Atomic structure Revision Notes - IIT JEE/NEET Preparation | Nucleon

Atomic structure

  • Introduction
  • Dalton’s concept of the indivisibility of the atom was completely discredited by a series of
    experimental evidences obtained by scientists. A number of new phenomena were brought to
    light and man’s idea about the natural world underwent a revolutionary change

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  • CATHODE RAYS
  • CATHODE RAYS  DISCOVERY OF ELECTRON

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  • PROPERTIES OF CATHODE RAYS
  • PROPERTIES OF CATHODE RAYS

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  • Cathode rays have the following properties
  • (i) Path of travelling is straight from the cathode with a very high velocity as it produces
    shadow of an object placed in its path.
    (ii) Cathode rays produce mechanical effects. If small light paddle wheel is placed between

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  • PRODUCTION OF ANODE RAYS (DISCOVERY OF PROTON)
  • Goldstein (1886) repeated the experiment with a discharge tube filled with a perforated cathode
    and found that new type of rays came out through the hole in the cathode

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  • PROPERTIES OF ANODE RAYS
  • Anode rays travel along straight paths and hence they cast shadows of object
    placed in their path

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  • DISCOVERY OF NEUTRON
  • Later, a need was felt for the presence of electrically neutral particles as one of the
    constituent of atom

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  • The NUCLEUS
  • Electrons, protons & neutrons are the fundamental particles present in all atoms,(except hydrogen)

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  • ATOMIC MODELS
  • An atom is electrically neutral. It contains positive charges (due to the presence of protons ) as well as negative
    charges (due to the presence of electrons). Hence, J.J. Thomson assumed that an atom is a uniform sphere of
    positive charges with electrons embedded in it.

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  • DRAWBACKS OF RUTHERFORD’S MODEL
  • This was not according to the classical theory of electromagnetism proposed by maxwell.
    According to this theory, every accelerated charged particle must emit radiations in
    the form of electromagnetic waves and loses it total energy

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  • PROPERTIES OF CHARGE
  • This does not mean that a proton has a greater charge but it implies that the charge is equal and opposite.
    Same charge repel each other and opposite charges attract each other.

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  • Size of the nucleus
  • The volume of the nucleus is very small and is only a minute fraction of the total volume of the
    atom.

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  • ATOMIC NUMBER
  • Atomic number is also known as proton number because the charge on the nucleus depends upon the
    number of protons

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  • Electromagnetic wave radiation
  • The oscillating electrical/magnetic field are electromagnetic radiations. Experimentally, the direction of oscillations of
    electrical and magnetic field are prependicular to each other

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  • Some important characteristics of a wave
  • Some important characteristics of a wave

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  • Particle Nature of Electromagnetic Radiation
  • Some of the experimental phenomenon such as diffraction and interference can be explained
    by the wave nature of the electromagnetic radiation. However, following are some of the
    observations which could not be explained

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  • Black Body Radiation
  • When solids are heated they emit radiation over a wide range of wavelengths.
    The ideal body, which emits and absorbs all frequencies, is called a black body and the radiation emitted by
    such a body is called black body radiation

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  • QUANTUM THEORY OF LIGHT
  • The smallest quantity of energy that can be emitted or absorbed in the form of electromagnetic
    radiation is called as quantum of light

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  • One electron volt
  • Energy gained by an electron when it is accelerated from rest through a potential difference of 1 volt.
    Note : Positive charge always moves from high potential to low potential and –ve charge always. moves
    from low potential to high potential if set free.

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  • Photoelectric Effect
  • When certain metals (for example Potassium, Rubidium, Caesium etc.) were exposed to a beam of light electrons
    were ejected as shown in Fig

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  • BOHR’S ATOMIC MODEL
  • There are certain orbits around the nucleus such that if electron will be revolving in these orbit, then it not
    emit any electromagnetic radiation. These are called stationary orbit.
    The neccessary centripetal force is produced by attraction forces of nucleus

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  • Calculation of velocity of an electron in Bohr’s orbit
  • Calculation of velocity of an electron in Bohr’s orbit

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  • Calculation of energy of an electron
  • The total energy of an electron revolving in a particular orbit is

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  • Failures / limitations of Bohr’s theory
  • (a) He could not explain the line spectra of atoms containing more than one electron.
    (b) He also could not explain the presence of multiple spectral lines.
    (c) He was unable to explain the splitting of spectral lines in magnetic field (Zeeman effect) and
    in electric field (Stark effect)

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  • Energy Level Diagram
  • (i) Orbit of lowest energy is placed at the bottom, and all other orbits are placed above this.
    (ii) The gap between two orbits is proportional to the energy difference of the orbits

