Structure of Atom GRADE 11DISCOVERY OF AN ELECTRON « Anelectron was discovered by cathode ray discharge tubes experiment. » Acathode ray tube is made of glass containing two thin pieces of metal called electrodes, sealed in it. 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 , the current starts to flow through a stream of particles moving in the tube from the nesaliie electrode to the positive one. These rays were called the cathode rays or cathode ray particles. » The flow of current from cathode to anode was further checked by making a hole in the anode and coasting the tube behind anode with phosphorescent material called zinc sulphide coating , a bright spot on the coating is developed . [wavy bchae om reneResults of the experiment hs cated rays start from cathode and move towards the anode, These rays were not visible but their behaviour could be observed with a certain kind of material called FLOUROSCENT OR PHOSPHORESCENT MATERIALS. In the absence of electrical or magnetic field these rays travel in straight lines. In the presence of electrical or magnetic field , the behaviour of cathode rays are similar to that expected from negatively charged particles suggesting that the cathode rays consist of negatively charged particles called electrons. The characteristics of cathode rays do not depend upon the material of electrodes nature of the gas present in the tube .CHARGE TO MASS RATIO OF ELECTRON The measure of mass ratio of an electrical charge (e) to the mass of an electron (m,) by using the cathode rays discharge tube and applying electrical and magnetic field perpendicular to each other as well as to the path of electrons. The amount of deviation of the particles from their path in the presence of electrical and magnetic field depend upon: The magnitude of the negative charge on the particle, greater the magnitude of the charge on the particle , greater is the interaction with the electric and magnetic field and thus greater is the deflection. The mass of the particle: lighter the particle , greater the deflection and vice versa. The strength of the electrical and magnetic field-the deflection of electrons from its original path increases with the increase in the voltage across the path electrodes or the strength of the magnetic field.« When only electric field is applied, the electrons deviate from their path and hit the cathode ray tube at point A. « When only magnetic field is applied electron strikes the cathode ray tube at point C. « When electrons deviate from their path, then both electrical and magnetic field is applied , it is possible to bring them back and electrons go and hit the screen at point B. e © =1.758820x10!!C kg? meDiscovery of protons and neutrons « Electrical discharge carried out in the modified cathode rays tube led to the discovery of particles carrying positive charge also known as canal rays. « The characteristics of these rays are: > Unlike cathode rays, the positively charges particles depend upon the nature of gas present in the cathode ray tube.These gases are simple positively charged ions. = The charge to mass ratio of the particles is found to depend on the gas from which they originate. = Some of the +vely charged particles carry a multiple of the fundamental unit of electrical charge. = The behaviour of these particles in the magnetic or electrical field is opposite to that observed for electron or cathode rays.« The smallest and lightest positive ion was obtained from hydrogen and was called the proton. « Discovery of neutrons: Chadwick felt that by bombarding a thin sheet of beryllium by a- particles. When electrically neutral particles having a mass slightly greater than that of the protons was emitted. He named these neutral particles as neutrons. Thus this discovery Was a very important discovery in the history of chemistry.Thomsons model of an atom « According to Thomson, atom was ina spherical in shape which had positive charged particle sand negative charged particles equally distribution and hence it was electrically neutral. « Its observation could be called as a plum pudding model or a watermelon.Rutherford’s model of atom « Famous experiment of Rutherford was the a- particle scattering experiment. « Astream of high energy a particles from a radioactive source was directed at a thin foil ofa gold metal. The thin foil had a circular fluorescent zinc sulphide screen around it .Whenever a- particles struck the screen, a tiny flash of light was produced at the point.