Chemistry Past Paper PDF

INTRODL CTIOY ll is a human instinct to classify, ie, to put things of one kind in one group and of anothe nd another group. low ards the end of the 18h and the beginning of the 19th centuries, more and more elements b discovered. and their individual properties and com...

INTRODL CTIOY ll is a human instinct to classify, ie, to put things of one kind in one group and of anothe nd another group. low ards the end of the 18h and the beginning of the 19th centuries, more and more elements b discovered. and their individual properties and compounds had begun to be studied. Soon. the nee classifying them so as to make their comparative study casier. 3.1. REASONS FOR CLASSIFIC ATION Obviously. scientists were in seh c: OF ELEMENTS characteristics of anelement that would ne nang: l. It helps in studyingthe elemenis in an organized It was William Prout who discovered that t ton manner. mass of an element neer differs and tihat coa2 2 Jt helps in correlating the properties of elemeat therefure. form a iruly scientific basis tor 4 with the fundamental properties of all Slales of satisfactory scheme of classification. Jnatter 5.2 DOBEREINER'S TRIADS 3. I helps in defining the relationship of on J. W. Dobereiner, a German chemist. obsene element with another. that certain elements displayed similar properues an In the beginning. attempts were made by chemists that they could be placed in groups of three element to classify elements on the basis of factors such as each. These groups of three elements each ueR density, malleability. ductility. etc.. and also to consider called triads. For example. lithium, sodium anu whether they were metals or non-metals. But none potassium were placed in one triad. of these early classifications proved satisfactory The three elements of a particular triad hac because : similar chemical properties. In fact. Dobereiner wa (i) the number of elements falling in a paricular the first scientist to show the clear relationshi, group were so large that it did not serve the between properties of an element and its atomic purpose of generalization. mass. (ii) some of the characteristics being considered varied under differing conditions. The atomic mass of the middle element of à (iii) certain elements showed metallic as well as Dobereiner's triad was approximately equal tO u non-metallic characteristics. arithmetic mean (average)of the atomic masses O the other two elements of that triad. 78  AhutneethAS nd tound that evefy ighth elenent had properties smilar to the properties of the first element. just us ih' iluv ns, caleium, s(ontiunn and the cighulh note of a musical octuve is the sume as iIN fust note. Bascd on this observation, Newla)u Ne hhs law of octaves for classification of eleents/ Acordny to Newland's law of octaves, when cemeN rc rngedby inceasing utomic mUss, tle Poperies of everv Cighth clement starting from unY clement ae dtepeiion of the propcties of the Western Do Re Musie Indian R M. Dha Ll Musie t h C. 4 , men alue of he alomme B ( Na Me S P K C. Mo Co and NI (u As Se b Take threelements chlorine (35:5), bromine Bi Rb Sr 80) and iodine (127); thev also tom a iad because: Newland divided the clements into horizontal rows of se\Cn clenments cach, as shown above (the noble, i.e. inert, gases were not known at that tinme). Merits of Newland's classification -SI5. u hich is pprovimlcly I. This sy slem worked quite well for the lighter clement. For eumple, lihium, sodium and potassiumn were brought together. Reasons for discarding the law of triad 2. ll relales the properties of the elements to their D'rme uled teIALge all the tlen known atomic masses. 3. For the irst time, il was shown that here is a distinct periodicityin the propertics of elements. Newland was honoured in I887 Ior umpl, taking halogens, viz. the first The Royal Society presented the Davy Medal to thiee membets, Fluorine (|9), Chlorine (35:5) Newland in 1887 for his work on classification of ihd Bromine SO), i i observed that the mean clements. the llomie nnasse ot Fluoine and Bromine 0s 719+ S0) = 40.5, n0t 35:5. Reasons for discarding the law of octaves " This classification did not work with hcavier So, Dobereiner's scheme of classification of clements) ie. those lying beyond calciunm. As Clment Was n0 \ery sucCesstul. though it did more and more elements were discovered. they Mn inportant insights and principle. could not be fitted into Newland's Octaves. S3 \LI \ND'SLIW OF OCTAV ES Newland adjusted two elements cobalt (Co) and ohn New lnd (a wientist and a lover of musie) nickel (Ni) in the same slot and these were placed anged elements in ascendling order of alomic mass in the same column as