STRUCTURE OF ATOM

STRUCTURE OF ATOM

Thomson model of atom :

 J.J Thomson in 1898, proposed that an atom possesses a spherical shape (radius approx. 10-10m) in which the positive charge is uniformly distributed. The electrons are embedded in it in such a manner as to give the most stable electrostatic arrangement. Many different names are given to this model, like plum pudding, raisin pudding and watermelon.

This model can be visualised as a pudding or watermelon of positive charge with plum and seeds embedded into it. An important feature of this model is that the mass of the atom is assumed to be uniformly distributed over the atom. This model was not able to explain the overall neutrality of the atom.

Rutherford α scattering experiment :

Rutherford’s famous α particle scattering experiment is bombarded very thin gold foil with α particle. A stream of high energy α particle from a radioactive source was directed at a very thin gold foil (thickness 100nm). The thin gold foil has a circular fluorescent zinc sulphide screen around it. Whenever, α- particles stuck to the screen, a tiny flash of light was produced at that point.

Observations of α- scattering experiment :

•  Most of the α - particle passed through the gold foil un-deflected.
• A small fraction of α - particle was deflected through a small angle.
•  A very few α - particle (approx. 1 in 20000) bounced back, that is, were deflected by an angle of 180°.

Conclusion of α - scattering experiment :

•  Most of the space in the atom is empty as most of the α - particles passed through the foil un-deflected.
• A few positively charged α - particles were deflected. The deflection must be due to enormous repulsive force showing that the positive charge of the atom is not spread throughout the atom as assumed by Thomson. The positive charge has to be concentrated in a very small volume that repelled and deflects the positive charged α - particles.
• Calculations by Rutherford showed that the volume occupied by the nucleus is negligibly small as compared to the total volume of the atom. The radius of the atom is about 10-10m while the radius of the nucleus is about 10-15m.

Rutherford’s model of an atom :

• The positive charge and most of the mass of an atom is densely concentrated in extremely small region. This small portion of the atom is known as NUCLEUS.
• The nucleus is surrounded by electrons that moves around the nucleus with a very high speed in a circular path called as orbits. Thus, Rutherford’s model f atom resembles the solar system in which the nucleus is sun and the electrons are the revolving planets.
• Electrons and the nucleus are held together by electrostatic force of attraction.

ATOMIC NUMBER : 

Atomic number is the number of protons present in an atom.
Atomic number = number of proton.
Atomic no. = no. of electron in a neutral atom.
Mass no. or atomic mass = no. of protons + no. of neutrons
No. Of neutrons = atomic mass - atomic no.

ISOTOPES : 

The species having same atomic number but different mass number are called Isotopes.

Exam: 1) 1H1, 1H2, 1H3.
2) 6C12, 6C14.

ISOBARS : 

The species having same mass number but different atomic number are called as Isobars.
Exam: 1) 7N14, 6C14.
2) 18Ar40, 20Ca40.

ISOTOMES : 

Species having same no. of neutrons are called as Isotomes.
Exam: 1) 6C14, 8O16.

ISOELECTRIC SPECIES : 

species having same no. of electrons are called as Iso-electric species.
Exam: 1) Na+, Ne.
2) F-, Mg2+.

ELECTROMAGNETIC WAVES : 

The wave which are produced when a particle is accelerated due to varying electric and magnetic field. EMW consist both electric and magnetic field perpendicular to each other as well as perpendicular to the direction of propagation of wave.
Properties of Electromagnetic waves are - - - -

• They do not require any medium to travel.
• When these waves are arranged from low wavelength to high wavelength region, then they forms a spectrum known as EM spectrum.
• Gamma waves are the lowest wavelength wave and radio waves are the highest wavelength wave of EM spectrum.
•  EM waves shows all the characteristics of wave like frequency, amplitude, wavelength etc.
• These waves follows the relation ---

                                                        f = 1/T and
                                                         c = f入
                                   {c=speed of light, f=frequency, 入=wavelength }
NOTE: Wave number = 1/λ.

PHOTOELECTRIC EFFECT :

 It is the emission of photo electron from the surface of metal when a light of certain minimum frequency falls over it.

