STRUCTURE OF ATOM (ISC CLASS 11)

STRUCTURE OF ATOM 

Subatomic particles or fundamental particles are electron proton and neutron 

Discovery Of Electron ( study of cathode rays)

The particles move from the negative electrode ( cathode ) to positive electrode ( anode ) the particle were called cathode ray because they are emitted from negative electrode ( cathode )

Properties Of Cathode Ray

• Cathode ray always travel in straight line
• Cathode ray rotate a light weight FAN in their path this shows cathode rays of particle nature which possess mass and kinetic energy

• In electric field cathode rays are deflected towards positive electrode this shows cathode rays are negatively charged particles electrons

• Cathode ray cause the  ionization of the gas 
• It Produces fluorescence  ( when it strikes to photographic plate )
• Cathode rays pass through the thin foil  
• Charge and mass of the electrons
• mass of electron is 1 /1837 of the mass of one hydrogen atoms (9.109 * 10-31KG)
• Charge on e- = 1.602 * 10-19 C (coulomb)

Discovery Of Proton

properties of positive rays ( Canal rays ) 

• they travel in straight line
• they have particle nature
• they produce fluorescence
• they ionises the gas and
• In electric field Canal rays are deflected towards cathode because Canal rays are positively charged particles
• Canal rays obtained from hydrogen gas only contains protons
• e/m or  electron mass ratio value of canal rays is lower in comparison to e/m value of cathode rays
• charge of proton = 1.602 * 10-19 C (coulomb)
• mass of proton = 1.672 * 10-27 KG

Discovery Of Neutron 

• it was discovered by chadwick

2He4+3Li7-------->0n1+5B10

2He4+4Be9-------->0n1+6C12

• Neutron are the neutral particle having no charge
• They are not deflected when we apply electric field
• Mass of neutron : 1.675 * 10-24 KG
• 
  

Atomic Model 

JJ Thomson model of atom

he proposed that an atom posses a spherical shape ( radius 10-10m ) in which the positive charge is uniformly distributed like red edible part of watermelon and the electrons are embedded into it in such a manner as to give the most stable electronic arrangement this model is also known as plum pudding model or raisen pudding  model or watermelon model

Rutherford Nuclear Model Of An Atom

The result of scattering experiment are

• Most of the Alpha particle pass through the gold foil undeflected
• A small fraction of the Alpha particle was deflected by small angles
• A very few Alpha particles are reflected back to their original path i.e, deflected by 180

• Nucleus is positively charged and is present at the centre of atom
• Nucleus is hard, dense and rigid
• The radius of nucleus is of the order of 10-15 m
• The radius of atom is of the order of 10-10m
• Since most of the Alpha particle pass through the thin gold foil without any deflection this shows that there is large empty space around the atom nucleus

Atomic Number And Mass Number 

Atomic Number 

• It is represented by Z  
• Z is equal to number of protons in the nucleus of an atom or number of electrons in the neutral atom

Mass Number 

• It is represented by A
• A  is equals to number of protons + number of neutrons

Example

8O16

Z = number of electron is equals to number of protons = 8 

A =number of electrons protons + number of neutron (n) = 16

16 =  8 + n

N = 16 - 8

n = 8 ( number of neutrons )

Isotopes 

the atom having same atomic number but different atomic masses are called isotopes

example

protium (1H1), deuterium  (1H2), tritium  (1H3)

Isobars 

The atom  having same mass number but different atomic numbers are called isobars

Example

6H14  ,  7H14

Drawbacks Of Rutherford Model 

when a body is moving in in an orbit it undergoes acceleration ( even if the body is moving with a constant speed in an orbit it must accelerate because of changing direction ) according to the electromagnetic theory ofMaxwell charged particles when accelerated should emit electromagnetic radiation. Therefore an electron in an Orbit will emit  radiation the orbit will thus continue to shrink and it takes 10-8 seconds to spiral into the nucleus

 Rutherford Model was unable to explain this concept that electron are revolve In A nucleus at the fix path  known as orbits this theory is explained by Bohr's

