Class 11 Structure of Atom Notes
Structure of Atom
DISCOVERY OF CATHODE RAYS :
- To produce cathode ray , discharged tube is used .
Discharged Tube : It is long glass tube filled with two electrode at ends of the tube and a vacuum pump .
Function of vacuum pump : To keep the desired pressure inside the tube .
- J.J Thomson took a discharged tube and applied a potential difference of 10000 volt across electrodes at a pressure of 10-2 Mm of Hg . He found some glowing behind anode .
It means some invisible rays produced at cathode strike behind anode and produce Fluorescence .
He named them cathode rays .
PROPERTIES OF CATHODE RAYS :
i) These rays have mechanical energy and travel in straight line .
ii) These rays are deflected towards positive plate of electric field . It means these are made up of negatively charged particle called electron
iii) Characteristics observed is independent from natural gas .
iv) Mulliken determined the charge on electron which is 1.602 x 10-19 C .
v) Specific Charge on electron is calculated by J.J Thomson .
Discovery of anode rays
Goldstein took a discharge tube having perforated cathode. He applied 10,000 volt potential difference at pressure of 10-2 mm of Hg. He observed something glowing behind cathode. This was due to striking of some invisible rays produced between anode and cathode.
These invisible rays are named as canal rays or anode rays.
Properties :
i) These rays deflect towards negative plate of applied electric field. It means these are made up of
positively charged particle.
Note: If H₂ gas is taken inside the discharge tube, then anode rays are made up of proton.
(ii) Property of anode rays depends on nature of gas.
(iii) These rays travel in straight line and have mechanical energy.
DISCOVERY OF NEUTRON :
Chadwick in 1932 found the evidence for the production of neutron in the following reaction :
49 Be + 24He à 612 C + 01n
Neutron is charge less particle and have mass slightly greater than that of proton .
# THOMSON'S MODEL OF ATOM :
J.J Thomson in 1898 , proposed a model of atom which looked more or less like plum pudding .
He assumed atom to be spherical body in which electrons are unevenly distributed in a sphere having positive charge which balance the electrons charge . it is called plum pudding model .
Important Feature of this Model : The mass of the atom is assumed to be uniformly distributed over whole atom .
Failure : It failed after Rutherford's Alpha Scattering experiment , which proved atom to be quite different .
RUTHERFORD'S MODEL :
- Rutherford's model is based on alpha scattering Experiment
Scattering of a-particles are interpreted by Rutherford from this experiment, he observed that
(i) Most of the a-particles pass without any deviation
(ii) Few a-particles deviate with small angle.
(iii) Very few a-particles retrace its path or show deflection greater than 90°.
On the basis of these observation, he proposed a model.
(i) Atom is of spherical shape having size of the order of 10-10 meters
(ii) Whole mass is concentrated in center called nucleus having size of the order of 10-15 meters.
(iii) Electron revolves around the nucleus in circular path like planets revolve around sun.
Limitation:
This model could not explain stability of atom. According to Maxwell's classic theory, an accelerated charged particle liberates energy. So, during revolution, it must radiate energy and by following the spiral path it should come on nucleus within 10-8 seconds.
Atomic Number and Mass Number
Atomic number: It is equal to the number of protons present in the nucleus of an atom
Atomic number is designated by the letter 'Z'
In case of neutral atom atomic number is equal to the number of protons and even equal to the number of
Z = Number of protons (p) = Number of electrons (e)
EXAMPLE :
In case of hydrogen atom
Number of protons (p) = 1
Number of electrons (e) = 1
Therefore, Z = 1
MASS NUMBER :
It is equal to the sum of the positively charged protons (p) and electrically neutral neutrons (n).
Mass number of an atom is designated by the letter 'A'
Nucleons: Protons and neutrons present in the nucleus of atom together constitute nucleons.
Mass number (A) = Number of protons (p or Z) + Number of neutrons (n) = Number of nucleons
ELECTROMAGNATIC WAVES :
- These are the waves which are produced by varying electric field and magnetic field which are perpendicular to each other in the direction perpendicular to both of them .
Properties :
i) These travel with velocity of light .
ii) These rays do not deflect in electric and magnetic field .
- Electromagnetic waves have two components :
i) Electric component
ii) Magnetic component
1) Wavelength : It is the distance between two consecutive crests or troughs and its denoted by ג.