fluorine, chlorine and 79  bromine which have very different properties Periodic table is a chart of elemen:s than these elements. in such a way that elements W repar smila Iron, which resembles cobalt and nickel in properties occur in the same veriic olum becau properties, has been placed far away from these (or group). It is called periodic imen elements. with similar properties Occur at fixe, ler tah 54 MENDELEEV'S PERIODIC TABLE (or periods); and it is called a CCau In 1869, Dmitri Ivanovich Mendeleev, a elements are arranged in tabular forn Russian chemist, arranged all the 63 elements Essential features of Mendeleev's per tabl known at that time in increasing order of their Mendeleev's Periodic table conia: verti atomic mass. Elements with similar properties were columns, called 'groups' and horizonta: Scali put one after the other in the same vertical column 'periods". (group). with blank spaces where the expected " There are in all eight groups, i.e. Grou periodicity of the properlies was disrupted, i.e., VIII. Each of these groups from Ito Vi divK. where the properties of an element did not tally into two sub-groups : A and B. Group lhas with the properties of the element placed above it. sub group. Inert gases were not known that uin: He observed that elements with similar properties " All elements of a sub-group (or of Group VI: occurred at regular intervals. He called this as the which has no sub-group) have similar propert: periodicity of the properties of elements. and show the same valency, which is equal to t Based on this, he propounded a law that is called called group number (for upto Group IV) or group nub: Mendeleev's Periodic Law. subtracted from eight (for Groups V-Vil). Mendeleev's Periodic Law " In a period, elements gradually change fror metallic to a non-metallic character or metal. Physical and chemical properties of elements character decreases as one moves from left to ng are a periodicfnction of their atomic masses. across the horizontal row. Mendeleev's Periodic Table was published in a German journal in 1872. He used letter R' to represent the element of that particular group. Hydride of any element of group IV is written as RH4. For example. hydrid: of carbon is CH4. Oxide of any element of the same group is RO, (CO,).  Mendeleev 's table (i) Tellurium with atomic mass 127-6 precedes Grouping of elements. He fthe elements then knowngeneralized the study iodine with atomic mass 26-9. to a study of mere cight groups. (2) Position of isotopes :., Gaps for Isotopes ofan elenent are aoms of thuL element undiscovered elements. In order Io make sure that clements having similar Laving similar chenical properties but different properties fell in the same vertical column or atomic masses. group. Mendeleev left some gaps in his According to Mendeleev's periodic law, isotopes periodic table. These gaps were left for of an element must be given separate places in the subsequent incluSion of elements not known at periodic table since they have different atomic that time. Mendeleev correctly thought that masses. But they were not assigned separate places. Such elements would be discovered later. (3) Grouping of chemically dissimilar clements: Prediction of properties of undiscovered Elements such as copper and silver bear no elements. He predicted the properties of the resemblance to alkali metals (lithium, sodium, ctc.) then unknown clements on the basis of the but they have been placed together in the first properties of elements lying adjacent to the group. vacant slots. He actually predicted the properties (4, Separation of chemicaliy similar clements: of some undiscovered elements in 1871. Elements that are chemically similar, such as For example : gold and platinum have been placed in separate ùEka aluminium (means cne place below groups. aluminium in the group): its atomic mass and (5) Eleciron arrangement : chemical properties are quite similar to those of the element gallium discovered later on in 1876. It does not explain the electron arrangement of elements. (ii) Properties of eka-silicon are the properties of germanium. (6) Position of hydrogen : Property Eka Gallium Eka- Germanium Hydrogen was not given a fixed position. It was aluminium silicon considered in Group IA as well as in Group VIIA ALomc mass 68 69.7 72 72.6 because it forms both a positive ion, viz. in HCI, and Dersty 5.9 g/cc 59] g/cc 5.5 g/cc 5.36 glcc a negative ion, viz. in NaH. 302 K High 123J K Melung point Low 4 5.5 ATO\MIC V VIBER AS BASISFOR MODERY 3 4 Valency 3 PERIODIC LAW Formula M,O, Ga,0, MO, GeO, The magnitude of positive charge present in the corrected. He was able nucleus of an atom was determined by Henry 