Bohr's Model Of Atom 

Neils Bohr: He improve the Rutherford Model of atom mainly he discovered two things

• Dual character of the electromagnetic radiation that is wave nature and particle nature
• Experimental results regarding atomic spectra which can be explained only by assuming quantized electronic energy levels in atom

Wave Nature Of Electromagnetic Radiation 

Electromagnetic Radiation 

When electrically charged particles moves under acceleration alternating electrical and magnetic fields are produced and transmitted these fields  transmitted in the form of wave known as electromagnetic wave radiation

Theory 

• energy is emitted continuously in the form of radiation waves when electric current is passed through a filament of a bulb bulb
• the radiation possess wave nature and its velocity is 3 * 10-8 m/s ( speed of light ) the radiation is known as electromagnetic radiation or electromagnetic waves
• it does not require any material medium ( travel in vacuum )

Velocity 

it is the linear distance travelled by the wave in one second

Wave Number

it is defined as the number of wave per unit length

Relation Between Speed Of Light,Frequrncy And Wavelength

Black Body Radiation 

the ideal body which emits and absorb radiations of all frequencies is called a black body and radiation emitted by such a body is called a black body radiation

Quantum (as a whole or number in fractions)

This word is coined by planck’s which represent the smallest quantity of energy that can be emitted or absorbed in the form of electromagnetic radiation

E = h*f

Or

E= h*v(nu)

where :-

E= energy of quantum of radiation

f =  frequency

h = planck’s constant (6.62 * 10-34JS)

Photoelectric Effect 

it is the phenomena of ejection of electrons from the metal surface when a light of suitable frequency strike the metal surface. Electrons are ejected from metal surface are called photoelectrons and the phenomenon is known as photoelectric effect.

1. there is no time lag
2. electronic ejection is directly proportional to brightness
3. v>v0

Threshold Energy 

Threshold energy is the minimum frequency above which Photoelectric effect takes place

(frequency at which photoelectric effect occur )v > v0 (threshold frequency)

Explanation Of Photoelectric Effect 

It was explained by Einstein on the basis of quantum theory(when a photon strikes a metal surface it transfer its energy instantaneously to the electron during the collision and the electron is ejected without delay)

Work Function 

some of the energy of photon is used up in ejecting the electron and the remaining energy is taken up by the electron in the form of kinetic energy

( Mathematically )

 energy of striking Photon = work function + kinetic energy emitted by electron e-

 Heisenberg Uncertainty Principle

• It states that it is impossible to determine simultaneously the exact position at exact Momentum or (velocity) of an electron
• The effect of Heisenberg Uncertainty Principle is significant only for motion of microscopic object and is negligible for the macroscopic object

Quantum numbers

Mainly there are four types of quantum numbers

1. Principal Quantum Number ( n )
2. azimuthal quantum number ( l )
3. magnetic quantum number ( m )
4. spin quantum number ( s )

To describe electrons in an atom indifferent orbital we use Quantum numbers

Principal Quantum Number

Principal Quantum Number guess the average distance of the electron from the nucleus and the energy associated with it

for example

Azimuthal Quantum Number ( l )

• It determines angular momentum of the electron
• the value of l give the subshell or sublevel
• it can have positive integer value from 0 to n -1 Where n is the principal quantum number

Magnetic Quantum Number

• The spatial orientation of the orbital with respect to standard set of co-ordinate Axes
• In influence of external magnetic field the electron in a given subshell Orient themselves in certain preferred regions of space around the nucleus these are called orbitals

Spin Quantum Number

• The quantum number describe the screen orientation of the electron which is either plus half(+1/2) or minus half (-½)
• any orbital can have only two electrons

ORBITALS

AUFBAU PRINCIPLE

Pauli Exclusion Principle

• Only two electron exist in the same orbital and these electron must have opposite spin
• The maximum number of electron in the shell with principal quantum number n = 2n2

Hund's Rule Of Maximum Multiplicity

• Pairing of electron in the orbital belonging to the same subshell p,d,f does not take place until each orbital belonging to that subshell has got one electron each i.e,  it is singly occupied

Note:- half filled and fully filled orbitals are more stable due to symmetry