Structure of atom (Class - 11 ) Cbse, Study notes of Chemistry

Download Structure of atom (Class - 11 ) Cbse and more Study notes Chemistry in PDF only on Docsity! Discovery of Electron—Discharge Tube Experiment In 1879, William Crooks studied the conduction of electricity through gases at low pressure. He performed the experiment in a discharge tube which is a cylindrical hard glass tube about 60 cm in length. It is sealed at both the ends and fitted with two metal electrodes as shown in Fig. 2.1. Structure of Atom Class |) Notes Chemistry. The electrical discharge through the gases could be observed only at very low pressures and at very high voltages. The pressure of different gases could be adjusted by evacuation, When sufficiently high voltage is applied across the electrodes, current starts flowing through a stream of particles moving in the tube from the negative electrode (cathode) to the positive electrode (anode). These were called cathode rays or cathode ray particles. « Properties of Cathode Rays Gi) Cathode rays travel in straight line. Gi) Cathode rays start from cathode and move towards the anode. Gii) These rays themselves are not visible but their behaviour can be observed with the help of certain kind of materials (fluorescent or phosphorescent) which glow when hit by them. Civ) Cathode rays consist of negatively charged particles. When electric field is applied on the cathode rays with the help of a pair of metal plates, these are found to be deflected towards the positive plate indicating the presence of negative charge. Cv) The characteristics of cathode rays do not depend upon the material of electrodes and the nature of gas present in the cathode ray tube. + Determination of Charge/Mass Celm) Ratio for Electrons J. J. Thomson for the first time experimentally determined charge/mass ratio called elm ratio for the electrons. For this, he subjected the beam of electrons released in the discharge tube as cathode rays to influence the electric and magnetic fields. These were acting perpendicular to one another as well as to the path followed by electrons. According to Thomson, the amount of deviation of the particles from their path in presence of electrical and magnetic field depends upon following factors: @ Greater the magnitude of the charge on the particle, greater is the interaction with the electric or magnetic field and thus greater is the deflection. Gi) The mass of the particle — lighter the particle, greater the deflection. Gii) The deflection of electrons from their original path increases with the increase in the voltage across the electrodes or strength of the magnetic field. By carrying out accurate measurements on the amount of deflections observed by the electrons on the electric field strength or magnetic field strength, Thomson was able to determine the value of efme = 1.758820 x 1011 € kg-l where me = Mass of the electron in kg e = magnitude of charge on the electron in coulomb Cc). - Charge on the Electron RA. Millikan devised a method known as oil drop experiment to determine the charge on the electrons. Structure of Atom Class |) Notes Chemistry Chapter 2 + Discovery of Proton—Anode Rays In 1886, Goldstein modified the discharge tube by using a perforated cathode. On reducing the pressure, he observed a new type of luminous rays passing through the holes or perforations of the cathode and moving in a direction opposite to the cathode rays. These rays were named as positive rays or anode rays or as canal rays. Anode rays are not emitted from the anode but from a space between anode and cathode. + Properties of Anode Rays @ The value of positive charge Ce) on the particles constituting anode rays depends upon the nature of the gas in the discharge tube. Gi) The charge to mass ratio of the particles is found to depend on the gas from which these originate. Gi) Some of the positively charged particles carry a multiple of the fundamental unit of electrical charge. Civ) The behaviour of these particles in the magnetic or electric field is opposite to that observed for electron or cathode rays. + Proton The smallest and lightest positive ion was obtained trom hydrogen and was called proton. Mass of proton = 1.676 x 10-27 kg Charge on a proton = (+) 1.602 x 10-19 C « Neutron It is a neutral particle. It was discovered by Chadwick (1932). around nucleus in an orbit is under acceleration. According to Maxwell’s electromagnetic theory, charged particles when accelerated must emit electromagnetic radiations. Therefore, an electron in an orbit will emit radiations, the energy carried by radiation comes from electronic motion. lis path will become closer to nucleus and ultimately should spiral into nucleus within . 