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  • DEFINITION VALID FOR SINGLE ELECTRON SYSTEM
  • DEFINITION VALID FOR SINGLE ELECTRON SYSTEM

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  • HYDROGEN SPECTRUM
  • Study of Emission and Absorption Spectra :
    An instrument used to separate the radiation of different wavelengths (or frequencies) is called spectroscope or
    a spectrograph. Photograph (or the pattern) of the emergent radiation recorded on the film is called a spectrogram
    or simply a spectrum of the given radiation The branch or science dealing with the study of spectra is called

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  • Absorption spectra
  • When white light from any source is first passed through the solution or vapours of a
    chemical substance and then analysed by the spectroscope, it is observed that some
    dark lines are obtained in the continuous spectrum

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  • EMISSION SPECTRUM OF HYDROGEN
  • When hydrogen gas at low pressure is taken in the discharge tube and the light emitted on
    passing electric discharge is examined with a spectroscope, the spectrum obtained is called
    the emission spectrum of hydrogen

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  • Line Spectrum of Hydrogen
  • Line spectrum of hydrogen is observed due to excitation or de-excitation of electron from one stationary orbit to
    another stationary orbit

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  • SPECTRA LINES OF HYDROGEN ATOM
  • SPECTRA LINES OF HYDROGEN ATOM

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  • PASCHEN SERIES
  • SPECTRA LINES OF HYDROGEN ATOM

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  • No of photons emitted by a sample of H atom
  • If an electron is in any higher state n = n and makes a transition to ground state, then total no. of

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  • Dual nature of electron
  • (a) Einstein had suggested that light can behave as a wave as well as like a particle i.e. it has dual character.
    (b) In 1924, de-Broglie proposed that an electron behaves both as a material particle and as a wave.
    (c) This proposed a new theory wave mechanical theory of matter. According to this theory, the electrons protons
    and even atom when in motion possess wave properties

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  • Dual nature of electron
  • (a) Einstein had suggested that light can behave as a wave as well as like a particle i.e. it has dual character.
    (b) In 1924, de-Broglie proposed that an electron behaves both as a material particle and as a wave.
    (c) This proposed a new theory wave mechanical theory of matter. According to this theory, the electrons protons
    and even atom when in motion possess wave properties

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  • Heisenbergs Uncertainty Principle
  • The exact position and momentum of a fast moving particle cannot be calculated precisely at the same moment
    of time. If Dx is the error in the measurement of position of the particle and if Dp is the error in the measurement
    of momentum of the particle, then

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  • Orbital
  • An orbital may be defined as the region of space around the nucleus where the probability of finding an electron
    is maximum

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  • Shape of the orbitals
  • Shape of the orbitals are related to the solutions of Schrodinger wave equation, and gives the space in which the
    probability of finding an electron is maximum

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  • QUANTUM NUMBERS
  • The set of four numbers required to define an electron completely in an atom are called quantum numbers. The
    first three have been derived from Schrodinger wave equation

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  • Electronic configuration
  • No two electrons in an atom can have the same set of all the four quantum numbers, i.e., an orbital cannot have
    more than 2 electrons because three quantum numbers (principal, azimuthal and magnetic) at the most may be
    same but the fourth must be different, i.e., spins must be in opposite directions.

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  • THE SCHRODINGER EQUATION
  • The de Broglie wave relation is the basis for predicting the behavior of freely moving particles. Shortly after it was
    proposed, Erwin Schrodinger demonstrated that the de Broglie expression could be generalized so as to apply
    to bound particles such as electrons in atoms.

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  • THE SCHRODINGER EQUATION
  • The de Broglie wave relation is the basis for predicting the behavior of freely moving particles. Shortly after it was
    proposed, Erwin Schrodinger demonstrated that the de Broglie expression could be generalized so as to apply
    to bound particles such as electrons in atoms.

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  • Nuclear Chemistry
  • Spontaneous disintegration of nuclei due to emission of radiations like a, b, g is called radioactivity.
    Radioactivity is a nuclei phenomenon

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  • Nuclear stability belt
  • Nuclear stability belt

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  • Group displacement law
  • When 1a emission takes place from a nuclie, new formed nuclie occupy two position left in periodic table

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  • Nuclear fission and nuclear fusion
  • In both processes, large amount of heat evolved due to conversion of some mass
    into energy

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  • Nuclear fusion
  • Nuclear fusion

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  • For objective questions
  • For objective questions

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  • Radioactive Disintegration Series
  • Radioactive Disintegration Series

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