ci tue of mater > pny of decor: Stay of atad > isa > moi odes > Daz of neutron > Ban'smad! of atom yf proton: anode rays > dui uber > Mas ber > Hw sctens ae cstibuted in iternt tis se: Eletronic configuration > Vacs ltrs > bes > bas chemistry when John Daltc Wehave studied in Chapter 3 that a great landmark took place in the progress proposed his atomic theory in 1803. Dalton’s concept that matte stoms ruled the scientific world for about 100 years. However, Dalt ty the experiments conducted by J J Thomson, Goldstein, Rutherford, e 20th century. The work of JJ Thomson and Emest Ruthe smaller sub-atomic particles andinthe beginning of beyond dout at atom is divisible and in fact, it consists of sti times from th fom very early experiments on the production b dry hair and then bring the ebonite rod is rubbed with silk or fur and then the rod. pat is th The electrical nature of matter is known fictional electricity. For example, when we com! tatracts small pieces of paper. Similarly, when 2 glass Of ught near an inflated balloon, the balloon is attracted towar electricity? Since it was produced by rubbing we associated with matter. Further, as matter is made up 28 atom, This was further confirmed by Michael Faraday Pased through an electrolytic solution, @ chemical reacto articles made ¥ atoms, this suggests the presence of charge in en he showed that when electricity is occurs at electrodes,Pao CEM eae me = Te os Wiliam Crookes in 1879 studied the condu ae 3 ong gis ie abut 6 fon) ele at oth enc was ted wt Bn Sec a dae Purpose, he t at the ends and also provided Of the gas inside the tube to any desired value as shown in Fig au ne When a high electric discharge of about 10,000 volts is applied betwe are made urrent flows betweer follo When the press ofthe gas inde the dchaige tbe. atm no ( fre the pressure ofthe gas about 20? tm about mm to 10 mm), ec caren sta mm), electric current between the electrodes and a coloured glow taken in the tube, (i) When the pressure is further reduced to about 10 atm ‘about 0.01 mm), the the electrodes disappears but the current kee cP on flowing The tube glows witha faint greenish light Ifa hole is made inthe anode and the tube behind the anode is cated with some fluorescent material lke ZnS, a bight spot is developed on the coating (the same thing happens in TV tube) cathode. These rays were called eathode ray 4.2.1 Properties of Cathode Rays 'ese experiments and found that cathode rays have the followin 44 Thomson (1897) and others repeated th properties: ) Cathode rays emer: i) If an object is put in the path of cathode rays, travel in a straight line. i) Ifa small paddle wheel is pla cathode rays are made of material particles. MY irien an electric field i applied on the cathode rays, these rays are deflected towards the positive pl of ihe applied electric field. Ths shows that cathode rays carry negative charge. Cathode rays ionise gases through which they pase (wi) Cathode rays can penetrate through thin metal foile io ens snaracteristic properties of cathode rays do not depend on the material of the electrodes and also the nature of the gas taken in the discharge tube, On the basis of these observations, it isco of atom, that carry a negative charge. negative electricity, The name was later changed to electron op Particle or atoms of electricity Electron is represented by the symbole or e* Origin of cathode rays: The cathode rays are the gas taken inside the discharge tube due to high speed electrons emanating from the cath 4.2.2 Charge on Electron P experiment. The negative cha? ¢su. This is the smallest measurable quantity of chai High Dc vo Fig. 4.1. Production of cathode ra 'ge from the cathode and move towards the anode. these rays cast a shadow. It means that cathode ray in their path, the paddle wheel starts mechanical motion. It means th {A SIMPLE STUDY OF CHEMISTRY ="OF this ctrodes tube. rations trodes, owin 9 Ne gas Q wing, rays that slate des eces s of ans, rom s by fore, called one Unit of electrical (pate relative charge on an elect, regative charge on electron = 1 zi pelative charge on electron = he F9e. Therefore, 