2) Frequency (V) : It is the number of waves passing through a given point in one second .
3) Wave number (⊽) : It is the number of waves present in a unit lenght
4) Amplitude : It is the height of crest or depth of trough
Intensity ∝ (Amplitude)2
5) Velocity : It is the distance travelled by one wave in one second .
C = Frequency x wavelength
Particle nature of electromagnetic radiation: Planck's Quantum Theory :
- Till now the electromagnetic radiations were thought to show wave like character. And no doubt, wave character was even able to clearly justify some of the phenomenon like diffraction and interference
Diffraction: It is the bending of wave around an obstacle
Interference: It is the combination of two waves of the same or different frequencies to give a new wave whose distribution at each point in space is the algebraic or vector sum of disturbances at that point resulting from each interfering wave.
But the wave nature of light could not justify some other observations like:
1. Black-body radiation: Emission and radiation over a wide range of wavelength from the hot bodies which includes change in color with temperature.
2. Photoelectric effect: The ejection of electrons from the metal surface when radiation strikes on it.
3. Variation of heat capacity of solids as a function of temperature.
4. Line spectra of atoms with special reference to hydrogen.
Black Body Radiations:
Black-body is an ideal body which emits and absorbs radiations of all frequencies. The radiation emitted by these bodies is called black-body radiation.
- Max planck in 1900 gave the concept of black body radiation . He said that when solids are heated , they emit radiation over wide range of wavelength .
This Graph shows the variation of intensity with wavelength at two temperature T1 and T2 ( T2 > T1)
PLANCK'S THEORY OR THEORY OF QUANTIZATION :
According to this theory, energy cannot be absorbed or released continuously but it is emitted or released in the form of small packets called quanta. In case of light this quanta is known as photon. This photon travels with speed of light. Energy of the photon is directly proportional to frequency
E ⋉ V
E = hv
PHOTOELECTIC EFFECT :
When light of a suitable frequency is allowed to incident on a metal, ejection of electrons take place. This phenomenon is known as photoelectric effect.
Einstein theory of photoelectric effect.
Light of a particular frequency strikes a clean metal surface inside a vacuum chamber. Electrons are ejected from the metal and are counted by a detector that measures their kinetic energy.
Main Point
1. One photon of suitable frequency can eject one electron
2. Energy of the photon can be utilised in two ways:
(i) Its one part is used to just eject the electron, this part is known as work function. Work function can be calculated as
W= hc / λο threshold wavelength
W = hv, where v, is threshold frequency
Threshold frequency: Minimum frequency that is required to just eject electron.
(ii) Remaining part of energy is used to provide kinetic energy.
So, Einstein equation is as given below :
BOHR'S MODEL
Main Postulate:
1. Atom is of spherical shape having size (of order 10-10 metre).
2. Whole mass is concentrated in centre called nucleus (having order of size 10-15 metre).
3. Electron revolves around nucleus only in limited circular path and he assumed that electron does not radiate energy during its revolution in permitted paths.
4. Only those orbits are allowed whose orbit angular momentum is integral multiple of h/2π
mvr = nh/2π , Where n = 1,2,3,4
5. When electron absorbs energy, it jumps to higher orbit and when it comes back, it radiates energy
This postulate explain spectra.
KZee-r2 = mv2 / r
Bohr Radius : rn = n2 h2 / 4π2 mKZe2
Velocity of moving electron : Vn = 2.19 X 10^8 Z /n
Number of revolutions per second : N = 4π2 . K2 Z2 e2 m / n3 h3
Energy of moving electron : En = -2π2 me4 K2 Z2 / n2 h2
Potential Energy : -4π2 me4 k2 z2 / n2 h2
Kinetic energy : En = -313.6Z2 / n2 ( kcal/mole)
SPECTRA
The most compelling evidence for the quantization of energy comes from spectroscopy. Spectrum word is taken from Latin word which means appearance.
The record of the intensity transmitted or scattered by a molecule as a function of frequency or wavelength is called its spectrum.
Limitation of Bohr Model: Bohr model does not explain
(1) Zeeman effect
(ii) Stark effect
(iii) Dual nature of matter
(iv) Heisenberg uncertainty principle
(v) Spectra of multielectronic system
(vi) Atoms are spherical (3-dimensional) while the path of electron is circular (2-dimensional). We deal with these factors one by one
Zeeman Effect: When spectral line (source) is placed in magnetic field, spectral lines split up into sublines. This is known as zeeman effect.
