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Atomic Structure

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Atomic Structure PDF Notes, Important Questions and Formulas

Atomic Structure

Different Models of Atom

1. Dalton's Atomic Model:

Dalton’s atomic model is one of the fundamentals of physics and chemistry. This theory of atomic composition was hypothesized and partially confirmed by the English chemist and Physicist John Dalton. Dalton came with his Atomic theory as a result of his research into gases. He discovered that certain gases only could be combined in certain proportions even if two different compounds shared the same common element or group of elements. Through deductive reasoning and experimentation, he made an interesting discovery. His findings led him to hypothesize that elements combine at the atomic level in fixed ratios.

This ratio would naturally differ in compounds due to the unique atomic weights of the elements being combined. This was a revolutionary idea but further experimentation by himself and others confirmed his theory. The findings became the basis of Dalton’s Atomic Laws or Model. These laws focus on five basic theorems.

  1. Pure Elements consist of particles called atoms.
  2. Atoms of an element are all the same for that element. That means gold is gold and oxygen is oxygen down to the last atom.
  3. Atoms of different elements can be told apart by their atomic weights.
  4. Atoms of elements unite to form chemical compounds.
  5. Atoms can neither be created nor destroyed in chemical reaction. The grouping only changes.

2. Thomson’s model or Plum Pudding model or Raisin Pudding model or water melon model:

Thomson stated that atom is uniform sphere of positive charge with negatively charged electron embedded in it in such a way that atom becomes electrically neutral.

Rutherford’s Nuclear Model of Atom:

α-Particle scattering experiment:-

 

A beam of α-Particle was projected towards this gold foil. The foil was surrounded by circular fluorescent ZnS screen which produces flash when α -Particle collides with it.

Observations of experiment:

  1. Most of α-Particle passed through the foil un deflected or deflected by very small angles [less then 10].
  2. Few particles were deflected through angles as large as .
  3. Very few particles[1 in 20 thousand]bounce back or rejected through an angle almost equal to 180o
  4. Angle of deflection increases with increase with increase in atomic number.

Conclusions:

  1. Since, most of the α -Particles passed through the foil un deflected, therefore there must be large empty space present in the atom.
  2. α -particles are positively charged and having considerable mass could be deflected by only some heavy positively charged centre by repulsion.[Rutherford named this positively charged heavy centre as nucleus]
  3. Since, very few α-particles were thrown back therefore the size of nucleus be very small and is should be rigid because α -particle can recoil back only if it undergo direct collision with the heavy positively charged centre.
  • Rutherford’s Model

On the basis of scattering experiment, Rutherford proposed an atomic model or nuclear atomic model or nuclear model of atoms.

 

  1. The electron are revolving around the nucleus in circular path and electrostatics force of attraction between the electrons and nucleus is balanced by the centrifugal force acting on revolving electron.
  2. The –ve charge on electrons are equal to the total positive charge on the nucleus and therefore as a whole atom is electrically neutral.
  3. Size of the nucleus is extremely small as compared to size of the atom.

Radius of nucleus

Radius of nucleus is of the order of 10-15m and radius of atom is order of 10-10mbegin mathsize 12px style straight r equals straight r subscript 0 straight A to the power of 1 divided by 3 end exponent end style

A= Mass number=no. of protons + no. of neutrons.

r0=constant =1.2×10-13cm.

Failures of Rutherford’s Model:

  1. According to maxwell’s wave theory, an accelerated charged particles (like electron) revolving in the field of another charged particle like nucleus loses energy in the form of electromagnetic radiation and if this happen then orbit of the revolving electron should keep on decreasing and ultimately electron should fall into the nucleus and atom should collapse but this doesn’t happen actually.
  2. Rutherford’s model didn’t give any idea about electronic arrangement of an atom and since he didn’t mention specific energy levels he couldn’t explain the discontinuous atomic hydrogen spectrum.

Waves and its characteristics:

It is a periodical disturbance causing transfer of energy without transfer of matter.

 

  • Characteristics of waves:

Amplitude: Maximum displacement from mean position it remains constant with distance except stationary or standing waves.

Wavelength: It is the distance between two adjacent crest or troughs.

Frequency (begin mathsize 12px style straight mu text  or  end text straight nu end style): The no of waves passing through a point in 1 second, unit-sec-1 or Hz

Wave number:  begin mathsize 12px style left parenthesis straight mu with bar on top text  or  end text straight nu with ⃛ on top right parenthesis end style No. of waves present in unit distance.

 begin mathsize 12px style straight mu with bar on top equals 1 over straight lambda equals metre to the power of negative 1 end exponent comma cm to the power of negative 1 end exponent end style etc.

Velocity: linear distance travelled by wave in one second.

 begin mathsize 12px style straight V equals μλ end style

NATURE OF LIGHT

  • Maxwell electromagnetic wave theory (wave nature of light):

An accelerated electrically charged particle produces and transmits electrical and magnetic field. These are transmitted in the form of waves known as electromagnetic waves or electromagnetic radiations. He stated that light also possess electrical and magnetic field and therefore it is also known as electromagnetic radiations or e.m.w.

  • Characteristics of Electromagnetic Radiations:
  1. In these electromagnetic radiation electrical and magnetic field oscillates ⊥ er to each other and it also propagates ⊥ er to both field.
  2. All these electromagnetic radiation do not require any medium and can travel in vacuum.
  3. Velocity of all electromagnetic radiation is 3×108m/s in vacuum.
  4. Energy of an electromagnetic wave is directly proportional to intensity and it is independent of frequency.
  5. There are also showing diffraction an interference and therefore. Maxwell concluded light to be wave nature.

            But Maxwell theory couldn’t explain the results of photoelectric effect and black body radiations.

Photon Energies for EM Spectrum

 

A Blackbody:

A blackbody is an object that emits a well-defined spectrum of radiation solely based on its temperature. We see from figure at right that the hotter the blackbody, the more intense it is, and the shorter the peak wavelength. The picture does not say anything about what the object is made of, or how heavy it is, etc. It doesn't matter! The only property that determines the spectrum of a blackbody is its temperature. Brick, iron or a dense gas will emit the same spectrum as long as they are at the same temperature. That spectrum will have a peak that lies at a particular wavelength.

 

  • Planck’s Quantum Theory:

[Particle nature of light];

He stated that a radiates energy in the form of discontinuous energy packets or bundles. Each bundle of energy is known as quantum and quantum of light is known as photons.

Energy of each quantum is directly proportional to frequency of radiation.

begin mathsize 12px style table attributes columnalign left end attributes row cell straight capital epsilon proportional to straight nu end cell row cell straight capital epsilon equals hν text              h=6.62 end text cross times text 10 end text to the power of negative 34 end exponent Js. end cell row cell text                                           end text downwards arrow end cell end table end style

Plank’s constant

Total energy absorbed or emitted by a body will be whole no. integral multiple of energy of quantum

begin mathsize 12px style straight E subscript abs text or Eemitted=nh end text straight nu end style

 

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