4) Incorrect atonmicmass Moseley, an English physicist. to correct the values of atomic mass oJ by placing these elements like gold andplatinumsimilarities In 1913, Moseley used anodes of different metals by Clements strictly on the basis of in their in a discharge tube and subjected them to attack cathode rays. Hefound that when cathode rays struck properties. anodes of different metals, the wavelengths of the Delects in Mendeleevv's periodic table rays rays produced change. The wavelength of these Anomalous pairs : did not follow was found to decrease in a regular manner on The pairs of elements changing the metal of the anode in order of their following Irndeleev's principlesatomic : mass 39.9 precedes position in the periodic table. By this, he concluded i) Argon with 39-]. that the number of positive charge present in the pPolassium with atomic mass precedes nickel nucleus of an atom is the most fundamental property 58-9 Cobalt with atomic mass of an atom. with atomic mass 58-6. 81  The nunmber of unit positive charge present in the Explanations for anomaies in Mendelee nucleus of an atom of a particular element is classification of clenents called the atomic number of that element. 1. Position of isotopes Some of the positive charge present in the Since all isotopes of an clement bave the sa nucleus is due to protons and the number of protons nunber of protons, theit atomic nunmber is also same. And since all isotopes of l clement huve l. is cqual to the atomic numbcr of that element. sane atomic number, they can be put at the s : Thus. Henry Moseley found that atomic position in the periodic table uumber is a better fiundunmental property of elements 2. Position of anonalous pairs COmpared lo ulonic mUss. This led to the modern periodic law. The atomic number of agon is l8 and its ma number is 40 while the atomic unber of potavsuun Modern periodic law : Pysical nd chemicul is 19 and its mass number is 39, Now, odinp o properties of elements arca periodic Junction o the Modern periodic law, clements ae arnpcd m heir atonnic nunnbers. increasing order of their atomic nunbe. Soaon. with its lower atomic nwnber, shoukl cone us, al If clenments are arranged in order of their potassium, with its hipher atommc nunber, should increasing atomic number, those with similar cOme l:ater. properties are repcated after regular intervals, In a sinilar way. the poition of cobaltall i.e. periodicity in the periodic table occurs based on the atomic numbers (number of protos). nickel are also justilicd.  corresponding periviiin And, of showshow increasing 5.6 ) (v) (iv) (u) respeetively. example, sodium, have (V) (i) (i) valence They : Consider since When n fouwOmbncd are are PERIODIC MODERN TABLE mpat stale. they atomic periodiciy e potassum, VeIy good have sott the the atomic elecirons ddes asi elementsare olor tCacUVe thmetals. e numbers tirst (regular a number es ducmg group numtes, With thto valency rubium,group curat e and of with , ot rpenl valene arTange arlents. tlame hy th e ne el en.drogen. l1. otthe when thus nd 19,NUN vnirhnDmT elements. ininYN ties tound tom 37. emAN they 0D0 DE aud S$ burn. manly unipositive tranum. able. Lithium, imiKIN and in S7 tor rTis oup. a same. perties, down configuration the configuration. Thichclectronic the of periodicity elements. of(Propertics properties decreasC nigur.ilions wierN outermost Ina periodicity outermost li So puticular group, there is irogen. h clements nod though al that is and regular s ell orbit ishave similar. reappear orbit the tha and the radual of the remins similar so intervals, same number thc phenonnenon i.e., clements hey Valeney the variation, at same number properties. have number the of recurrence rcgular with of same, group are simlal shells also called I.e. intervals, Nilr of is Thus, knOwninerease clectron(s) sinilar have i.e. electrons remains of eases clectronie perivdic POe" he cletoIC similar or cause ltes. the as Or n in in  colleeively (a) Main Goup 5.7.3. t, 5.7.2. d)() (b) ) (e) (d) () (b) ()Main () (} (0 \ifttt lom Dot They They atories.Acinides boiling points. They TheInner compounds coloured Most MoOst Some They characteristics bolng pontsand Most magnet. characternsticAl and10,1L Theyvalences. vanableSone Non and COnmods Mlealli salsnvng found , Tansition 7. Metal, hadttish clements thesc are fom show are ie. of of of are are ctals, cleity. known transition of heaver the these thee thee these good inare coloured hcaVy clements nuded 0 whheh natue nature. all variable lanthanides Conductorswhh wh clements as of clements clements clementselements, conductosof clemcnts sof adoacuve metalsinnerofinner elenentsthe of armcaseon e are non ith ions. sixth are n regrevcntatvc hey 1 valencies ransition Groups present ne good mctals ta ransition exhibit metals (d-block with transition to m are are hhe presentof and and are clements. hig h seventh t he (-block coloured vanable usedattracted heat nheatconductors made clements: w 3. n clements. as and ith elements): 4, and and n OvW! elementsacinides, meling high Poups oups clements in elements): catalysts. 