's-1 (minus 10 We say that ele one)” thet electron has 1 unit of coulomb ative charge | Mass of an Electron jut accurate measurements of the g He manson was able to determine charge (erent Bee aim, = 1.7588 « 10% hgh OMS @ = magnitude of ch hee. eh Of charge on elect lectrons under the (m) ratio of el vence of electric and magnetic Bese ol clecwon bss tron in coulomb (C) 1.6022 x 1078 ¢ 1.7588 x10" Ckg® | « im, = 9.412 * 10 kg or 8.11 x 10 g Jfucmssof an electron is negligibly small and is approxi | . ; tely 1/1840 times the mass of a hydrogen atom yaiElectron is a Basic Constituent of an Atom a feathat electrons are found regardless of what gas is tal peetectrodes, suggested that they are universal gasis taken in the cscharge ube and whats the material Setfants of all atoms. jgment found in all atoms. Therefore, electrons are the | tasie const 3 DISCOVERY OF PROTON: ANODE RAYS ie an atom is electrically neutral and the presence of _H.. Catade mie 2S ace Me coming charged part of an atom, i, electron was clearly tablished, it was quite reasonable to think that some | esely charged particles must also be present in the fom The search for such a particle soon began. Goldst ( in 1886 performed discharge tube experiments witha | . ~ x svete puters Hyg) tow | fated cathode as shown in Fig, 42 eae Hig ee J Ongsing high electric discharge through the ges ee pi/adveamiord ueeiats Fresure, it was observed that some rays We" Fred ys Were Fe perforated cathode and produced green fluorescence On the | fe side 3d throu: eee in ai a tnese rays were called anode rays. These.rays were co called ray$ as they | jas | cdated with zinc sulphic id ti harge. oo ; a” ea ano ess ea of a tays moving : rom them and producing positively | Shards the anode, collide with gaseous cathode in the form of anode rays. | | Gated particles, These positively charged P=! | Properties of Anode Rays al rode rays were found to have the following character Shoode rays travel in straight lines al particles. iP tiey are made up of mater .aseous ions Bese parcls ar poste) Cr Oe patie ost the nature ofthe gas taken Te manitude ofthe pOstN® ere cage on ese Pa though positive, i found to be an integral inside the discharge tube. However re produced when high speed ¢\ stoms knod rticles move king out electrons towards thean electron, ie. these particles may carry on ‘multiple of the magnitude of charge on Units of positive charge (The mass ofthese particles constituting the anode rays also depends & However, its value is found to be nearly equal : harged particles were produced ds on the nature of the « 10 the mass of the ato the discharge tube. hen hydrogen gas is 1. Experiments showed that the lightest positvel eae o) inside the discharge tube. The postive charge on these particles s found to be save ne ie, 2.6 = 10" coulomb or 1 unit of positive charge. These sv to be same in magnitude as on ej alled proton.” Mass of Proton Therefore, mass of proton = —_1-6%10™ coulomb _ atomic particle. The proton(s) is (are) present in all atoms. Gey 2. What are cathode rays? 2. What are canal rays? 3: Give the mass and charge on electron and proton. Ue yany After having established that an atom is composed of two sub-atomic Patticles, i, electrons and protons, the next task before the scientists was fo know how these particles are arranged within the atom, i, what is the structure 6f the atom, 4.4.1 Thomson Model of Atom JJ Thorson in 1904 proposed that #An atom is a uniform sphere of positive electricity in which electrons are embedded. == * The number of electrons embedded is such that the total oa) negative charge on all the electrons is equal to the positive __/¢ discovered election and coe charge. So, the atom as a whole is electrically neutral. the first model of the atom This model is sometimes called plum-pudding model, where the pudding represents the sphere of positive electricity and the bits of plums scattered in the pudding are electrons. The model is also, sometimes, called watermelon model because the red, edible portion may be thought to represent the positive sphere and the seeds are ike electrons