Structure Of Atom
Here you will get complete notes of class - 11 chapter-2 Structure Of Atom
Structure Of Atom
Discovery Of Electron - J.J.Thomson (1869)Discovery Of Proton - E. Goldstein(1886)
Discovery Of Neutron - J. Chadwick (1932)
J.J.Thomson Experiment (Discovery Of Electron)
Sir J.J.Thomson discovered electron by the study of electric discharge in the discharge tube. Discharge tube contains a glass tube with metal electrode(cathode and anode ) fused in the wall. Air is evacuated from the tube with the help of vaccum pump.The conditions inside the tube are high voltage ( 105 volt) and very low pressure( 10-2 to 10-4 atm). When the electric discharge passes between the electrodes,the residual gasses in the tube glow with green colour and the ray which produces at cathode ad moves in straight line with high velocity towards anode are called Cathode rays.
Electron
It is defined as the fundamental particle of an atom which contains one unit -ve charge (1.6x10-19 coulumbs) and mass (9.1x10-31kg) 1/1837th of mass of H-atom.
Properties Of Cathode Rays / Electrons
1. They travel in straight line towards anode with high velocity approximately 107 to 109 m/sec.
2. Cathode rays have negative charge as they are produced from negative electrode.
3. They produce a shadow when a metallic object is placed in their path.
4. They are made up of material particles which have mass and velocity.
5. They are defelected by electric and magnetic fields.
6. They produce X - Rays ,when they strike on the surface of hard metals.
7. They produce green fluoroscent light on the glass wall or on ZnS screen.
8. They effect photigraphic plate.
9. They ionise the gas through which they passed.
10. They can pass thruogh the thin sheets of material.
E.Goldstein Experiment ( Discovery Of Proton)
Goldstein used perforated cathode in discharge tube. He observed that there were colored rays produced simultaneously which passed through the cathode and caused a glowon the wall opposite to the anode. These positive ions passed through the perforated cathode and appears as a +ve rays.They are generated from gas between the cathode and anode ,not from the material of anode.
Proton
It is defined as the fundamental particle of an atom which carries one unit +ve charge (1.6x10-19 coulumbs) and has mass (1.67x10-27kg)nearly equal to the mass of H-atom.
Properties Of Anode Rays / Protons
1. They move in straight lines.
2. They are made of material particles.
3. They carry +ve charge.
4. Unlike cathode rays, the value of e/m ration depends upon nature of gas taken in discharge tube.
5. The magnitude of charge on these particles depends upon nature of gas taken in discharge tube.
6. When hydrogen gas is taken in discharge tube e/m ratio is found maximum.
Discovery Of Neutron
It was discovered by Chadwick in 1932 by the bumbardment of alpha particle on Be atom.
4B9 + 2α4 ------> 6C12 + 0n
Neutron
A neutron is defined as that fundamental particle which carries no charge but has a mass nearly equal to that of hydrogen atom or proton.
Rutherford's Scattering Experiment (Discovery Of Nucleus)
Rutherford bombarded alpha particles (from radium) on thin sheet of metal like gold which was surrounded by ZnS screen.
Observations
1. Most of alpha particles about 99% passes through gold foil undefeclected.
2. Few particles were deflected by small angles.
3. A very few particles {1 out of 20,000} suffer major deflection means greater than 90 degree and even came back in same direction.
Conclusions
1. Since most of alpha particles passed through the foil undeflected,there must be sufficient empty space with in the atom.
2. Since few alpha particles were deflected through small angles ,it means there is some positively charged boby/mass present with in the atom which deflect these positively charged alpha particles which pass very close to this +ve charged body.
3. Since very few alpha particles deflected back therefore these particles being heavy could be deflected back only when they strike some heavy body inside the atom.
4. Since number of these deflected particles is extreamly small this shows that the heavily body present with in atom must occupy a very small volume.
This small heavy +vely charged body present with in the atom was called Neuleus.
According to Rutherford's Model Of Atom , Atom contains two parts :
1. Nucleus:- It carries +ve charge and entire mass of atom is concentrated in nucleus .It's size is very small.
2. Extra Nuclear Part :- It is the space around the nucleus in which the electrons are present.
Drawbacks Of Rutherford's Model
According to Maxwell's Electromagnetic theory, whenever a charge particle revolves in field of force.It loses energy continuously in the form of electro-magnetic radiations .Thus , in Rutherford's modelof atom the revolving electrons come closer to nucleus and ultimately fall on nucleus.It means electrons loses their energy but it doesn't happens .So,there is some drawback in Rutherford's model of atom.