5, eleeticity. lead, Some periods valencies. ions electricity. towards 6, down special nelting I of 7. exhibit oand are and : are and 8, and heat ay of a 9. 2 shellsclectrons clectron to increases GROUPS () 5.10 5.9 MERITS5.OF 8 summaze (Cvept other which 2. I. (4) (3) (2) )elements. typical called cements The Iypical Suhcements Be ot Bridge As PERIODIC TABLE GENERAL elements, Group SCobBacc propcrties m Numbers into propertics I DEFECTS PositionTABLE thrd Penod Penod 3 and the Ilcmets fauls reproduce. understand Modern clements and group. a It (oatommic Position better even mass.to clemcnts remains we th e thwdpeood AL the arithmetically and shows its is penod the elemens:Ilemcnts B Hc, go main to of based and whh electronic properties downshells ofTRENIDS elementina aCCOMmOdate i.e. hvdrogen relate periodic of of cqual OF on regular THEMODERN elements, Na ecalledbridye S de given body the moving fundanental an on leadmy bsonl he lanthanides to THE element of Zen belongs. ogroup, aand atonnic thecoC, clement of both is tableconfiguration. changes hen Na, M Ber Wtheletser the valence the still MOIDERN acrOSs toadiagonal of ) IN Group is tespeciVe elements, thc oup nunber numbervalen of periodic the not in number, My. 0hey casier property P:.RIODICIBL the AI 13 sec Ct thAlso, cquals numbre number THE andsatisfactory. a AI. (ond whh electrons inner I period in table S, o of the and PERIODI toproperties which proup relationship. v I number the MODERY table. remember P 14 land peIod trans1tionGroup actinide. compard is S ol or and gr she doW related nd shuy : as is a M d  uniformly down nuhc tey tons Properties tements Ctronic ().is (GroupValencyIGroupValency(GroupValency(GroupValencyValencyValency Ptassium SxdumLathium Element the n clextllrons cleatrons balency: groupof havevery Elements hunge group. tlhat n gIven a Valency Groupgroup Valencyof mpl. particular Valeney tableelenents A B CI Flement crrodthe inpresent their configurations. number number number ot number of number of of of ol Group valency of group group group K Na below. their shoWn below as to milurplhysical icalproperties,nd respective inof group group group group 1 ingrOUp in an of period equalsthe whehit elements respective of a number shellsNo.of 17: 16. VIA 15; VA 14: IVA 13. lIlA I A lA its clement VIIA anygven 6 3 2 18 elements18 clements clements configuration 14 is413- clements I8 elements Electronie valence ero elements clements 2, 2,I haVe 8. 8. belongs. outermost group : - - - - alkali Because (or 17= 16 15 10 10 8, I 2, 1 group valence shells, anequals = = Is = = is is shell. 2) is 3) 4) 3) is metal Group ) is 3 3 2 I cqual shells poSSess I 2 is 2, 2.I8,7 2. 8,8,7 2. 2,7 ElectroicConsider configuration the valenceNo, of isthe Since the 8, 8, electrons 18, 18, is number number I, same. number number 32, I8, the as elements halogens similar shown valency 18,7 7 same. For 18) of of of we of Character ofThe character : ofarsenic Elements of Note electronic very have character: 1, acidic Oxide Group elements become metal. aGroup typical and Us (5) lithiumtoSIZe increase caesium. "CCessive increases. elements For This group, (4) Halogen(b) Examples: (a) the above oxides move one Atonmic It character increases Metallic Metallic 1ncreases AtomiC SIZe exanmple, increases is As degree fluorine iodine. to group.the tAlkali he small I : is 15, Li, of metals size, metals. alkali elements account as in number moves reactivity atomsare electron. NO,: a Group one of : Reactivity increseS of has group. down the FBe>Li> period of of of character increases) (Acidic elements from Metalloid and Al,O, period become non-mnetallic. B, C, from the most elements atomsperiod, pull an : of left gradually left then mostReactivity in an of (increase atom the For to : the of reactive : ofelement its are alkaliwhich 3rd progressively SiO, elements. reactive to increases. electrons in decreases 0xides N,2nd elements : the example, right Non-metal decreases,right outermost decreases atoms prot the in period period The smallest P,O, and of 0, NeF metals, first non-metals. in depend number when last have considerfinally the SO, elementsIn less element i.e. given a Group Thus valence decreases while in orbit it a unon thesize). moVing of CI.0. chan: basic chang: perio tho perio laty Si