CHEMISTRY 1903 SHI-LING KOU 2011

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1 CHEMISTRY 1903 SHI-LING KOU 2011 THE ORIGINS OF MODERN CHEMISTRY Evlutin f the atmic thery Daltn s Atmic Thery (1808) Matter cnsists f indivisible particles (atms) Atms f ne element can t be cnverted int atms f anther element Atms f an element are identical and unique cmpared t ther elements atms Cmpunds result frm chemical cmbinatin f different elements atms in specific ratis Current Atmic Thery Atms are made f subatmic particles (prtns, neutrns, electrns) Atms can be intercnverted thrugh nuclear reactins Atms f an element have identical atmic number, but have different atmic masses (istpes) Generally true (exceptins ccur) Spectrscpy and Spectrmetry Atms and mlecules are identified by a set f techniques knwn cllectively as spectrscpy Techniques can invlve examining Electrns in the species Atmic and mlecular mtins Atmic and mlecular masses are measured by mass spectrmetry (develped by FW Astn) Mass Spectrmetry Based n principle that when a mlecule is inised, it can be accelerated thrugh an electric/magnetic field where ins f different mass are deflected t different extents 3 Key steps f mass spectrmetry: 1. Vaprisatin, then inisatin f the sample by bmbarding with electrical discharge Mlecule M + high energy e " M + + 2e M is a radical catin 2. Separatin f resulting ins 3. Detectin f ins Ins fllw curved path thrugh magnetic field smallest ins deflected mst (sharp angle), mst massive ins deflected least Ins cunted by detectr " abundance pltted n graph against mass t charge ratis m/z Where nly singly charged ins are present a mass spectrum lks like this: Astn s results established the existence f istpes (previusly un-shwn fr stable elements) Measured 2 istpes f Ne (20 and 22) and 3 f S (32, 33, 34) etc Recrded as impact n strip f phtgraphic film (nw digital current/luminescence detectrs) NUCLEAR & RADIATION CHEMISTRY Nuclens, nuclides and istpes The unit f mass fr nuclens is the amu, defined by setting the mass f 12 6C t exactly " 1 amu ~ 1.66 x kg

2 Particle Symbl Charge Mass(amu) Prtn p Neutrn n Electrn e Psitrn* e *nt present in stable atms; when meeting an electrn, will annihilate and prduce a gamma ray Nuclide: an atm with a particular mass number and atmic number Istpe: ne f tw r mre atms with the same atmic number but different atmic masses Atmic mass f an element: the average f the atmic masses and abundances f each f the naturally-ccurring istpes Wrk ut by multiplying the % abundance with the respective mass and adding tgether Nuclegenesis the rigin f atms All elements are generated frm H, by nuclear reactins Cluds f atmic H gravitate twards each ther " heat as cmpressed " eventually temperatures high enugh fr nuclear fusin " ignites as a star Fundamental nuclear reactin: Rapidly fllwed by: The verall hydrgen burning reactin releases energy int the surrundings as heat (exthermic) and radiatin (γ rays and neutrins ν) # γ is a high energy, shrt wavelength radiatin with n mass r charge As the star exhausts its H, it begins He burning " frm heavier nuclei increasingly larger atms Heavier nuclei eg 13 C, 13 N, 16 N, 16 O etc are prduced by red giants, even heavier nuclei in supergiants. Can t fuse beynd Ni csts energy and desn t make energy True heavy metals eg 79 Au frm in supernvae Where des the energy cme frm? Overall H burning reactin is: Mass(LHS) = 4 x = 4.031g/ml Mass(RHS) = x = 4.003g/ml Mass(LHS) Mass(RHS) = m = g/ml = x10 5 kg/ml Therefre as E = mc 2 (c = x10 8 ms 1 ) E = x10 5 kg/ml x (2.9979x10 8 ms 1 ) 2 = 2.5x10 12 J/ml energy released The CNO Cycle takes place in mre massive stars: E = 2.5 x J/ml f reactin, divide by 4 (fr each atm) = 6.2 x J/ml f H atms Nuclear burning liberates 4mn times mre energy than burning O with H Natural radiactivity radiactive decay prcesses 4 mst imprtant prcesses: α decay, β decay, β + decay, electrn capture Unstable istpes underg radiactive decay t fall int the zne f stability (see graph)

