Topic 2a Mass Spectrometry Revision Notes

Transcription

1 1) Mass Spectrmetry Tpic 2a Mass Spectrmetry Revisin Ntes The mass spectrum f an element gives the fllwing infrmatin: Number f peaks = number f istpes M/z f peak = mass number f istpe Size f peak = relative abundance (i.e. percentage) The mass spectrum fr zircnium The 5 peaks in the mass spectrum shws that there are 5 istpes f zircnium - with relative istpic masses f 90, 91, 92, 94 and 96 n the 12 C scale. In this case, the five species detected (with their relative abundances) are: 90 Zr %, 91 Zr %, 92 Zr %, 94 Zr %, 96 Zr + 2.8% a) Operatin Fur prcesses take place inside a mass spectrmeter; inisatin, acceleratin, deflectin and detectin The sample is vaprised and then inised using high energy electrns frm an electrn gun. X(g) + e - X + (g) + 2e - Inisatin is necessary t allw the remaining prcesses t take place. The energy f the electrns frm the gun is cntrlled s that nly ne electrn is kncked ff each atm r mlecule in the sample i.e. nly 1+ ins are frmed The psitive ins are accelerated by a negatively charged plates which als fcus the ins int a beam The ins are deflected by a magnetic field. The amunt f deflectin depends n mass:charge rati (m/z). As the ins have the same charge, deflectin depends n mass with heavier ins deflected less and lighter ins deflected mre. The strength f the magnetic field can be altered s that ins f different m/z are detected When an in hits the detectr a small current is prduced which is recrded. The size f the current is prprtinal t relative abundance

2 b) Uses A mass spectrum can be used t calculate the relative atmic mass f an element (see belw) Mass spectrmetry can be used t identify elements frm the mass numbers f the istpes e.g. in space prbes On anther planet, such as Mars, the number f istpes and their mass numbers will be the same as n Earth. Hwever, the relative abundances may well be different s the relative atmic mass will be different as well. Mass spectrmetry can als be used t determine the relative mlecular mass f a cvalent mlecule (see Tpic 16 Analytical Techniques fr mre details) 2) Relative atmic mass Relative atmic mass is the weighted average mass f an atm f an element taking the mixture f istpes int accunt. Hwever, learn the technical definitin frm definitins sheet T calculate relative atmic mass, add tgether (mass number x abundance/ttal abundance) fr each istpe Abundances can be given as percentages r may have t be wrked ut frm the line heights n the spectrum Example 1: Calculate the relative atmic mass f Cl frm the fllwing infrmatin. 75% f Cl atms have a mass number f 35 25% f Cl atms have a mass number f 37 Ttal abundance = 100% Average mass f a Cl atm = (mass n x percent/100) + (mass n x percent/100) = (35 x 75/100) + (37 x 25/100) = 35.5 Example 2: Calculate the relative atmic mass f unknwn metal Z frm the fllwing infrmatin. M/z Abundance Ttal abundance = 11.5 Average mass f a Z atm = (188 x 1.5/11.5) + (189 x 2.5/11.5) + (190 x 3.0/11.5) + (192 x 4.5/11.5) = = 190.3

3 Example 3: A mass spectrum f a sample f indium shws tw peaks at m/z = 113 and m/z = 115. The relative atmic mass f indium is Calculate the relative abundances f these tw istpes. Difference between mass numbers = = 2 Relative atmic mass lies 1.5/2 alng this difference i.e. 75% f the way Abundances are m/z =113, 25%, m/z 115, 75%

4 Tpic 2b Electrnic Structure Revisin Ntes 1) Orbitals An rbital is the regin in which an electrn can be fund Each rbital can hld up t tw electrns f ppsite spin Orbitals have different shapes called s, p, d, and f S rbitals are spherical in shape and cme in sets f ne P rbitals are hur-glass r egg-timer shaped and cme in sets f three (which can hld up t 6 electrns) D rbitals cme in sets f five and f rbitals cme in sets f seven (which can hld up t 10 and 14 electrns, respectively) 2) Energy levels (r shells) The first energy level (r shell) nly cntains an s rbital, labelled 1s The secnd energy level cntains an s rbital and three p rbitals, labelled 2s and 2p The third energy level cntains an s rbital, three p rbitals and five d rbitals, labelled 3s, 3p and 3d The rder in which the rbitals are filled is as fllws: 1s 2s 2p 3s 3p 4s 3d 4p Nte that the 4s fills befre the 3d Sme examples f electrnic structures are shwn belw.

5 Hydrgen 1 electrn 1s 1 Nitrgen 7 electrns 1s 2 2s 2 2p 3 Sdium 11 electrns 1s 2 2s 2 2p 6 3s 1 Sulphur 16 electrns 1s 2 2s 2 2p 6 3s 2 3p 4 Calcium 20 electrns 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 r [Ar] 4s 2 Irn 26 electrns 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6 r [Ar] 4s 2 3d 6 There are 2 exceptins t the pattern: chrmium and cpper Cr is 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 5 and Cu is 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 10 These electrnic structures are mre stable than the alternative structures that fllw the pattern When transitin metals, like irn and cpper, frm ins they lse their 4s electrns befre their 3d electrns Fe 2+ is 1s 2 2s 2 2p 6 3s 2 3p 6 3d 6 r 1s 2 2s 2 2p 6 3s 2 3p 6 4s 0 3d 6 There are varius mdels t illustrate atmic structure. The mdel chsen depends n what we are trying t explain e.g. bnding can be explained by representing Na as and Cl as with bth seeking t gain a full uter shell. Explaining why cpper(ii) sulphate is blue requires us t use the sub-shell mdel