According to this theory , atom is the ultimate particle of matter , also known as Dalton’sAtomic theory (1808).
Cathode ray discharge tube experiments: –
1.Cathode rays start from cathode and move toward anode.
2.These rays are not visible but there behaviour can be observed with fluorescent or phosphorus sent material.
3.In the absence of magnetic or electric field these travels in strait lines
4.In the presence or magnetic or electric field the behaviour of cathode rays in similar TO Negatively charged particleswhich suggest that these rays contain negatively charge particles called electron
5.Cathode rays (electrons) do not depend onthe martial of the electrode andnature of the gas tin the tube so electro us are basic constituent of all atoms.
Charge (e) to mass (me) ratio of electron
ðMeasured byJ. J. Thomson (1897).
ðBy using cathode ray tube ; applying electrical & magnetic field perpendicular to each other also perpendicular to path of electrons.
ðHe proposeddeviation of particles from their path in presence of magnetic or electrical field depend upon the following
1.Magnetic of– vecharge on particle
i.e. it magnitude of charge on particles is greater than interaction with magnetic or electric field is greater so deflection is also grater.
2.Mass of particles
i.e. particle is lighter then deflection is greater.
3.Strength of magnetic or electric field.
i.e. it strength of magneticfield or voltage at electron is increases then deflection ofe– also increases
=>value of e/me = 1.758820× 1011 C kg-1
Charge of electron
=> Determine byR. A. Millikan
=> By oil drop experiment (1906-1914)
=> Charge on e– =-1.6× 10-19C
=> Present accepted value , e– = -1.6022× 10-19 C
Mass of electron
From charge on e– & e/me
Me = 9.1094 ´10-31 kg
Discovery of protons
=> Discovered by E. Goldstein.
=> In modified cathode ray tube gives +ve charge carrying particles known as canal rays.
=> Lithest & smallest +ve ion obtained from Hydrogen called proton.
1)Depend upon, nature of gas present in cathode ray tube.
2)Charge to mass ratio of particles depends on gas from which these originate.
3)Some of +ve charged particles carry a multiple unit of electrical charge.
4)Behavior of protons in magnetic or electric field is opposite to election behavior.
Discovery of neutrons
=> Discovered by Chadwick (1932).
=> By bombarding a thin sheet of beryllium by alpha particles.
=> Electrically neutral particles were emitted known as neutrons.
Thomson model of atom
=> Give by J.J Thomson (1898)
=> According to J.J. Thomson atoms posses a spherical shape with radius about 10-10 m, in which + ve charge is uniformly distributed.
=> Electrons are embedded in such a manner to give most stable electrostatic arrangement.
=> Other names of this model plum pudding raisin pudding watermelon.
=> Mass is assumed to be uniformly distributed in atom.
Rutherford’s nuclear model of atom
=> Given by Rutherford & his students Ernest Marsden and Hans Geiger.
=> By α- particles scattering experiment-
Rutherford’s nuclear model of atom
=> When beam of high energy α- particles was directed at gold foil then tiny flash of light observed at photographic plate.
Rutherford observed that-
1)Most of the α-practicalpassedthrough gold foil undeflected :
2)A small fraction of α- particles was deflected by small angles.
3)A very few α- particles (about 1 in 20000) bounced back means deflected by nearly 1800
From above observations he concludes the structure of atom.
1)Most of space in atom is empty because most of α- particles passed undeflected.
2)Few +ve charged α- particles were deflected.
Because + ve charge of the atom present in center in very small volume that repelled & deflected the +ve charged α- particles.
3)Volume of nucleus is negligible as compared to total volume of atom
i.e.radius of atom = 10-10m (approx)
radius of nucleus =10-15m(approx)
On the basis of observation &conclusion Rutherford proposed model of atom as–
1)+ve charge & most of mass present in the center of atom known as nucleus.
2)Electrons moves around nucleus with very high speed in circular paths known as orbits.
3)Electrons and nucleus (protons) are held together by electrostatic force of attraction .
Atomic number (Z)= no of protons in the nucleus of an atom
= no of electrons in a neutral atom
Mass number(A) = number of protons (z) + number of electron (n)
These are atoms with same mass number but different atomic number.
These are atoms with same atomic number butdifferent atomic mass no.
Wave nature of electromagnetic radiations: –
First explanation gives by James Maxwell (1870)
1)Oscillating magnetic & electric fields produced by the oscillating charged particles are perpendicular to each other and both also perpendicular to the wave direction of propagation.
2)These waves do not require medium i.e. electromagnetic wave can travel in vacuum.
3)Electromagnetic radiation differs from one another in frequency or wavelength gives electromagnetic spectrum.
4)Different units are used to represent electromagnetic radiation.
l = wavelength.