Properties of photoelectric effect :

•  The energy required to just take out electron from the metal surface is called work function (W₀).
• The minimum frequency of light below which the photoelectric effect does not takes place is called THRESHOLD FREQUENCY.
•  Number of photo electron depends on the brightness or intensity of light striking on the metal surface. Brighter the light greater the number of photo electron emitted.
•  As soon as the beam of light strikes the metal surface, photo electrons emitted by the metal surface. It means there is no time gap between striking of light and emission of photo electron.
• The energy of beam of light striking on metal surface is divided into two parts one for the emission of electron (work function) and the remaining as the kinetic energy of the photo electron.

                                           Energy = work function + kinetic energy
                                                     hν = hν₀ + 1/2 mv2
{ hν= energy of light source, hν₀ = work function, 1/2 mv2 = kinetic energy of photo electron}
                                        hν = W₀ + 1/2 mv2
                                        1/2 mv2 = (hν - hν₀)
                                      kinetic energy = h(ν - ν₀)
{m= mass of electron, h=plank’s constant(6.62 X 10-34 J/Sec), ν=frequency of light, ν₀= threshold frequency}

                                     W₀ = hν₀ (where, ν₀ = c/λ₀).

SPECTRUM :  

The collection of waves of different frequency or wavelength in increasing or decreasing order of their wavelength or frequency is called as spectrum.
Depending on nature waves are of two type:

a) Continuous spectrum : A spectrum in which different regions are not separated by dark lines are called as continuous spectrum.

b) Line spectrum : The spectrum in which different regions are separated by dark fine lines are called as line spectrum.
Line spectrum are also of two type:

1) Atom emission spectrum : It is obtained from the radiation emitted from atom in its excited state (higher energy state of atom).
The radiation of excited atom are made to fall on prism so that a line emission spectrum of an atom is obtained.

2) Atom absorption spectrum : It is obtained from an atom in its ground state (lower most energy state).
When a white light is passed through an atom and it is taken up to prism after passing through an atom then it splits up and form a line absorption spectrum of an atom.

HYDROGEN SPECTRUM :

When electric current is passed through hydrogen gas then its molecules splits up into atoms in excited state. The spectrum obtained from these these atoms of hydrogen is called hydrogen spectrum.

In hydrogen spectrum Balmer region was first of all identified because it is found in visible region. After the advancement in spectrum other regions in series are identified, which are as follow -

• Lyman series.
• Balmer series.
• Paschen series.
• Brackket series.
• P-fund series.

Hydrogen spectrum

NOTE :- Wave number for electronic transition in hydrogen spectrum is given by-

109677 = Rydberg constant.(cm-1).

BOHR’S MODEL OF ATOM

• Electron revolve around the nucleus in a circular path called as orbit.
• Energy of electron do not changes while moving in an energy level but it changes when it shows absorption or emission type of transition. In absorption electron absorb energy to move from its lower energy state to higher energy state, whereas, in emission electron moves from its higher energy state to its lower energy state.
• Frequency of radiation during absorption or emission is given by –

                        ΔE =hν
                       ν= ΔE/h

• Energy of an electron in its stationary state (orbit) can be given by the following formula –

                                       En = −2.18 x10−18n2 J

• Electron can move in those orbit in which their angular momentum is quantized. It means the angular momentum of an electron in that orbit must be an integral multiple of h2π.

                                  Angular momentum = mevr.
                                   Quantization of angular momentum –
                                   mevr = nh2π

• Where, n is an orbit in which electron can move, r = radius of orbit, v = velocity of electron, m= mass of electron.
•  This model can be used for all those species which have single electron (He+, Li+, Be+2).

                 For such species energy of electron in nth orbit can be calculated by-
                                         En = −2.18 X 10−18(Z2)n2

•  Radius of nth orbit can be calculated by –

                            Rn = 5.29xn2Z {where Z is an atomic number}

• It is impossible to calculate the velocity of electron moving in those orbit.

Limitation of Bohr’s model

•  It is applicable for single electron species only.
• It could not explain the Zeeman effect and Stark effect. Zeeman effect is the splitting of spectral lines in the presence of magnetic field. Stark effect, it is the splitting of spectral lines in the presence of electric field.

• It could not explain the doublet and triplet of spectral lines.

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