Atomic Number(Z) : It is the number of protons in an atom or number of electrons in a neutral atom .
Atomic Number = NO. Of Protons = NO. Of Electrons
Mass Number(A): It is equal to the sum of NO. of protons and NO. of neutrons in an atom.
Mass Number = NO. Of Protons + NO. Of Neutrons = NO. Of Nucleons
Isotopes : These are atoms of some elements which have same atomoic number but different mass number. For Ex. Isotopes of Hydrogen (H) are 1H1,1H2,1H3.They differ in no.of neutrons.
Isobars : The atoms of different elements having different atomic no. but same mass no.For Ex. 18Ar40,20Ca40.
Isotones: These are the atoms of different elements which contain same number of neutrons For Ex. 6C14,7N15,8O16.
IMPORTANT TERMS FOR ELECTRO - MAGNETIC WAVE THEORY
* One wave contains one crest and one trough.
Wavelength : It is defined as the distance between two successive crests or troughs in a wave.
Frequency : The no. of waves passing through a particular point in one second.
Velocity : The distance travelled by wave in one second.
Wave number : Number of waves present in one cm length.
Amplitude: It is defined as the height of crest or depth of trough.
Electromagnetic wave theory(Maxwell Theory)
1. The energy is emitted from any source continously in form of waves or radiations.It is called radiant energy.
2. Radiations consists of electric and magnetic field oscillating perpendicular to each other as well as perpendicular to the direction of propogation of waves.
3. Radiations have wave character and travel with velocity of light.
4. These waves don't require material medium for propagation.
Electromagnetic spectrum : The arrangement of different types of electromagnetic radiations in order of the increasing wavelength oe decreasing frequency.The different radiations are Cosmic rays,X-rays,UV rays,Visible rays,Infrared rays,Microwaves ,Radiowaves etc.
Energy order : Cosmic rays < γ rays < X-rays < UV rays < Visible rays < Infrared rays < Microwaves < Radiowaves.
There are two types of Electromagnetic spectrum : 1. Emission spectrum 2. Absorption spectrum
Emission spectrum
When the radiation emitted from some source is passed directly through the prism and then received on the photographic plate ,the spectrum obtained is called emission spectrum.
Emission spectrum is of two types
(a)Continous spectra : When white light from any source such as sun,a bulb is analysed by passing through a prism,it is observed that it splits up into seven bands of colours from violet to red.These colours are so continuous that each of them merges into the next. The spectrum is called continuous spectrum.
(b)Line spectra : When some volatile salt is placed in the bunsen flame is passed through a gas at low pressure,the light emitted. For ex. Sodium salt emit yellow light .If this light is resolved in spectroscope,it is found that no continous spectrumis obtained but some isolated coloured lines are obtained on photographic plate. This spectrum is called line spectrum. Each line in the spectrum corresponds to a particular wavelength. The spectra of the elements are described as their finger prints different from each other like finger prints of human beings.
Absorption spectra
When white light from any source is first passed through the solution and then analysed by the spectroscope ,some dark lines are obtained.These lines are supposed to result from the fact that when white light is passed through the solution,radiations of certain wavelength are absorbed and the dark lines are at the same place where coloured lines are obtained in emission spectra.The spectrum thus obtained is called absorption spectrum.
Black Body Radiation
If a substance being heated is a black body,the radiation emitted is called black body radiation. According to elecromagnetic theory ,the energy is emitted or absorbed continuously.Hence the energy of any electromagnetic radiation is proportional to its intensity and the radiatio emitted by the body being heated should have the same colour. However, the change of colour shows that on heating the frequency of radiation emitted is increasing.
Photoelectric effect
When radiation with certain minimum frequency strike the surface of metal,the electrons are ejected from the surface of metal. This phenomenon is called photoelectric effect.
1. The electrons are ejected only if the radiation striking the surface of metal has at least a minimum frequency (v0). This v0 is called Threshold frequency and the minimum energy is called work function (w0).
2. The velocity of electron ejected depends upon the frequency of incident radiation and is independent of intensity.
3. The no. of photoelectrons ejected is propotional to intensity of incident radiation.
* NOTE - Black Body Radiation and Photoelectric effects can't be explained by wave nature.
