of electrons than usual (normally to make it stable)

• process is called ionisation said to have been ionised

Radioactive decay

• unstable isotopes tend to decay into other elements and give out radiation as they try to become more stable

• Radioactive substances emit ionising radiation

Ionising radiation

• Radiation that knocks electrons off atoms creating positive ions

• the ionising power of a radiation source tell you how easily it can do this

• type of radiation - beta,gamma,alpha

Alpha decay

• alpha particles (helium nucleus) has two neutrons and two protons

• -2 protons number - 4 mass number

• relatively big,heavy and slow moving meaning they dont penertrate very far into materials and are stopped quickly

• due to size they are strongly ionising the bash into lots of atoms and knock electrons off (creates lot of ions)

Beta decay

• beta particle fast moving electron released by a nucleus

• neutron decays into a proton and electron

• no mass relative charge of -1

• neutron turns into a protons in the nucleus this increase the charge on the nucleus by 1 but leaves mass number unchanged

• beta particles move quite fast and are quite small

• moderately ionising and penetrate moderately far into material before colliding

Gamma decay

• very shot wavelength electromagnetic waves (pure energy)

• no mass and no charge

• penetrate far into material without being stopped and pass straight through air

• weakly ionising as they tend to pass then collide

Nuclear Equation

• alpha particles are helium nuclei 2a4

• beta particles are electrons (0 e -1 charge)

Activity

• Geiger Muller tube and counter measures count rate

• number of radiation counts reaching it per second

• half life can be used to find the rate at which a source decays , activity is measured in becquerels , Bq where 1 bq is 1 decay per second

Half Life

• Half life is the time it takes for number of nuclei of a radioactive isotope in a sample to halve

• activity never reaches zero which is why we have to use the idea of half life to measure how quickly it drops off

• radioactivity of sample decreases over time, each time it decays the activity as a whole will decrease so less radiation emitted

Short Half life

• actiivty falls quickly

• nuclei are very unstable and rapidly decay

• dangerous because of high amount of radiation they emit at the start

• they quickly become safe

Long Half Life

• acitivty falls more slowly because most of nuclei doesnt decay for a long time

• release small amount of radiation over long period of time

• nearby area are exposed for years

Calculating half life + graph

• divide by 2 to get how many half lifes

• time in hours x axis bq acitivty y axis half life plot then line of best fit

Irradiated

Exposted to radiation, doesnt not make it reactive but radiation can be harmful to living things

Saftey Precautions

• lead line boxes

• standing behind barriers

• being in a different room

• uses tongs when picking up radioactive substance

• wear overalls/goggles/gloves

Contamination

• unwanted radioactive atoms get onto or into a material then it is said to be contaminated

• gloves and tongs should be work to avoid particles getting stuck to skin or under your nails

Different particles and how dangerous + best way to imrpove research

• oustide beta and gamma as they can go through your skin alpha cant penertrate

• inside alpha as it is the most ionising while beta and gamma just pass through contamintion rather than irradiation is the major concern when working with alpha sources

• important to publish discoveries and peer review leading to improvement in use of radioactive sources

Radiation Dose

Measure of the risk of harm to your body due to exposure to radiation.

• depends on type and amount of radiation youve been exposed to

• higher dose more risk of cancer

• Measured in sieverts (Sv) 

• Background radiation small so normall in millisieverts (1Sv=1000mSv)

Background Radiation Sources

• low level radiation that is present at all time

• radioacitivty of naturally occurring unstable isotopes which are around us

• radiation from space which is know as cosmic rays these come mostly from the sun, earth atmosphere absorbs most of them but at high altitudes some get trhough

• radiation due to man made sources, nuclear weapons, nuclear reactors

Effect of location on radiation

• Underground rocks like granite can release higher levels of radon gas which gets trapped inside peoples houses

• radon detector can be used to check and radon pipe can be used to keep level down

• People who live at high altitudes are exposed to more background comic rays that people who live at sea level

Effect of occupation on radiation

• Nuclear industry workers and uranium miners are exposed to 10x the normal amount

• Wear protective clothes and face marks to stop from touching and inhaling material and monitor radiation dose with special badges and check ups

• Radiographers work in hospitals using ionising radiation and so have a high risk of radiation expose,wear lead aprons and stand behind lead screen to protect them

• Underground the radiation dose increases because of the rocks all around

Effect of radiation on living cells

• Particles will enter living cells and collide with molecules causing ionisation which damages or destroys the molecules

• low does tend to cause minor damage giving rise to mutant cells that divide uncontrollably, which eventually forms a tumor

• higher doses kill cells completely causes radiation sickness if too many are killed at once

Harmful effect of radiation depend on

• how much exposure you have to the radiation

• the energy and penetration of the radiation since some types are more hazardous than others

Medical Tracers

• Isotopes that are injected to progress around the body followed by an external dectector

• Help to see if the patients interal organs are functioning as normal

Radiotherapy

• radiation targets at cancer cells using ionisation radiation to kill it

• patient will feel sick after as lots of cells are killed even some living ones near the tumor

Nuclear fission

• Splitting up of a large and unstable nucleus into smaller nuclei releasing energu

• neutron is fired at unstable nucleus causing it to split apart

• unstable nucleus splits into 2 daughter nucleu and 2/3 neutrons.

• Emits a large amount of energy as gamma radiation

• 2 or 3 neutrons that were released are then abosrbed by other unstable nuclei and this process repeats

• process occurs over and over in a chain reaction

• boron control rods can be lowered into nuclear reactopm to absorb neutrons and slow chain reaction

• uncontrolled chain ractions lead to nuclear explosion

Nuclear Fusion

• fusion of two lighter nuclei to make a single heavier nucleus

• a lot of energy is emiited in form of em radiation

• only happens at high temp/pressure as it need to overcome the repelling force of similar positvely charged particles

Nuclear power station how generate electricity?

• unstable nuclei split apart release lots of gamma radiation

• this gamma radiation is then used to heat liquid water into steam

• as the steam rises it turn turbines and the kinetic energy of those turbines is converted into electrical energy by a generator