Stark Effect: If splitting of spectral lines take place in electric field, then it is known as stark effect.
Dual Nature: Einstein told that light have two type of nature.
(1) Wave nature - Proved by interference, reflection, refraction etc.
(ii) Particle nature-Proved by black body radiation, photoelectric effect.
HEISENBERG'S UNCERTAINTY PRINCIPLE :
It is impossible to measure simultaneously the exact position and exact momentum of a microscope particle .
if uncertainty in position = ΔX
uncertainty in momentum = ΔP
Note : When both are measured simultaneously.
Δx. ΔP >= h/4π
QUANTUM MODEL ;
Bohr's model considered only particular nature of electron . Quantum model of atom depends on
i) Heisenberg's uncertainty principle
ii) De broglie equation
WAVE FUNCTION :
It is the solution of schrodinger equation . But all solution are not accepted . only those solution accepted which have
i) Wave function must be continuous .
ii) Wave function must be finite
iii) Wave function must be single valued
iv) First derivative of wave function must be continuous
QUANTUM NUMBERS :
The solution of the non relativistic schrodinger equation for the hydrogen atom are set of wave function described by three quantum number : n , l , ml . These wave function are called atomic orbital .
1) Principle Quantum Number
ii) Azimuthal Quantum Number
iii) Magnetic Quantum Number
iv) Spain Quantum Number
PRINCIPLE QUANTUM NUMBER :
It is indicated by "n" and its valued are 1,2,3,4...........
SIGNIFICANCE :
1)
n = 1 K
n = 2 L
n = 3 M
n = 4 N
2) It gives information about size of an orbital
3) It gives information about energy of e- in a hydrogen atom
E = -1312 / n2
4) It gives information about maximum no. of e- filled in the main energy level . Number of e- = 2n2
5) It tells about no. of orbitals in main energy level
AZIMUTHAL QUANTUM NUMBER :
Azimuthal quantum number or Angular quantum number or Subsidiary Quantum number is indicated by " l "
Its value are from 0 to n - 1
SIGNIFICANCE :
1) It gives information about the sub energy level to which e- belongs .
0 s
1 p
2 d
3 f
4 g
2) It gives information about shape of the orbital .
3) It gives information about maximum no of e- in any subshell
MAGNETIC QUANTUM NUMBER :
it explain Zeeman Effect
it is indicated by " m " . Its value lies between -l to +l
so, total value of m = (2l + 1 )
SIGNIFICANCE :
It is described the orientation of orbital in which probability of finding e- is maximum
for p subshell , l = 1 so , m = -1 , 0 , 1
SPIN QUANTUM NUMBER :
Three quantum number are necessary to describe the spatial distribution of electron in atom . to completely describe an electron in an atom , fourth quantum number ms clalled the spin quantum number , it must be specified . This is because every electron is rotating on its own axis , either in clockwise or in anticlockwise direction , which is quantized in one of two possible orientation . The allowed value of spin quantum number are +1/2 , -1/2 /
ELECTRONIC CONFIGURATION :
Distribution of electron in various orbitals is known as electronic configuration . The electrons filled in orbitals must obey the Following rules :
1) Aufbau's Principle
2) Pauli's Exclusion Principle
3) Hund's Rule of maximum multiplicity
1) Aufbau's Principle
According to this principle , orbitals with lowest energy are filled before the orbitals having higher energy .
Energy Order :
1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p
2) Pauli's Exclusion Principle :
According to this principle , in an atom , no two electron have same value of all the four quantum number
In the same orbital , electron always accommodate in opposite spins .
some conclusion are :
i) The maximum capacity of electron in an orbit is 2n2
ii) The maximum capacity of electron in a subshell is 2(2l + 1)
iii) An orbital can have a maximum of two electrons with opposite spin
3) Hund's Rule of Maximum multiplicity :
According to this rule , electron are distributed among the orbitals of a subshell in such a way soo as to give the maximum number of unpaired electrons with a parallel spin
* Spin multiplicity , S = 2S+1 , where S is the magnitude of sum total spin quantum number of electron present
* Fully filled orbitals are always most stable due to spherical symmetry . Thereafter , half filled orbitals are more stable than other configuration , due to maximum spin multiplicity .