embedded init Thus, Thomson's model assumes uniform fg. 43 Tyomeong : distribution of mass and charge throughout ee Brn the volume of the atom. ‘A SiMPUE STUDY OF CHEMISTAVES)etc, sidle gas, ken, the b- ks of Thomson's Mode} fomamsens model Wa able to o-fay scattering experimen the over ir the overall ney erford O-ray Scatterin, Vo rast Rutherford, in igi me es at an extre ‘ partic mely'thin (abo OW be ig evacsted chamber Alpha peri spo¥t 0.00006 cm Tint oa Bees me of 4 u (four times that of; lang eh ithe presence of particles around pron) and eMieet Tee Ms the help of a circular Macrae 8th" metal fl wos cent Zn since ‘4 Waly of soon he atom, it failed to explain the results se n rejected, in erimen dtected a nave seovery of Nucleus helium n nucle Bi. the point at which an c-particle strikes the cecus no i ight is see. 8 the screen, a flash of ro 7S r Sir Emoat Rutherford ve ier - 4937) own a5 father of nuclear physics, he Lead sit Salen cls c worked with 3. Thomson in 1895 at cpetor Cavendish Laboratory. He researched a mmr University of McGill, Canada and Manchester University, London. He won Nobel Prize in chemistry in 1908. His os metable nudents Include Patrick aries isc, Nts Bohr, ames. chadik FF Mane cige Otto Hahn and Frederick aM Sold, Ze sue Fig. 4.5 Rutherford a-ray scattering experiment serait IRaresIC6F series of experiments, Rutherford observed that fst of the o.-particles passed through the gold fel without suffering any deflectiog) in their path We ofthe a-particles were deflected through wider angles. Bey few o-particles (only one in 20,000) were deflected through very large angles and some even {traversed ir original path. eds i ich cannon shell at words of Rutherford, "This result wae ® of d it comes bac! obs ae ‘ explained by Thomson's model because according to Thomson's model, the servations could not Pe f the atom and hence, a-particles while and charge are uniformly distributed throughov! seat of through the gold foil Yyould not suffer large de! ns Rutherford Drew from the obssniatlons: se Since most of the a-particles Pass through the gold fil without th space inside the at0™ OPP yo us of ostve a9 ‘Alpha particles which are hey, and arty Tye the atom. heavy and positively charged bon oe ck, this heavy and positively charged body inside the Since pha eaten F a ines was called nucleus. ection from their path, most of .e could only be deflected by some figid and Pp:Fig. 4.6 Scattering of a-patcies by (a) a layer of atoms (b) a single ato 1 f.4.8-Rutherford’s Nuclear Model of Atom tering experiment, Rutherford proposed the following mer ‘On the basis of the observations of his a-ray for an atom. BAT atom consists of a very small, positively charged centre called the nucleus. : i) The total positive charge and almost the entire mass of the afom is contained in the nucleus, The p. the protons present in it, Ns are revolving around the nucleus at extrem cular mot Charge on the nucleus is due to Gi Arrequat-number oF negatively charged elect h Fipest a Sates aS ae Sa eT Ee balances the electronaforce of atvacion bewieen mle 18a eer Rutherford compared his model of atom with oar sytem, As the planets move around the sun, the elec Move around the muces, Therefore Ruthertords model cometites clea leneee/ osck eee Path of electron oround the nce Co ee Gr the ade nt hs been found thatthe rat of atoms are of the order of 10° im, The ado nude of stoms an af the erder of 30° m. Thus, the sizeof the miclus is 1/3,00,000th the see of the aan entated in the nucleus, the density of nuclear matter is vey high, Le. of te ‘© As almost the entire mass of an atom is conc Model of Atom ity of the atom. AcZerding to laws of physics, the electron, while moving around the accelerated and a charged particle lke electron wh of energy, the electron will slow down and move dl finally would spiral into the nucleus. This wo this would happen in only 10-* second. Fortunately, Rutherford's model fails to explain the stability of the ater 1 Another Serious drawback ofthis models that it says nothing about the distribution of electrons around the nucleus.prediction Was Proved to