Planck's Quantum Theory
1. Energy is emitted or absorbed from any source not continously but dis continously in form of energy packets called Quanta. If the energy is emitted in the form of light the term 'Photon' used in place of quantum.
2. Energy emitted is directly proportional to frequency of radiation i.e. E = hv
3. The total amount of energy emitted or absorbed is eqaul to whole number multiple of a quanta E = nhv
Dual Nature Of Light : Some experiments like interference and diffraction show that light has wave character but some other like black body radiation and photo-electric effect show the particle nature of light.Thus light is considered to be of dual nature.It has wave as well as particle nature.
De Broglie Equation
I. In case when photon is of wave character,
E = hv {by planck's quantum theory}
II. In case when photon is considered to have particle nature then according to Einstein's theory,
E = mc2, where m is mass and c is the velocity of particle
Therefore,both the energies must be equal
hv = mc2
OR hc/λ = mc2
λ = h/mc
If c is replaced by v of matter wave, then λ = h/mv or λ = h/p
Significance of De Broglie Equation :- Although De Broglie Equation is applicable to all material particles but it has significance only in case of microscopic particles.
Heisenberg's Uncertainty Principle
It is impossible to measure simultanuosly both the position and of a microscopic particle with obsolute accuracy or certainty i.e. Mathematically δx . δp > h/4π {where δ x = uncertainty in position , δ p = uncertainty in momentum}
Or δx.m δ v = h/4π
If an attempt is made to measure one of these two quantities with higher accuracy then other becomes less accurate.
Explaination Of Heisenberg's Unceratainty Principle
To measure the position of an object light or some suitable wavelength is made to fll on the object which scatter the light. When this scattered light enters ratina of our eye we can see the object.For scattering to take place the wavelength of light used to locate the object should be of same order as the size of object.now if the object is large its position and velocity will not change by striking photons and hence it will be possible to know both accurate position and velocity of object simultanously. In case of microscopic particle,position can be located by using super microscope or by using light of very small wavelength.
Orbital
Three dimensional region around the nucleus in which the probability of finding the electron is maximum.
Quantum Numbers
A set of four numbers which specify position and energy of an electron ,size ,shape,direction of orbital to which particular electron belong.
1. Principle Quantum Number(n) :(i) It determines main energy shells to which electron belong.
Energy Order : K < L < M < N
(ii)The energy of an electron in nth shell of H-atom is given by En= -1312/n2KJ/mole
(iii)It helps in calculating maximum no. of electrons in given principle shell.
2. Azimuthal Quantum Number (l) : (i)It provides information about No. of subshells present with in same principle shell.
(ii) For a given value of (n),the values of l are from o to n-1.
(iii) Relative energy of various subshells of particular shell in the order s < p < d < f.
3. Magnetic Quantum Number (m) : This quantum number helps us to determine preferred orientations of electrons present in a subshell.
(ii)For a given value of l.the value of m are from -l to +l including 0.The total no. of m are equal to (2l+1).
4. spin Quantum number (s): It indicates the orientation of the intrinsic angular momentum of an electron in an atom.It can have values +1/2 or - 1/2.
Shapes Of Orbitals
Shape Of S - Orbitals : A S-orbital has zero angular momentum(l=0) and the probability of finding an electron at a particular distance from the nucleus is same in all directions So, S-orbital has spherical shape.
Shape Of P - Orbitals: It has Dumb-bell shape. In P - orbital,electrons clouds are present in two lobes in opposite direction around the nucleus.
For P - orbital, l = 1 and m = -1,0,+1.It has three orientations px,py,pz.
It means d-orbitals have five directions.These orbitals have equal energy.dx,dy,dz are similar in shape and consists of four lobes of high electron density which lie in xy,yz,xz planes respectively.They have clover leaf shape.
dx2-y2 also consists of four lobes but lie along the axes.dz2 has a dumbbell shape with a doughnut shaped electron cloud in the centre.
1. Aufbau Principle : Electrons are filled in increasing order of their energies in various orbitals starting with lowest energy orbital.
Energy Order : 1s < 2s < 2p <3s < 3p < 4s < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s < 5f < 6d < 7p.
Hund's Rule : Electron pairing will not take place in an orbital of same subshell untill the orbitals of that subshell are singly occupied.
Pauli's Exclusion Principle : No two electrons in the same atom can have identical values for all four of their quantum numbers.