« The results of this experiment were unexpected. e (1) most of the a-particles passed through the gold foil undeflected. ¢ (2)a small fraction of a-particles was deflected by small angles e (3) avery few a-particles (-1 in 20,000) bounced back, that is were deflected by nearly 180 degree. Observations: « Most of the space in the atom is empty as most of the a-particles passed through the foil undeflected. « A few +vely charged a-particles were deflected. « The deflection must be due to enormous repulsive force showing the positive charge in the atom. « The positive charge has to be concentrated in a very small volume that repelled and deflected the positively charged a-particles .« Conclusions: « The positive charge and most of the massof the atom was densely concentrated in extremely small region. This concentrated region was called nucleus. « The nucleus was surrounded by electrons moving ina very high speed in circular paths called orbits. » Electrons and the nucleus are held together by the electrostatic forces of attraction.Atomic number and mass number « Atomic number(Z)=number of protons present in the nucleus = number of electrons in the neutral atom. « Electrons and protons together ina nucleus are / were called nucleons. « Mass number (A)= number of protons (z)=number of neutrons(n)Isobars and isotopes « Isobars are elements having the same mass number but different atomic number. « Whereas isotopes are elements having same atomic number but a different mass number. « Hydrogen has 3 isotopes: protium , deuterium and tritium. Chemical properties of atoms are controlled by the number of protons in the nucleus therefore they show similar chemical properties and similar chemical behaviourDraw backs of Rutherford’s model of atom. « It could not explain the gravitational force in nature. e It could not explain planetary motion under the influence of gravity. « It could not explain Maxwell's electromagnetic radiation property. « It could not explain quantum mechanics as a whole.The Electromagnetic Spectrum ¢ Visible light is a small portion of the electromagnetic radiation spectrum detected by our eyes. ¢ Electromagnetic radiation a includes radio waves, microwaves and X-rays. e Described as a wave traveling through space. e There are two components to electromagnetic radiation, an electric field and magnetic field.The Wave Nature of Light « Wavelength, A, is the distance between two corresponding points on a wave. « Amplitude is the size or “height” of a wave. « Frequency, v, is the number of cycles of the wave passing a given point per second, usually expressed in Hz.Planck’s Quantum Theory « Higher T = shorter A (higher E) maximum. e Couldn’t explain with classical physics As Tt, the Wavelength of maximum intensity shifts toward the blue 5780 K (sunlight)Electromagnetic Radiation « In 1900 Max Planck studied black body radiation and realized that to explain the energy spectrum he had to assume that: energy is quantized light has particle character 1 2, « Planck’s equation is E=hv or E=-< A h = Planck’s constant= 6.626x 10 J-sThe Wave Nature of Light © The fourth variable of light is velocity. ¢ Alllight has the same speed in a vacuum. © €=2,99792458 x 108 m/s e The product of the frequency and wavelength is the speed of light. c=AvV e Frequency is inversely proportional to wavelength.The Wave Nature of Light e Electromagnetic radiation can be categorized in terms of wavelength or frequency. « Visible light is a small portion of the entire electromagnetic spectrum. Energy Increases 10 102 go! gol to 0!!! aot to? 0? (te) H 1 1 1 ! ! H 1 1 1 H i 1 yrays X-rays | UV IR ‘Microwave R| [AM] Long radio waves Radiowaves T T T T 7 1 T + T 1 1 “ lL oS ues Visible spectrum 400 300 0 00 Energy increases Wavelength increases A(am)The Particulate Nature of Light « Photoelectric effect: light striking a metal surface generates photoelectrons. o The light’s energy is transferred to electrons in metal. o With sufficient energy, electrons “break free” of the metal. e Electrons given more energy move faster (have higher kinetic energy) when they leave the metal.The Particulate Nature of Light « Photoelectric effect is used in photocathodes. e Light strikes the ode at frequency v. Electrons are ejected if v exceeds the threshold value vy. @ Electrons are collected at the anode. Current flow is used to monitor light intensity. eExample Problem 6.1 « Neon lights emit an orange-red colored glow. This light has a wavelength of 670 nm. What is the frequency of this light? « We know that c=vA Therefore c/A=v