by e true in pombarded by c-particles, a high) 922 by Ja eqreatty no electrical charge on Penetrating ncnaaick called neutrons. The mass of "° "e om He disc scovered that when beryflium is emitte f Tt actually consists of new SS nearly equal to that of proton. These kg, Thus, neutron is slightly heavier rm : mic particles, i.e. electron, prot icleus of the atom, They ar pat They are collectvel particles. Relative properties Pea Wellton ate sornetiia tro ; \ are someti Of these particles are summ es). co | ex i HRS Neeataly capes m se hy sively charged 1/1840 _ 1 . pat are alpha particles? ‘observation of Rutherford’s a-ray scattering experiment led to the fact that there i tom? ithe nature of charge on the nucleus of the atom? Why is it so? Jscovered neutron? Give the absolute value of mass of a neutron. ly called nucleons. 5 called elementary particles oF narised in Table 4.1 NEUTRON James Chadwick Neutral, no charge Zero 14675 «10 g is large empty space inside jhodel of atom or Bohr's theory of atom wes developed by a Danish physicist and Nobel laureate ‘Bohr in 1913 to overcome the ‘drawbacks of Rutherford's model of atom. Postulates of Bohr’s Th this theory are: Neen ane re nuceusin rato ons revolve around the nuc jar orbits, the electron f Out of the several possible aes ae : stain permissi eodated with a definite fixed a ee ay Ore is are caled stationary states oF e allowed to te letters K. Ls M, on energy levels or shells 3°, design is it is nearest to the “s lowest ener9V: is and has order of energies of thes? shells is 2 MeN i Fig. 4.10 Electronic configurations of is “i io and neurons Inside t . snide % (gpl PROBLEM 4.3: What is atomic nu cleus. Write down its electronic co the nucieus, (The imber of Na‘? Calculate the number of neutrons present in rina, ber of neutrons present | UTION: ifeatomic number (Z) of sodium is 11 and its mass number (A) is 23 [fnwbore, number of neutrons = (A- = 23-11= 2 si is he electronic configuration of Na (Z = 11) is 2, 8,1. |[wisobtained when 1 electroh is lost from the sodium atom. Therefore, Na* has 8 extranuclear electrons. |: a Jnreemelpyy re Tieefore electronic configuration of Na’ is 2, 8. Dorotget confused to say that atomic number of Na’ is 10 because it has 10 electrons, Remember that atomic [Inter is equal to number of electrons in an atom. But Nar isan fon } VALENCE ELECTRONS hi ‘and molecules, we studied that valency of an element is its combining Pe eo nss det “he number of hydrogen atoms or chlorine atoms which combine ‘pxaty with other elements. It was defined as the Miprestorrof thet clement. For example, valency of nitrogen (W) in ammonn (NH) is 3 Because an atom Nicombines with 3 atoms of H. Nose and Lewis and Langmuir studied the electron configuration of different elements. Itis observed that is ration and normaly, the electrons present in the outermost shell Sere oct seto coo sce ears el eee ce ecie Therefore, the outermost shel ‘pf an atom is known as the valence shell and te ey rr the outermost shell are known 25 valence electrons. ns present ns and, Chemical Pro! erties of the Elements ee ippdetermin ig cassity ly-take part fonic configuration of these ele Sees 4 a E lements is as shown belo . AG) 2 ‘| al valen 8 2 “ Da : ‘ Paz a BiValencies of the elements are as follows B=4,C=1,D=1andE=1 and D are metals while B, C and E are nonmentals. Lihat is octet rule? cin Zia is the maximum number of electrons that c re there In Si and L shells, find Rs atomic accommodated soso) How many valence electrons 2 fan atom has completely filed K umber of ators Ww Bratire most elements have 2 4 ~ E same atomic numb but sitferant mast numbers re called of the same element which have Bepes ber ie. th Heeisciopes have same atomic NUMBE! Eons in ferent num othe presence of di ees h have same atomic number but diffrent mass number er of protons the difference in mass number isProtium Deuterium Tritium Hor D H or Protium, Because of historical reasons, oth are simply representec erscripts. For example, the two isotopes © Carbon neve same atomic Mp ber Lomi eyiiave came rn proton erica ee ero locos Wile caesar ee es thee Same element have same numberof electrons he te ‘pumber of electrons is same the isotopes of the same element have same electro Configuration and therefore, they displ) identical chemicat properties. 