90 Th He γ

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1 Nam Hnrs Chmistry / / Nuclar Chmistry Th prprtis f th nuclus ar nt f primary imprtanc t chmists. Fr th mst part, th nuclus can b rgardd as a cllctin f nuclns (nutrns and prtns). Th numbr f prtns in a particular nuclus is calld th atmic numbr (Z) and th sum f nutrns and prtns is th mass numbr (A). Atms that hav idntical atmic numbrs but diffrnt mass numbr valus ar calld istps. Th trm nuclid is a uniqu atm, rprsntd by th symbl: A Z X. Typs f Radiactiv Dcay Sm imprtant bsrvatins abut radiactiv dcay. All nuclids with 84 r mr prtns ar unstabl with rspct t radiactiv dcay. Light nuclid ar stabl whn Z quals (A - Z), that is whn th nutrn/prtn rati is 1. In all cass f nuclar dcay th mass numbr and atmic numbr bth cnsrv mass. An alpha particl r α particl is a hlium nuclus. ( 4 2 H). Alpha particl prductin is a vry cmmn md f dcay fr havy radiactiv nuclids. Alpha particls hav a mass f 4 amu, ar psitivly chargd and ar th mst asily stppd f th thr particls (a sht f papr will stp thm). It is th last dangrus. Uranium 238 dcays by alpha particl prductin: 238 U Th H Bta particl r β-particl prductin is th mst cmmn dcay prcss in which th mass numbr f th dcaying nuclus rmains cnstant. Bta particls ar high spd lctrns ( 0 1 ), with a mass f amu, a ngativ charg and can b stppd by a sht f aluminum. Thy ar mr pntrating and mr dangrus than alpha particls. Thrium-234 dcmpss by lsing a bta particl: Th 91 Pa Th bta particl is assignd th mass numbr 0, sinc its mass is tiny cmpard with that f a prtn r nutrn. Bta particl prductin changs nutrns int prtns. A gamma ray, r γ ray, rfrs t a high nrgy phtn. Gamma rays hav n mass r charg, and ar th mst pntrating (svral cm s f lad ar ndd t stp thm). Thy can caus svr damag. Frquntly, gamma ray prductin accmpanis nuclar dcays, such as in th alpha particl dcay f uranium-238: 238 U Th H γ Th missin f gamma rays is n way a nuclus with xcss nrgy (in an xcitd nuclar stat) can rlax t its grund stat. Yu will nt b xpctd t prdict gamma ray prductin. Psitrn prductin ccurs fr nuclids whs nutrn/prtn ratis ar t small). Th psitrn is a particl with th sam mass as th lctrn but ppsit charg ( 0 1 ). Psitrn prductin ccurs in sdium-22: Na 10 N In psitrn prductin a prtn is changd t a nutrn. Th psitrn is th antiparticl f th lctrn. Summary: Typ f Dcay Exampl Why? Alpha Bta P 82 Pb H Occurs in havir nuclids Ac 90 Th Th mst cmmn nuclar dcay. A nutrn changs t a prtn. Gamma Excitd nuclus Grund Stat Nuclus γ T rlas xcss nrgy. Accmpanis thr typs f dcay Psitrn Prductin 13 7 N 13 6 C Occurs whn n/p + rati is lss than 1. A prtn bcms a nutrn. 1

2 Half-Lif Th half-lif (t ½ ) f a radiactiv sampl is dfind as th tim rquird fr th nuclids t rach half th riginal valu. Exampl: Th half-lif f mlybdnum-99 is 67.0 hurs. Hw much f a 100.-mg sampl is lft aftr 335 h? = 5 half livs T dtrmin hw much will b lft aftr 5 half-livs, it is asy t sktch a graph. 100 > 50 > 25 > 12.5 > 6.25 > ½ lif 2 ½ livs 3 ½ livs 4 ½ livs 5 ½ livs Th frmula usd t calcit half-lif is: A=Ai( ½) t/h (mmriz this frmula) Whr: A = th amunt yu hav n hand, Ai = th riginal amunt, t = tim r ag f th sampl H = Half lif Exampl: Calculat th amunt f mass lft aftr 2.0 minuts f grams f X is lft t dcay. Th sampl has a half-lif f 45.0 scnds. A=Ai(0.50) t/h 2.0 minuts x 60 scnds/minuts = 120 scnds A= 200.0(0.50) [120/45.0] A= grams Th half -livs f radiactiv nuclids vary vr a trmndus rang. Fr xampl, Nd has a half lif f 5 x yars, whil P has a half-lif f 2 x 10-4 scnd. Nuclar Transfrmatins Nuclar transfrmatin invlvs changing n lmnt int anthr. In 1919 Lrd Ruthrfrd discvrd that by bmbarding 14 7 N with alpha particls, th nuclid 17 8 O culd b prducd N H 17 8 O H Particl acclratrs ar dvics usd t giv particls vry high vlcitis. Bcaus f th lctrstatic rpulsin btwn th targt nuclus and a psitiv in, acclratrs ar ndd whn psitiv ins ar usd as bmbarding particls. Th particl, acclratd t a vry high vlcity, can vrcm th rpulsin and pntrat th targt nuclus, thus ffcting th transfrmatin. On typ f particl acclratr is th cycltrn. By using nutrn and psitiv-in bmbardmnt, scintists hav bn abl t xtnd th pridic tabl. Sinc 1940, th lmnts with atmic numbrs , calld th transuranium lmnts hav bn synthsizd. Dtctin and Uss f Radiactivity Th mst cmmn instrumnt usd t masur radiactivity lvls is th Gigr cuntr. Th prb f th Gigr cuntr is filld with argn gas, which can b inizd by a rapidly mving particl: highnrgy Ar particl Ar + (g) + - Argn gas ds nt nrmally cnduct a currnt whn an lctrical ptntial is applid. Hwvr, th frmatin f ins and lctrns prducd by th passag f high-nrgy particls allws a mmntary currnt t flw. Elctrnic dvics dtct this currnt flw, and th numbr f ths vnts ar cuntd. Dating by Radiactivity A mthd that has bn vry imprtant fr dating ancint articls mad frm wd r clth is radicarbn dating, r carbn-14 dating. Radicarbn dating is basd n th radiactivity f th nuclid 14 6 C, which dcays via β-particl prductin: 14 6 C 14 7 N Carbn-14 dating can b usd t dat wd and clth artifacts bcaus th 14 6 C, alng with th thr carbn istps in th atmsphr, racts with xygn t frm carbn dixid. A living plant cnsums carbn dixid in th phtsynthsis prcss and incrprats th carbn, including 14 6 C int its mlculs. As 2