10-8 s. But actually this does not happen. Thus, Rutherford’s model cannot explain the stability of atom if the motion of electrons is described on the basis of classical mechanics and electromagnetic theory. Gi) Rutherford’s model does not give any idea about distribution of electrons around the nucleus and about their energies. + Developments Leading to the Bohr's Model of Atom Two developments played a major role in the formulation of Bohr’s model of atom. These were: G@ Dual character of the electromagnetic radiation which means that radiations possess both wave like and particle like properties. Gi) Experimental results regarding atomic spectra which can be explained only by assuming quantized electronic energy levels in atoms. ‘ Nature of Electromagnetic Radiation (Electromagnetic Wave Theory) This theory was put forward by James Clark Maxwell in 1864, The main points of this theory are as follows: Gi) The energy is emitted from any source Clike the heated rod or the filament of a bulb through which electric current is passed) continuously in the form of radiations and is called the radiant energy. Gi) The radiations consist of electric and magnetic fields oscillating perpendicular to each other and both perpendicular to the direction of propagation of the radiation. Gili) The radiations possess wave character and travel with the velocity of light 3 x 108 m/sec. Civ) These waves do not require any material medium for propagation. For example, rays from the sun reach us through space which is a non-material medium. + Characteristics of a Wave Wavelength: It is defined as the distance between any two consecutive crests or troughs. It is represented by X and its S.J. unit is metre. “4 Frequency: Frequency of a wave is defined as the number of waves passing through a point in one second. It is represented by v Cnu) and is expressed in Hertz (Hz). | Hz = 1 cycle/sec. Velocity: Velocity of a wave is defined as the linear distance travelled by the wave in one second. It is represented by c and is expressed in cm/sec or m/sec. Amplitude: Amplitude of a wave is the height of the crest or the depth of the through. It is represented by V and is expressed in the units of length. Wave Number: It is defined as the number of waves present in | metre length. Evidently it will be equal to the reciprocal of the wavelength. It is represented by bar v (read as nu bar). Electromagnetic Spectrum: When electromagnetic radiations are arranged in order of their increasing wavelengths or decreasing frequencies, the complete spectrum obtained is called electromagnetic spectrum. + Limitations of Electromagnetic Wave Theory Electromagnetic wave theory was successful in explaining properties of light such as interference, diffraction etc; but it could not explain the following: G The phenomenon of black body radiation. Gi) The photoelectric effect. Gili) The variation of heat capacity of solids as a function of temperature. Civ) The line spectra of atoms with reference to hydrogen. + Black Body Radiation 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, The, exact frequency distribution of the emitted radiation from a black body depends only on its temperature, At a given temperature, intensity of radiation emitted increases with decrease of wavelength, reaches a maximum value at a given wavelength and then starts decreasing with further decrease of wavelength as shown. * Planck’s Quantum Theory To explain the phenomenon of ‘Black body radiation’ and photoelectric effect, Max Planck in 1900, put forward a theory known as Planck’s Quantum Theory. This theory was further extended by Einstein in 1905. The main points of this theory was as follows: , Gi) The radiant energy emitted or absorbed in the form of small packets of energy. Each such packets of energy is called a quantum. Gi) The energy of each quantum is directly proportional to the frequency of the radiation where h is a proportionality constant, called Planck’s constant. Its value is equal to 6.626 x 10-34 Isec, + Photoelectric Effect Hertz, in 1887, discovered that when a beam of light of certain frequency strikes the surface of some metals, electrons are emitted or ejected from the metal surface. The phenomenon is called photoelectric effect. Observations in Photoelectric Effect Gi) Only photons of light of certain minimum frequency called threshold frequency (v0) can cause the photoelectric effect. The value of v0 is different for different metals. Gi) The kinetic energy of the electrons which are emitted is directly proportional to the frequency of the striking photons and is guite independent of their intensity. Gi) The number of electrons that are ejected per second from the metal surface depends upon the intensity of the striking photons or radiations and not upon their frequency. Explanation of Photoelectric Effect Einstein in (1905) was able to give an explanation of the different points of the photoelectric effect using Planck’s quantum theory as under: GW Photoelectrons are ejected only when the incident light has a certain minimum frequency (threshold frequency v0) Gi) If the frequency of the incident light Cv) is more than the threshold frequency (v0), the excess energy Chv — hv0) is imparted to the electron as kinetic energy. KE. of the ejected electron energy of the emitted electron. Gi) On increasing the intensity of light, more electrons are ejected but the energies of the electrons are not altered. G@) In an atom, the electrons revolve around the nucleus in certain definite circular paths called orbits. Gi) Each orbit is associated with definite energy and therefore these are known as energy levels or energy shells. These are numbered as 1, 2, 3, Qos or Ky Ly My Nusa Gii) Only those energy orbits are permitted for the electron in which angular momentum of the electron is a whole number multiple of h/27 Angular momentum of electron Cmvr) = nh/2n (n = |, 2, 3, 4 etc). m = mass of the electron. " v = tangential velocity of the revolving