Particle nature of electromagnetic radiation :-
Also known as Planck’sQuantum theory
=> Planck suggested that the atoms and molecules can absorb or emit energy in discrete quantities nit in continuous manner. Planck gives it name as quantum. Energy (E) ofquantum ofradiation is directly proportional to its frequency(n)
Where,h = planks constant = 6.626× 10-34 js
Photo electric effect:-
=> given by H. Hertz(1887)
=> When a beam of light strike a metal surface then electrons were ejected. This phenomena is known as photo electric effect.
1.Electrons ejected from metal surface when beam oflight strike the metal surface
2.Number of electron ejected is directly proportional to intensity (or brightness) of light
3.There is characteristic minimum frequency (n0threshold frequency) below which photoelectric effect is not observed.
4.If n> n0then electrons comes out with kinetic energy which increases with increase in frequency of light.
Kinetic energy of ejected electrons is given by-
h n= h n0+ ½(meV2)
study ofabsorption or emission spectra is called spectroscopy .
Bohr’s model for hydrogen atom:-
=> Explain by nails Bohr (1913).
=> Postulates for Bohr’s modal are,
1.Electron in hydrogen atom move around nucleus in circular path of fixed radius and energy. these paths are called orbits
2.Energy of e does not change with time.
However, when electron move from lower to higher stationary state it absorbed sub amount of energy and energy release when it comes back.
3.Frequency of radiations emitted or absorbed when transition of e occur is given by
Where, e1 & e2 is lower & higher energy state.
4.Angular momentum of n electron in given stationary state is given by
[Where n =1,2,3…..]
Limitation of Bohr’s model:-
1.Bohr model fail to explain finer detail of hydrogen atom spectrum observed by spectroscopic, techniques.
2.It fails to explain spectrum of other atom except hydrogen atom.
3.It fails to explain splitting of the spectral lines in presence of electric (stark effect) or magnetic field ( Zeeman effect )
4.Fell to explain formation of molecules from atoms by chemical bonding.
Dual behaviour of matter :-
=> Explain by de Broglie (1924)
=> He explain that matter also behave like radiation and exhibit dual behavior means both like particle and wave like properties .
Where l =wavelength.
m = mass of particle ,
v = velocity of particle,
p = momentum
Heisenberg’s uncertainty principle:-
Given by Werner Heisenberg (1927)
He explain that it is impossible to determine simultaneously the exact positive and exact momentum (or velocity) of an electron
Where,Dx= uncertaintyin position
DVx = uncertainty in velocity or momentum
Quantum mechanical model of atom:-
Branches of science which explain duel behavior of Metter is called quantum mechanics .
ðQuantum mechanics independently developed by Werner Heisenberg and Erwin Schrodinger (1926)
Fundamental equation developed by Schrodinger (won Nobel Prize 1933)
Equation for a system (atom or molecules was energy does not change with time)
Principle quantum number ‘n’ :-
·It is a positive Integer with value of n = 1,2,3……
·It determine size and energy of orbital
·It also identifies the shell with increase in an , number of allowed orbital increase. And given by n2
Shell = k,l,m,l……
·Size of orbital increase with increase in ann.
Azimuthal quantum no.‘p’:-
·It is also known as orbital angular momentum or subsidiary quantum no.
·It defined 3d shape of orbital of orbital
·For given value of n possible value of
L= 0,1,2,3,4,5,———-(n-1) ,
Ex :- ifn=1thenl=0
·Each shell consists of one or more sub-shells or sub-shells.
·No of sub-shells = value ofn
Ifn= 1then1 sub-shell =(l=0)
Ifn= 2then2 sub-shell =(l=0,1)
Ifn= 3then3 sub-shell =(l=0,1,2)
·Value ofl =0,1,2,3,4,5———-
Notation for sub-shell= s, p, d,f,g, h————–
Magnetic orbital quantum no ‘mi’ :-
·This quantum no (mi) gives information about orientation oftheorbital .
·Ml = (2l+1) i.e. if value oflis1 then value ofml = 2×1+1=3=(-1,0,1)
Value of p
No of orbital’s
Electron spin quantum (ms):-
·Proposed by G. Uhlen beck & S. Goodsmit (1925)
·Electrons spins around its own axis
·Ms have two value +1/2 & -1/2
·Ms gives information about orientation of the spin ofthe electron.
Aufbau principle :-
According to this principle in the ground state of the atoms the orbital’s are filled in order of their increasing energies means electrons enter higher energy orbital’sso order in which orbital’s are filled is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.
Pauli exclusion principle:-
·given by W. Pauli (1926).
·Two electrons in an atoms can’t have same set of 4-quantum no.
·Only two electrons may exist in same orbital and these electrons must have opposite spin.
Hund’srule of maximum multiplicity:-.
·According to this rule pairing ofelectron in the orbital’s belonging to the same sub-shell (p, d or f)
·Does not take place until each orbital belonging to that sub-shell has got one electron each i.e. it is singly occupied.