and solve.The Particulate Nature of Light » The photoelectric effect is not explained using a wave description but is explained by modeling light as a particle. » Wave-particle duality - depending on the situation, light is best described as a wave or a particle. « Light is best described as a particle when light is imparting energy to another object. © Particles of light are called photons. « Neither waves nor particles provide an accurate description of all the properties of light. Use the model that best describes the properties being examined.Photoelectric Experiments © For v > vo, the number of electrons emitted is independent of frequency. Value of vy depends on metal used. As light intensity increases, the number of photoelectrons increases. As frequency increases, kinetic energy of emitted electrons increases linearly. The kinetic energy of emitted electrons is independent of light intensity.The Particulate Nature of Light « The energy of a photon (E£) is proportional to the frequency (v). » and is inversely proportional to the wavelength a « h=Planck’s constant = 6.626 x 104] s _ he Aa E=hvThe Particulate Nature of Light « Binding Energy - energy holding an electron toa metal. e Threshold EAU , v, - minimum frequency of light needed to emit an electron. e For frequencies below the threshold frequency, no electrons are emitted. For frequencies above the threshold frequency, extra energy Is imparted to the electrons as kinetic energy. * Eshoton = Binding E + Kinetic E e This explains the photoelectric effect.Example Problem 6.2 © The laser in a standard laser printer emits light with a wavelength of 780.0 nm. What is the energy of a photon of this light?The Wave Nature of Light is the bending of light when it passes from one medium to another of different density. « Speed of light changes. e Light bends at an angle depending on its wavelength. White lightExample Problem 6.3 « Ina photoelectric experiment, ultraviolet light with a wavelength of 337 nm was directed at the surface of a piece of potassium metal. The kinetic energy of the ejected electrons was measured as 2.30 x 10°19 J. What is the electron binding energy for potassium?Atomic Spectra « Atomic Spectra: the particular pattern of wavelengths absorbed and emitted by an element. e Wavelengths are well separated or discrete. e Wavelengths vary from one element to the next. « Atoms can only exist in a few states with very specific energies. e When light is emitted, the atom goes from a higher energy state to a lower energy state.Atomic Spectra « Electrical current dissociates molecular H, into excited atoms which emit light with 4 wavelengths.Atomic Spectra and the Bohr Atom « Every element has a unique spectrum. « Thus we can use spectra to identify elements. e This can be done in the lab, stars, fireworks, etc.Example Problem 6.4 « When a hydrogen atom undergoes a transition from E; to E,, it emits a photon with 4 = 102.6 nm. Similarly, if the atom undergoes a transition from E; to E,, it emits a photon with 4 = 656.3 nm. Find the wavelength of light emitted by an atom making a transition from E, to E,. SSS SS By 2 = 656.3 nm 2. = 102.6 nm EyThe Bohr Atom « Bohr model - electrons orbit the nucleus in stable orbits. Although not a completely accurate model, it can be used to explain absorption and emission. e Electrons move from low energy to higher energy orbits by absorbing energy. e Electrons move from high energy to lower energy orbits by emitting energy. e Lower energy orbits are closer to the nucleus due to electrostatics.The Bohr Atom « Ground state: the lowest state (orbital)where the electrons are initially present is called the ground state. « Excited state: the state where the electrons on gaining energy I subjected to go toa higher energy level is called the excited state. Atoms return to the ground state by emitting energy as light.Atomic Spectra and the Bohr Atom « The Rydberg 1 14 equation is an == et = +] empirical equation 4 m TM that relates the R is the Rydberg constant wavelengths of the R=1.097«10? m? lines in the hydrogen spectrum. n’s refer tothe numbers n, > > The region where this probability of finding an electron reduces to zero is called nodal surfaces or_simply nodes. In general, for an ns orbital , an ns orbital has (n-1)nodes, that is number of nodes increases with the increase of quantum number n.therefore for 2s it will be 1 and for 3s it will be 2. Boundary surface diagrams of constant probability density for different orbitals give a fairly good representation of the shapes of the orbitals . In this representation, a boundary surface or a contour surface is drawn in space for an orbital on which the value of probability density is constant.