AQA GCSE Physics. Topic 4: Atomic Structure. Notes. (Content in bold is for Higher Tier only)

Transcription

1 AQA GCSE Physics Tpic 4: Atmic Structure Ntes (Cntent in bld is fr Higher Tier nly)

2 Atmic Structure Psitively charged nucleus (which cntains neutrns and prtns) surrunded by negatively charged electrns. Subatmic Particle Relative Mass Relative Charge Prtn 1 +1 Nucleus 1 0 Electrn 0 (0.0005) -1 Typical radius f an atm: metres - And the radius f the nucleus is times smaller - Mst (nearly all) the mass f the atm is cncentrated at the nucleus Electrn Arrangement: - Electrns lie at different distances frm the nucleus (different energy levels). The electrn arrangements may change with the interactin with EM radiatin. Istpes and Elements - All atms f the same element have the same number f prtns - Neutral atms have the same number f electrns and prtns - Istpes are atms f the same element, but with different masses, which have the same number f prtns but different number f neutrns. - Fr example Carbn-12, Carbn-13 and Carbn-14 AA ZZXX ±nn - X is the letter f their element - A is the mass number (number f neutrns and prtns) - Z is the prtn number - N is the charge On a nrmal neutral atm, electrns = prtns, s cancels ut If N mre electrns than prtns, then N charge If N fewer electrns than prtns, then +N charge The number f prtns des nt change fr a certain element Atms and EM Radiatin - When electrns change rbit (mve clser r further frm the nucleus) When electrns mve t a higher rbit (further frm the nucleus) The atm has absrbed EM radiatin When the electrns falls t a lwer rbit (clser t the nucleus) The atms has emitted EM radiatin - If an electrn gains enugh energy, it can leave the atm t frm a psitive in.

3 Hw and why the atmic mdel has changed ver time Daltn said everything was made f tiny spheres (atms) that culd nt be divided JJ Thmsn discvered the electrn The Plum Pudding Mdel was frmed The verall charge f an atm is neutral, s the negative electrns were dispersed thrugh the psitive pudding t cancel ut the charges Rutherfrd realised mst f the atm was empty space Gld Fil Experiment Mst αα particles went straight thrugh - S mst f atm is empty space Sme αα particles were slightly deflected - S nucleus must be charged, deflecting psitive αα Few αα particles were deflected by >90 - S nucleus cntained mst f the mass kcmcgann.tripd.cm/gldfil.jpg This experiment was carried ut by Geiger and Marsden, specifics nt needed Rutherfrd Mdel Nw there is a psitive nucleus at the centre f the atm, and negative electrns existing in a clud arund the nucleus uplad.wikimedia.rg

4 1913 Bhr prduced the final mdel f the atm If Rutherfrd was right, the electrns in the clud clse t the nucleus wuld get attracted, and cause the atm t cllapse. S nw the electrns exist in fixed rbitals Later n: - Psitive charge f nucleus culd be subdivided int smaller particles, each with the same amunt f charge the prtn - 20 years after the nucleus was an accepted scientific idea, James Chadwick prvided evidence t prve neutrns existed (dn t need specifics) Radiatin Sme atmic nuclei are unstable. The nucleus gives ut radiatin as it changes t becme mre stable. This is a randm prcess called radiactive decay. Activity is the rate at which a surce f unstable nuclei decays - S a sample with high activity has a fast rate f decay - Measured in Becquerel, Bq Cunt-rate is the number f decays recrded by a detectr per secnd. - E.g. a Geiger-Muller Tube - Frms f decay Alpha α (a helium nucleus) Highly inising Weakly penetrating (~5cm f air) Beta Minus β (electrn) Medium inising Medium penetratin (~50cm f air, sheet f paper) Nuclear Equatins Nuclear equatins are used t represent radiactive decay. 4 AAAAAAhaa PPPPPPPPPPPPPPPP: 2HHHH BBBBBBBB PPPPPPPPPPPPPPPP: 1 0 ee Gamma γ (radiatin) Lw inising Highly penetratin (very far in air, few cm f lead) Neutrns