electron. it) ry = radius of the orbit. h = Planck’s constant. n is an integer. iv) As long as electron is present in a particular orbit, it neither absorbs nor loses energy and its energy, therefore, remains constant. Cv) When energy is supplied to an electron, it absorbs energy only in fixed amounts as quanta and jumps to higher energy state away from the nucleus known as excited state. The excited state is unstable, the electron may jump back to the lower energy state and in doing so, it emits the same amount of energy. (AE = E2 - El). . Achievements of Bohr’s Theory 1. Bohr’s theory has explained the stability of an atom. 2, Bohr’s theory has helped in calculating the energy of electron in hydrogen atom and one electron species. The mathematical expression for the energy in the nth orbit is, « Limitations of Bohr’s Model Gi) The theory could not explain the atomic spectra of the atoms containing more than one electron or multielectron atoms. Gi) Bohv?s theory failed to explain the fine structure of the spectral lines. Gii) Bohr’s theory could not offer any satisfactory explanation of Zeeman effect and Stark effect. Gv) Bohr’s theory failed to explain the ability of atoms to form molecule formed by chemical bonds. Cv) It was not in accordance with the Heisenberg’s uncertainty principle. « Dual Behaviour of Matter Cde Broglie Equation) de Broglie in 1924, proposed that matter, like radiation, should also exhibit dual behaviour i.e., both particle like and wave like properties. This means that like photons, electrons also have momentum as well as wavelength. From this analogy, de Broglie gave the following relation between wavelength CA) and momentum (p) of a material particle. + Heisenberg’s Uncertainty Principle It states that, “It is impossible to determine simultaneously, the exact position and exact momentum Cor velocity) of an electron”. + Significance of Uncertainty Principle @ It rules out existence of definite paths or trajectories of electrons and other similar particles, Gi) The effect of Heisenberg’s uncertainty principle is significant only for microscopic objects and is negligible for macroscopic objects. + Reasons for the Failure of Bohr Model G) The wave character of the electron is not considered in Bohr Model. Gi) According to Bohr Model an orbit is a clearly defined path and this path can completely be defined only if both the position and the velocity of the electron are known exactly at the same time. This is not possible according to the Heisenberg’s uncertainty principle. + Quantum Mechanical Mode! of Atom Quantum mechanics: Quantum mechanics is a theoretical science that deals with the study of the motions of the microscopic objects that have both observable wave like and particle like properties. Important Features of Quantum Mechanical Model of Atom Gi) The energy of electrons in atom is quantized i.e, can only have certain values. Gi) The existence of quantized electronic energy level is a direct result of the wave like properties of electrons. Gi) Both the exact position and exact velocity of an electron in an atom cannot be determined simultaneously. Civ) An atomic orbital has wave function @. There are many orbitals in an atom, Electron occupy an atomic orbital which has definite energy. An orbital cannot have more than two electrons. The orbitals are filled in increasing order of energy. All the information about the electron in an atom is stored in orbital wave function @. () The probability of finding electron at a point within an atom is proportional to square of orbital wave function i.e. |p2]at that point. It is known as probability density and is always positive. From the value of 92 at different points within atom, it is possible to predict the region around the nucleus where electron most probably will be found. + Quantum Numbers Atomic orbitals can be specified by giving their corresponding energies and angular momentums which are quantized (i.e, they have specific values). The quantized values can be expressed in terms of quantum number. These are used to get complete information about electron i.e, its location, energy, spin etc. Principal Quantum Number (n) It is the most important guantum number since it tells the principal energy level or shell to which the electron belongs. It is denoted by the letter V and can have any integral value except zero, he, N= I, 2, 3, Guu CLC. The various principal energy shells are also designated by the letters, K, L, M, N, O, P ..... etc. Starting from the nucleus. The principal guantum number gives us the following information: G It gives the average distance of the electron from the nucleus. Gi) It completely determines the energy of the electron in hydrogen atom and hydrogen like particles. Gi) The maximum number of electrons present in any principal shell is given by 2n2 where n is the number of the principal shell. Azimuthal or Subsidiary or Orbital Angular Quantum Number (i) It is found that the spectra of the elements contain not only the main lines but there ave many fine lines also present. This number helps to explain the fine lines of the spectrum. The azimuthal quantum number gives the following information: G@ The number of subshells present in the main shell. Gi) The angular momentum of the electron present in any subshell. Cin) The relative energies of various subshells.