3 α decay: the α particle is an He nucleus. Happens when atm is unstably large eg abve Bi β decay: neutrn changes int a prtn and ejects an electrn. Happens when t many neutrns β + decay: nuclei is t psitive (nt enugh neutrns), s ejects a psitive charge (p = n + e + ). The β + usually quickly cllides with its antiparticle β in the surrunding envirnment " annihilatin, γ ray prduced...fllwed by Electrn capture: e plucked frm 1s shell, placed in nucleus t reduce psitivity. e in uter shells shuffle t fill inner hle " prduces X-rays Randm cl facts: ur slar system is the recndensatin f supernvae type II. Earth s radiactivity keeps the cre mlten. Natural radiactivity examples Over time, unstable nuclei spntaneusly decay thrugh a series f unstable intermediates, frming a family f decay prducts (daughter istpes) in a decay series Eg 238 U decays int... decay series α decay is a decrease f 2 prtns (Z) and 2 neutrns (N) β decay is a decrease f Naturally 1 neutrn ccurring and an U cntains increase U-238 f s 1 therefre prtn will als cntain cmpnents f the Nuclear stability 2 main factrs determining nuclear stability: size f nucleus, and cmpsitin f nucleus (N:Z) Size: there are n stable nuclei heavier than Bi (Bi s half life is 20bn,bn years, s technically it isn t stable either =P) Neutrn t Prtn rati: all knwn stable nuclei fall inside the zne f stability which has a N:Z rati f 1:1 t 1.52:1 (mre neutrns needed per prtn as nucleus gets larger) That is, unstable istpes must decay twards the zne f stability, finally falling belw 209 Bi N/Z t high (t many N) " β decay N/Z t lw (nt enugh N) " β + decay

4 Example: carbn istpes Fr example, 11 C will emit psitrns since it s n the right side f the line and N/Z is t lw while 14 C will emit electrns since N/Z is t high. Each nuclide decays twards the zne f stability Heavier nuclides than 209Bi decay by a cmbinatin f mechanisms, using α decay and ther mechanisms t change N/Z α decay by itself will nly reduce mass but wn t get atm int zne f stability β decay must als ccur t change N/Z Fe is the mst stable element (fr really cmplicated reasns) Electrn capture ( inverse β decay ) nly ccurs in heavier, higher density atms Nuclear stability rigin f decay mechanisms T understand reasns behind the empirical stability rule, cnsider frces between the nuclens Stability f a nucleus invlves cmpetitin between 2 frces: Culmb (electrstatic repulsin) between prtns pushes them apart ver a lng range Strng nuclear frce, a shrt range attractins between all nuclens The main functin f neutrns is t cntribute t the binding f the nucleus withut als cntributing t the electrstatic destabilisatin Takes huge amunts f energy t bring prtns clse tgether nly gets energy back when strng frce kicks in at the shrt range In nuclides with t few neutrns, the electrstatic repulsins verwhelm the strng frce As the nucleus gets larger, the lng-range electrstatic repulsin between prtns accumulates and eventually verwhelms the strng frce, even if N/Z is ptimised Neutrns want t decay Neutrns frm the sun nly last 10mins in space, nly very high speed nes reach earth N weighs mre s when decaying, releases energy (entrpic frce) Hwever, wn t decay if making a prtn will cause an imbalance in the nucleus Unstable nuclei are present in nature because: Unstable nuclides cntinue t be frmed by nuclear reactins Sme unstable nuclides have very lng half-lives, s they simply haven t decayed yet Fr each parent nuclide that decays, a daughter nuclide and a particle r γ is prduced

5 Half-Life Half-life: the time required fr half f the nuclei t underg a decay event We als realise that the RATE f decay (ie the number f disintegratins per sec) halves ver the span f every half-life Disappearance f a radinuclide by radiactive decay: T determine half life, put N(t) as N 0 /2 and slve fr t (fr a knwn λ) t 1/2 = ln2/ λ Activity Activity A: rate f emissin, ie the negative f the rate f decay f nuclide A sample s activity is prprtinal t the number f nuclei present Fundamental units f activity: measured in becquerels, r decay events per secnd Bq nt as useful as scale is t small Curies (Ci) are mre practical units 1 Ci equals the number f nuclei disintegrating each secnd in 1g f 226Ra 1 Ci = 3.7 x Bq (r cunts per secnd) Half-life and activity are related lw activity = lng half-life, high activity = shrt halflife Mlar activity = activity per mle Specific activity = activity per gram Mlar activity, specific activity and half-life are all independent f the amunt f radiactive material present in the sample Example: what is the mlar activity f 13N, which has a half-life f 9.96 minutes? 1. Cnvert minutes t secnds: 9.96mins = 598sec 2. Find λ thrugh the frmula: λ = ln2/ Slve fr A M : A M = ln2/598 x x where N A = number f atms per mle = 6.98 x Bq/ml = (6.98 x )/(3.7 x ) Ci/ml = 1.88 x Ci ml 1 In a decay series, each step has its wn half-life Example U-238: U-238 s half-life is 4.5x109 years, s many f the atms present when the earth was frmed still haven t decayed yet Hwever, daughter nuclides (ther elements) can decay much mre quickly Half-lives can be as lng as billins f years r as shrt as ms r less Carbn dating 14 C is cntinuusly prduced n earth 99% f the naturally abundant 14 C is prduced in the upper atmsphere when high-speed neutrns frm the sun react with 14 N; 14 C enters bisphere thrugh frming CO 2, disslving in rain etc Prductin rate is 2.5atms/cm 2 /sec with a glbal inventry f 3 x C atms (90% ceans, 8% bisphere, 2% atmsphere) Typical [ 14 C] in sea water is 1.2 x 10 9 atms/l (2 x M) Other surces f 14 C: in-situ (0.1%) prduced by spallatin reactins, undergrund radigenic ie by decay f U and Th (useful in studying hydrlgical envirnments),