3 lng as th plant livs, th 14 6 C/ 12 6 C rati in its mlculs rmains th sam as in th atmsphr bcaus f th cntinuus uptak f carbn. Hwvr as sn as a tr r plant is cut, th 14 6 C/ 12 6 C rati bgins t dcras bcaus f th radiactiv dcay f 14 6 C ( 12 6 C is stabl). Th half-lif f 14 6 C is 5730 yars. Itms with a 14 6 C/ 12 6 C rati half f what is fund in currntly living trs is apprximatly 5730 yars ld. On drawback f radicarbn dating is that a fairly larg pic f th bjct (frm a half t svral grams) must b burnd t frm carbn dixid, which is thn analyzd fr radiactivity. Anthr mthd, using a mass spctrmtr, allws fr a vry accurat dtrminatin f th 14 6 C/ 12 6 C rati and nly rquirs abut 10-3 g. Nuclar Fissin and Nuclar Fusin Cmbining f tw light nucli t frm a havir, mr stabl nuclus is calld fusin. Splitting a havy nuclus int tw nucli with smallr mass numbrs is calld fissin. Bcaus f th bnding nrgis invlvd in hlding th nuclus tgthr, bth ths prcsss invlv nrgy changs mr than a millin tims largr than ths assciatd with chmical ractins. Nuclar Fissin Th fissin f 238 U prducs abut 26 millin tims mr nrgy than th cmbustin f mthan. 238 U n T + 40 Zr n In additin t th prduct nuclids, nutrns ar als prducd in th fissin ractins f 238 U. This maks it pssibl t hav a slf-sustaining fissin prcss - a chain ractin. Fr th fissin prcss t b slfsustaining, at last n nutrn frm ach fissin vnt must g n t split anthr nuclus. If n avrag, lss than n nutrn causs anthr fissin vnt, th prcss dis ut and th ractin is said t b subcritical. If xactly n nutrn frm ach fissin vnt causs anthr fissin vnt, th prcss sustains itslf at th sam lvl and is said t b critical. If mr than n nutrn frm ach fissin vnt causs anthr fissin vnt, th prcss rapidly scalats and th hat buildup causs a vilnt xplsin. This situatin is dscribd as suprcritical. T achiv th critical stat, a crtain mass f fissinabl matrial, calld th critical mass, is ndd. Hmwrk: 1. Dfin nucln. 2. What is an alpha particl? 3. Which nuclids usually xprinc alpha dcay? 3

4 4. What is a bta particl? 5. What transfrmatin happns in bta particl prductin? 6. What is a gamma ray? 7. What transfrmatin ccurs in psitrn prductin? 8. Writ balancd quatins fr ach f th fllwing prcsss C prducs a psitrn Bi prducs a bta particl Np prducs an alpha particl Cm prducs a bta particl 238 U prducs an alpha particl P prducs tw alpha particls and tw bta particls Cf n X + (fill in th missing infrmatin) 14 6 C 14 7 N + (fill in th missing infrmatin) 235 U n Cs n + (fill in th missing infrmatin) 9. Radiactiv idin-131 has a half-lif f ight days. Th amunt f a gram sampl lft aftr 32 days wuld b: 10. Idin-131 is a radiactiv istp with a half-lif f 8 days. Hw many grams f a 64 g sampl f idin-131 will rmain at th nd f 24 days? 11. Actinium-225 has a ½ lif f 10 days. Hw many grams f a 2.00 gram sampl will rmain at th nd f 60 days? 4

5 12. Th half-lif f 55 Cr is abut 2.0 hurs. Th dlivry f a sampl f this istp frm th ractr t a crtain labratry rquirs 12 hurs. Abut what mass f such matrial shuld b shippd in rdr that 1.0 mg f 55 Cr is dlivrd t th labratry? 13. Silvr-111 has a half-lif f 7.45 days. If yu hav grams nw, hw much will rmain aftr 30.0 days? 14. Sdium-24 has a half-lif f hurs. If yu hav grams nw, hw much will rmain aftr 5.00 hurs? 15. Tungstn-181 has a half-lif f days. If yu hav 2013 grams nw, hw much will rmain aftr 365 days? 16. Strntium-90 has a half-lif f yars. If yu hav 2.00 micrgrams nw, hw much will rmain aftr yars? 17. Answr ach f th fllwing qustins rgarding radiactivity. (a) Writ th nuclar quatin fr dcay f Plutnium-234 by alpha missin. (b) Dscrib hw alpha, bta, and gamma rays ach bhav whn thy pass thrugh an lctric fild. Us th diagram blw t illustrat yur answr. (c) Why is it nt pssibl t liminat th hazard f nuclar wast by th prcss f incinratin? 5