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STRUCTURE OF ATOM Class IX

\Background : Jhon Dalton considered atoms an indivisible  entity , but his concept was discarded   in 19th century when scientist were able to find  existence of charge particles as electron and proton and  neutral particle as neutrons in atoms These particles are known as sub- atomic particles.

DISCOVERY OF  ELECTRONS  ( CATHODE RAYS  ) BY  J.J. THOMSON. He  explained the presence of electrons during the study of cathode rays . The facts  charge on electron is – 1.6x 10^-19  coulomb, mass of electron was calculated by Robrt E  Millikan  as 9.1 x10^-27 kg

DISCOVERY OF PROTONS  is done by E Goldstein by his famous anode rays/ canal rays  experiment was able to detect the presence of positively charged particles called protons in the atom it’s charge is + 1.6×10^-19 coulomb mass is 1.673x 10^-27kg of 184times of mass of electrons

DISCOVERY OF NEUTRONS  is done by  J. CHADWICK when he  bombarded  lighter elements  like lithium/ boron with alpha particles  and observed emission of new particles having zero charge but mass equal to that of proton. These  particles  were came to be known as neutron. Neutron are absent in protium ( isotopes of hydrogen ) 1H1 as the mass of electron is  negligible  as compared to proton / neutron hence sum of mass of proton and mass of neutron will give us  atomic mass

ATOMIC MASS= MASS OF PROTON + MASS OF NEUTRON

ATOMIC MODELS  : by the knowledge of presence  of sub atomic particles that is electron, proton and neutron various atomic models were proposed by different scientists such as THOMSON ATOMIC MODEL , RUTHERFORDS MODEL OF ATOM , BOHR’S ATOMIC MODEL The most trusted and  scientifically established model of atom which is adopted was QUANTUM MECHANICAL MODEL OF ATOM

THOMOSON’S ATOMIC MODEL : It is also called Water Melon  Model . He predicted the presence of electrons inside the positive sphere  ( made of proton )just as seeds of watermelon are embedded  in the  edible part  of watermelon .

Although  this  model explained neutrality of atom  but couldn’t  able to explain other  scientific experiments  conducted on atom . but was discarded .

RUTHERFORD’S ATOMIC MODEL : It is famous  alpha rays scattering experiment  Rutherford bombarded  alpha rays upon thin gold foil .He made the following observation from the experiment that i) Most  of alpha particles  passed through  the gold foil undeflected ii) Some of the particles deflected by foil  by small angles iii) One out of 10000 particles retraces  it’s path

Rutherford Conclusions : Atom  consist of predominantly empty space  as most of the alpha particles  gold foil undeflected . Atom contains centrally placed positively charged the nucleus carrying the +ively charged particles  because few  alpha  particles wee deflected and bounced back .Since minute  fraction of alpha particles suffered deflection and very few  retraces it’s path this lead to conclusion that most of the space  an atom  is empty  and the space is  occupied by nucleus is negligible compared to the empty space

The size of nucleus  was about 10^-5 times that of size of the atom. Whole of  the atomic  mass is concentrated in the nucleus. On the basis of the experiment Rutheford proposed model of the atom having following features :There is positively charged particles in the nucleus of the atom Nearly all the mass resides in the nucleus  ( Proton + Neutron ). The electrons revolves  round the  nucleus in the well defined orbits and the size of nucleus is very small compared to the size of atom.

DRAWBACKS OF RUTHERFORD ‘S MODEL : According to Rutherford  electrons revolve  round the nucleus in well -defined orbits, but electrons being charged particles will lose their energy and finally will fall into  the nucleus this make the atom highly unstable This was the major drawback which remain unexplained by him

To over the drawbacks of Rutherford’s model Neil Bohr in 1912are  purposed modified model of structure of atom. He made the fallowing assumptions  that only certain  special orbits known as discrete orbits of electrons orbits the electrons’ d not radiate the energy. The energy is emitted or absorbed by  atom only when an electron moves from one orbit to other

ATOMIC NUMBER : It is the number of proton lying in the nucleus  of any atom is called the  atomic number.  An atomic number  is the identity  of the atom, changing atomic number changing of the atom . The atomic number  is denoted by Z = number of proton for neutral atom no.of  proton is equal to no of  electron

MASS NUMBER : It is the sum of total number of proton and neutron lying in the nucleus of atom  Mass Number = Number of Proton + Number of Neutron  Represented by A                         13  Al ^27  here 27 is mass number and 13 is atomic number electron = proton = 13  number of neutron 27-13 = 14

17 Cl^35 =>  no.of proton = no.of electron = 17  >> no. of neutron = mass number- no of proton = 35 -17 = 18

11 Na ^23  no of proton= no of electron  = 11  >> no. of neutron = 23 -11 = 12

DISTRIBUTION OF ELECTRONS  It is called Bohr Burry  scheme  of electron distribution. The filling  of electrons  in an atom is done in accordance to 2n^2  where n is number of shell      if n=1 (k) =  2 electrons , if n=2 (L ) = 8 electrons if  n=3 (M) >> 18 electrons. but  last orbit has  only not more than 8 electrons and inner shell /antepenultimate shell  has not more than 18 electrons. for example Na 11 = 2 (k) 8(L) 1(M) electrons distribution.

Al 13    k= 2 , L=8 M= 3 electros   Ne 10 = k=2 M=2

VALENCE SHELL AND VALENCY  : From  Bohr Burry  sequence, we know that maximum number of election which can be accommodated in the  outermost shell is 8 . Every element tends to have  8 electron in the  outermost shell ,in achieving 8 electrons atom  can either gain  electrons / loose electron . The number of electrons lost / gained  by the element in achieving 8 electrons determine the valency The electron in the outer most shell is called valence  electrons

  1. C  no. of electrons = 6  distribution of electrons 2,4   valence electrons =4  valency  4
  2.  N  no of electrons =7  distribution of electrons = 2 ,5  valence electron  5  valency = 3
  3.  O no of electron =8  distribution of electrons= 2,6  valence electron =6 valency = 2

ISOTOPES  :  They are the atoms of same elements having same atomic number but different  mass number  isotopes of hydrogen is 1H^1 , 1H^2 , 1H^3   isotopes of chlorine  17 Cl ^35 , 17Cl^37

Uranium isotopes  is used as fuel  in nuclear  reactor , Isotopes of Cobalt is useful in treatment of cancer, an isotopes of iodine is used in the treatment of goiter. Carbon -14 is used in carbon dating .

RELATIVE ATOMIC MASS   In any  mixture  of pure  chlorine 75% of Cl^35 and 25% of Cl^37

average atomic mass = .75×35+ .25×37 = 35.5

ISOBARS :  Atoms of those elements which  have the  same mass number but different atomic number are called isobars. Ca  mass no.=40 and Ar mass no =40 but atomic number are different  Na mass no =24 and Mg mass number =24

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