« In the boundary surface diagrams the nucleus is taken to be at the origin or rather it is . Here, , diagrams are not spherical like the s-orbital. Here the p-orbital consists of two section s called lobes. That are on either sides of the plane where the two lobes touch each other. The size, shape and energy of the three orbitals are identical . « Since the lobes are considered along the x, y and the z axis they are designated as the above:- 2B: 2Py2pz e It should be understood that there is no relation between first magnetic quantum number and x, y, z directionsBoundary surface diagrams for 2p orbitals 2p, 2p, * z 2p,d- orbitals e The 5 d-orbitals are designated as:- dyz Apq,0 252 and d,2 The shapes of the first four d-orbitals are similar to each other , where as the fifth one is different form others, but all 5 have 3d- orbitals and are equivalent in energy. The d- orbitals for which n is greater than 3 also have shapes similar to 3d orbital , but differ in energy. When two nodal planes pass through the same origin and bisecting the xy-pane and z-plane these nodes are called angular nodes.« Angular nodes are denoted by ’¢. There are one angular node for the p-orbitals and 2 angular nodes for d-orbitals The total number of nodes are given by (n-1)i.e the sum of ¢ angular nodes are (n-¢-1) radial nodes. Energies of orbitals:- Energy increasing order in the orbitals is given as follows:- 1s<2s=2p<3s=3p=3d<4s=4p=4d=4f< As 2p and 2s orbitals are different, an electron has the same energy as it is present in the 2p or the 2s orbital._ The orbitals which have the same energy are called degenerate. The 1s orbital in a hydrogen atom corresponds to the most stable condition and is called the ground state. And an electron residing in this orbital is strongly held by the nucleus _ The electrons residing in the 2s, 2p or higher orbitals in the hydrogen atom are said to be in the excited state. _ The attractive interactions of an electron increases with the increase of the positive charge (Ze) on the nucleus.® ® @ Due to the presence of electrons in the inner shells, the electron in the outer shells will not experience full positive charge of the nucleus. The effect will be lowered due to the partial screening of positive charge on the nucleus by the inner shell electrons. This is known as shielding of outer electrons from the nucleus by the inner shell electrons and the net positive charge experienced by the outer electrons is known as effective nuclear charge.(Z.¢<) In other word the energy of interaction b/w the nucleus and electron decreases with the increase of atomic number Z.Aufbau principle e Aufbau principle deals with filling up of electrons. The principle states:- In the ground state of the atoms , the orbitals are filled in order of the increasing energies. e In other words, electrons first occupy the lowest energy orbitals available tot hem and then enter into higher energy orbital only after lower energy orbital is filled. « Order of increasing order of energies in the orbital is as follows:- 1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,4f,5d,6p,7s........ts Order of filling up of electroPauli exclusion principle « According to this principle :- no two electrons in this atom have the same set of four quantum numbers. Or It can also be stated otherwise as only two electrons may exist in the same orbital and these orbital must have opposite spins The maximum number of electrons which can be accommodated in the shell with the quantum number n is according to the 7,2 rule.Hund’s rule of maximum multiplicity e This rule deals filling of electrons in the orbitals belonging to the same subshells of equal energy called degenerate orbitals. « It states that pairing of electrons in the orbitals belonging to the same sub shell (p,d, or f) does not take place until each orbital of that sub shell gets one electron that is singly occupied. e Some of the orbitals acquire extra stability due to their symmetry.Electronic configuration « The distribution of electrons into orbitals of an atom is called its electronic configuration. « Electronic configuration can be represented in two ways:- (a)Normal notation and (b) orbital diagram As given in the textbook. The electron in the completely filled electronic shell with the highest principal quantum number are called valence electrons.