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Nov 6, 2018

Atomic Structure

Atomic Structure:

Constituent Particles of an Atom:
Dalton’s Model to Modern Structure Of Atom: 
1.All the matter is made of atoms are indivisible and indestructible. 
2. All the atoms of a given element are identical mass and properties. 
3. Compounds are formed by combination of two or more same or different kind of atoms. 
4. A chemical reaction is rearrangement of atoms. 
Rutherford has remarked that it is not in the nature of things for any one man to make sudden violet discovery. Science goes step by step and every man depends on the work of his predecessor. 
The journey from Dalton model of the atom to modern structure of atom was long and arduous one. At turn this century many valuable information were being compiled. 
This clearly indicates that Dalton’s atomic model no longer enjoyed the exalted position to grant it. 
Today an atom is considered to made up of a tiny nucleus carrying neutrons and protons. This tiny nucleus has around itself a certain number of negatively charged particle carrying negligible mass, called electrons, arranged in a definite order.

Discussion on the Fundamental Particles of Atom:
Cathode rays - Discovery of Electron:
Gases at low pressures, when subjected to high potential, becomes conducting and various luminous effect were observed. When the pressure is quite low(0.01 mm), the tube remains dark (Crooks dark space) but a streak of rays, named cathode rays, emanate from the cathode. This is confirmed from the fact that a fluorescence is produced on the opposite wall where the rays impinge. The cathode rays have been very carefully studied for their many definite characteristics.
Cathode rays
Deflection of Cathode Rays on Electric Field
Characteristics of Cathode Rays:
(i) Cathode rays excite fluorescence on the glass walls where they impinge.
(ii) The rays have travel in straight lines, confirmed by the shadows of objects placed in their path.
(iii) The rays have penetrating power and can pass through thin metal foils.
(iv) They also possess considerable momentum, small paddle wheels placed in their path rotate from the impact with the rays.
(v) The cathode rays are deflected from their path by the application of magnetic or electrostatic field. From the direction of deflection, the charge accompanying the rays is a negative one.
(vi) When the rays impinge on a metal target, called anti cathode, and placed on the path , a different type of radiation, the X-rays is produced. these new radiation not deflected in an electric or magnetic field. X-rays are really electromagnetic radiation of very short wave length. 
Thus the cathode rays is a stream of negatively charged particles, called electrons and this was first established by J.J. Thomson in 1897
Charge of an Electron:
The electron carrying negatively charged and the charge of an electron(e), 4.8 × 10 ⁻¹⁰ esu = 1.60 × 10⁻¹⁹ coulomb.
Mass of an Electron:
Let the mass of an electron = m and charge = e, then e/m = 1.76 × 10⁸ Coulomb/gram.
Mass of an electron = (1.60 × 10⁻¹⁹)/(1.76 × 10⁸) gram
= 9.11 × 10⁻²⁸ gram.
Coulometric Determination of the Electronic Charge:


Electrode position of silver from an aqueous solution of a silver salt is a suitable experiment for the determination of the electronic charge.
Faraday's Low are readily interpreted by reference to the electrolysis of Silver Nitrate. The change at the cathode requires one electron for every Silver ion reduced. 
That is, Ag⁺ + e Ag 
If the electrons consumed at this electrode is equal to Avogadro Number (6.023 × 10²³ mol⁻¹), 1 mole of Silver metal (107.9 gm) is produced. At the same time, 1 mole of electrons is removed from the anode and 1 mole of nitrate ions is discharged.
Thus, 96500 coulomb of electricity which is necessary to produce 1 equivalent mass of a substance at the electrode will be total charge carried by 1 mole of electrons.
Hence Charge carried by each electron is given by,
e = (96500 C mol⁻¹)/(6.023 × 10²³ mol⁻¹)
= 1.60 × 10⁻¹⁹ C

Positive Ray - Discovery of Protons: 
Since the Electrons contribute negligible to the total mass of the atom and the atom is electrically neutralThat the nucleus must carry particles which will account both for the mass and positive charge of the atom.
We have so far deliberately restricted the discussion on the discharge phenomena at low gas pressure. The production of cathode rays in discharge tubes inspired physicists to look for the oppositely charged ions, namely positive ions.
Goldstein added a new feature to the discharge tubes by using holes in the cathode. With this modification it is observed that on operating such discharge tubes there appeared not only cathode rays travelling from the cathode to anode but also a beam of positively charged ions travelling from around anode to cathode. Some of the positively charged particles passed through the hole in the cathode and produces a spot on the far end of the discharge tube.
Positive Ray - Discovery of Protons
Goldstein Experiment (Discharge Tubes Using Holes In The Cathode)
The nature of these positive rays is extensively investigated by Thomson. It proved much more difficult to analyse the beam of the positive rays than to analyse a beam of electrons.
On deflection by a magnetic and electric field the positive ray beam produced a large defuse spot indicating that the e/m ratio of the constituents of the beam was not same and that the particles moved with different velocities.
Thomson further demonstrated that each different gas placed in the apparatus gave a different assortment of e/m.
Since a H⁺ is produced from a hydrogen atom by the loss of one electron, which has but a negligible mass, it follows that the mass of a H⁺ is same as that of the hydrogen atom(=1).
The particle represented by H⁺ is called a Proton and is considered a fundamental constituent that accounts for the positive charge of the nucleus.
Charge of a Proton:
The Proton carrying Positively charged and the charge of a proton(p), 4.8 × 10 ⁻¹⁰ esu = 1.60 × 10⁻¹⁹ coulomb.
Mass of an Proton:
Let the mass of an Proton = and charge = e, then e/m = 9.3 × 10⁴ Coulomb/gram.
∴ Mass of an Proton = 1.6725 × 10⁻²⁴ gm.
Discovery Of Neutron:
Attempts were now directed towards a correlation of atomic mass number ( = integer nearest to the atomic weight) and nuclear charge (= atomic number). If A stands for the mass number and Z for the nuclear charge of an element, then Z units of nuclear charge means Z number of proton inside the nucleus. But Z protons can contribute only Z mass units. 
The shortfall of the (A - Z) mass units bothered chemists and physicists for the quite same time. Rutherford then suggested this shortfall must be made up by another fundamental particle. This particle has electrically neutral, and mass equal to that of the proton, namely 1. He named the particle in advance as neutron.
The glory of discovering the neutron went to Chadwick, one of Rutherford Students.
An interpretation of the atomic nuclei on the basis of neutrons and protons is now a simple affair. Taking oxygen of mass number 16, for example and recalling that the atomic number of the element is 8, we have an atomic nucleus composed of 8 protons and 8 neutrons.
Since neutrons contribute only to the mass of the element but does nothing towards charge it follows that there may exist species with the same number of protons but varying numbers of neutrons inside the nucleus. 


Such species must belong to the same element and must vary only in their mass numbers. They are called isotopes.
Thus ₁H¹, ₁H², 1H³ are three isotopes of hydrogen, and ₈O¹⁶, ₈O¹⁷ and ₈O¹⁸ are three isotopes of oxygen.

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