2 Isotopes have different element have different have different physical properties, id N are different as shown STUDY OF CHEM 23fractional Atomic Masses of Some £1 ee ene Elements ort te pyerage atomic mass = “= @ It however, fra ‘ Iverage atomic mass = =e sof the isotopes can be employed instead of HONORS ate calculations, mass numbers of the SEP cnn of ato j For approximn masses. Mass oF is 15.9949 u. It topes with — Average atom : of bromine (Br) having two is o Dans csi ect ca ne ao 79 an : Stier ore Arerage atomic mass of ® is ; a a in Table 4 heled radioisotopes. Applications of Radioisotopes C14 is used to estimate the age of cathode red by J) Th ici qa eee B Bictron sa sub-atomic particle and it to ocelgt (pil 10°" ks). Bride eas were discovered by E Goldstein D Proton is another sub- Je thas one unt of post cla sea r model of atom proposed that electrons are embedded in a postive sphere a of atom proposed iy hares vine Iheatom at the centre and electrons m fovered neutron which is 3 w-atomie particle, Neatom i newteal and ha Bir teory of atom postulates ta ect on BS aie merng a itera Kh < MN B The shells are designated is Kt MN protons peesent B Atomic number (2) is equal See el Bostcnic number is equal to the number B Mass number (4) = Number of Pr he numbers Bilietopes are avoms of the #2 : > lhabars are 2 heeReg SUMMATIVE ASSIGNMENT ‘A. Very Short Answer (VSA) Type Questions (2 mark each) Name an isotop Name the isotope o How many elec Je pre h Atomic numb B. Short Answer-I (SA-I) Type Questi ns (2 marks each) How does a proton differ from an electron? mpare neutron and proton with respect to their mass and charge What conclusion Rutherford drew from his e-ray scatter 1g experiments’ Calculate the number of valence electrons in (i) Na’, and (i) oxide ion (O° Draw a picture of Thoms: 3 model of atom and point out its drawback. An atom of an element 'Y’ contains ber of Y? 3 electrons and 14 neutrons. (a) What is the atomic n (b) What is the valenc (© Is Ya metal or ani metal? ive one similarity and one difference between a pair of isotopes, What are isobars? Explain giving examples Explain one use of each of the following radioisotopes: {a) Cobalt-60 and () Uranium-235 Which is more reactive, Na or Na"? Explain giving reasons, _A SIMPLE STUDY OF CHEMISTRY =8; Mion 2 denen! * A what js the at What isthe ‘ toe phn pe number - hy ave Bi Which one is cation Bp Which one is anion? Bp Which of these representa pir of so An atom of an element has 3 electrons in i ig) the electronic configuratior of the : he atomic number fp the number of protons. FG) its valency I the name of the element its nature, metal or nonmetal Bip Answer (LA) Type Question 5 mark BiGompare the characteristics of * Briar conclusions were derived ia) Most of o-particles just {b) Some a-particles ie) Very few a-partic B Describe Rutherford’s mode! BF Ga) What are valence electrons? Ib) What is the number ‘of warence ectsone in thi e shell {€) Name the valence ‘of this atom by giving © fa) What are isotore*” Expla - {B) Give the number of P neutron Slate the postulates °° Bol Be pane bury heme Jectron, proton . by Rutherford om passed tou te Jes just mnples(oF « are present in hydrogen atom? A) Do is he same element? nost shell N his the outermo electrons in M He imilar chemical properte the electron distributor 2 Mention three imp air of atoms known as yy chemical met te isotopes of an element by che EU Ease say 3.P Rutherford A. 1. (i) Electron (ii) Proton and (jii) Neutron 2. J Thomson 3. Proton, Goldstein 4, Neutron 6. Alpha particles 7. Neutron 8. No 9, I faled to exglarg eon Sete ain the stability of the atom, 10 9 M1. Twelve 12. Eight 13. Eighteen 14. Thirteen 15. Five’ 16.2.8, 7 17, Paes sa ey cairo number of neutrons in their nuclei. 19. Isotopes 20. Two 21. Sixteen 20) Cong 24. None, o-particle 25. 2, 8, 8 ee Zero 3. (a) 12(b)2 4. % of B-1 20 and B-11 = 80 A SIMPLE STUDY OF CHEMISTRY 9