5 The emissin f the different types f nuclear radiatin may cause a change in the mass and /r the charge f the nucleus. Alpha Decay: AA AA 4 4 ZZXX ZZ 2YY + 2HHHH S alpha decay causes bth the mass and charge f the nucleus t decrease Beta Decay: AA ZZXX ZZ+1 AA YY ee S beta decay des nt cause the mass f the nucleus t change but des cause the charge f the nucleus t increase. Gamma Decay: Des nt cause the mass r charge t change. Half Life - The half-life f an istpe is the time taken fr half the nuclei in a sample t decay r the time taken fr the activity r cunt rate f a sample t decay by half. - It cannt be predicted when any ne nucleus will decay, but the half-life is a cnstant that enables the activity f a very large number f nuclei t be predicted during the decay. - S if 80 atms falls t 20 ver 10mins, the half-life? 80/2 = 40 40/2 = 20 s tw half lives in 10mins S half-life is 5mins - A shrt half-life The surce presents less f a risk, as it des nt remain strngly radiactive This means initially it is very radiactive, but quickly dies dwn S presents less f a lng-term risk - Lng half-life The surce remains weakly radiactive fr a lng perid f time Americium has a half-life f 432 years It is an alpha emitter, and used in smke alarms It is emitted int the air arund the alarm, and des nt reach far because alpha is weakly penetrating If smke reaches the alarm, the amunt f alpha particles in the surrunding air drps This causes the alarm t sund It is suitable because it will nt need t be replenished, and its weak activity means it wn t be harmful t anyne. The number f atms ver time tends t 0.

6 Net Decline - Calculate the rati f net decline f radiactive nuclei after X half-lives Half the initial number f nuclei, and keep ding s X number f times NNeeee DDeeeeeeeeeeee = iiiiiiiiiiiiii nnnnnnnnnnnn nnnnnnnnnnnn aaaaeeeenn XX hhiieeaa eeeellllll eeeeeeeeeeiiii nnnnnnnnnnnn Cntaminatin - Cntaminatin Lasts fr a lng perid f time The surce f the radiatin is transferred t an bject Radiactive cntaminatin is the unwanted presence f radiactive atms n ther materials the hazard is the decaying f the cntaminated atms releasing radiatin E.g. radiactive dust settling n yur skin (yur skin becmes cntaminated) - Irradiatin Lasts nly fr a shrt perid f time The surce emits radiatin, which reaches the bject Expsing an bject t nuclear radiatin, but des nt make it radiactive E.g. radiactive dust emitting beta radiatin, which irradiates yur skin Medical items are irradiated smetimes t kill bacteria n its surface, but nt t make the medical tls themselves radiactive Scientific Reprts Published need t be peer reviewed - If they are n the effects f radiatin n humans, peer review is essential If initial studies gt measurements wrng, safety levels based n the study may cause peple t die. Backgrund Radiatin (Physics nly) - Weak radiatin that can be detected frm natural / external surces Csmic rays Radiatin frm undergrund rcks Nuclear fallut Medical rays - The level f backgrund radiatin and radiatin dse may be affected by ccupatin and/r lcatin. - Measurement f Radiatin Dse, Sieverts (Sv). Uses (Physics nly) - Tracers Technetium is used as a medical tracer Half-life f 6hrs Decays int a safe istpe that can be excreted by the bdy It is injected/swallwed and there is enugh time fr it flw thrugh the bdy and be detected befre it decays away but is nly present fr a shrt enugh time t minimise any harm frm radiatin. It is a gamma emitter, s can pass thrugh the bdy tissue withut being absrbed (as it is the mst penetrating)

7 - Chemtherapy Gamma emitters are used t emit gamma rays, which are directed nt certain areas f the bdy with cancerus cells, which absrb the energy and die, cntrlling the disease It is used t cntrl any ther unwanted tissue t Hwever, as it is hard t direct accurately, surrunding healthy cells may als be irradiated, and their destructin causes unhealthy side effects Nuclear Fissin (Physics nly) - Nuclear fissin is the splitting f a large and unstable nucleus (eg uranium r plutnium). - Spntaneus fissin is rare. Usually, fr fissin t ccur the unstable nucleus must first absrb a neutrn. - When the unstable nuclei absrbs a neutrn it splits int tw smaller nuclei, rughly equal in size and it then emits tw r three neutrns and gamma rays. - Energy is released by the fissin reactin. - This neutrn may cllide with anther radiactive nucleus. - This nucleus absrbs the neutrn and becmes unstable - This nucleus splits, releasing anther neutrn and prduces mre energy - This is a chain reactin, as energy is being released and ne split causes anther t ccur A neutrn needs t be absrbed initially befre it splits, because it needs a little extra energy befre it can split and release lts f energy like a ramp n a cliff, the neutrn gives it enugh f a kick t get ver the ramp t fall - After the nucleus splits, it leaves tw smaller nuclei rughly equal in size - And they bth (alng with the neutrns) have kinetic energy - If the chain reactin is nt cntrlled, it will increase at an expnential rate which is what happens in a nuclear weapn - This prcess is used in nuclear fissin, with uranium nuclei Nuclear Fusin (Physics nly) - This is when tw small nuclei fuse t frm a heavier nucleus, releasing (lts f) energy - The sum f the masses f the tw nuclei is mre than the mass f the heavier nucleus - Sme f the mass is cnverted int energy. (released as radiatin) - The sun is a natural fusin reactr. - Fusin wuld be a much mre efficient way f prducing energy cmpared t fissin hwever n design has been prduced that culd accmplish psitive net energy n earth.