Structure of the Atom PDF

# Structure of the Atom ## The presence of positively charged particles in an atom has been predicted by Goldstein (1886) - based on the electrical neutrality of an atom. - The discovery of the proton by Goldstein was done based on the cathode ray experiment conducted by using a perforated catho...

# Structure of the Atom ## The presence of positively charged particles in an atom has been predicted by Goldstein (1886) - based on the electrical neutrality of an atom. - The discovery of the proton by Goldstein was done based on the cathode ray experiment conducted by using a perforated cathode. - In his experiment some rays were found to emanate from an anode and pass through the perforations in the cathode, without deflection by the cathode. These are called anode rays or canal rays. ## Now days we understand Goldstein's experiment as follows - When an electric discharge is passed through the gas, some of the molecules of the gas ionised and produce cathode rays. - Cathode rays consist of electrons. - These electrons move with high speed towards the anode. - As they move they collide with the remaining molecules of the gas in the tube causing them to lose electrons resulting in positive ions. - The positive ions formed are attracted by the perforated cathode. - The stream of these positive ions causes a glow on the glass wall of the discharge tube. - On the other side of the discharge tube, the cathode rays produce a green light. - The stream of positive ions so formed constitutes the positive or anode rays. ### Figure 3.7 Anode rays traveling through perforations in the cathode resulting in red glow on the cathode ray tube ### Properties of Anode rays: 1. Anode rays travel in straight lines. 2. They consisted of material particles. 3. They are deflected in electric and magnetic field in opposite to that of cathode rays. 4. The nature or e/m ratio of anode rays depends upon the nature of gas present in the cathode ray tube. 5. They are (particles of anode rays) simply positively charged gaseous ions. ## 3.4.2 Discovery of the Electron ### Activity 3.8 - Students form groups and discuss the following questions. - Present your discussion points to the class. 1. What happens to the electric bulb when you turn on the switch in your home? 2. What do you think happened inside the bulb? 3. Why do some electric bulbs give yellowish light and others white light? ### The Crooke's Discharge Tube or Cathode Ray Tube - In 1855 the German inventor Heinrich Geissler developed the mercury pump and produced the first good vacuum tubes. - These tubes, as modified by Sir William Crookes, became the first to produce cathode rays, leading eventually to the discovery of the electron. - Sir William Crookes was the first scientist who designed the discharge tube which was called the Crooke's Discharge Tube or Cathode Ray Tube. - The discharge tube consists of a glass tube from which most of the air has been evacuated having two metal plates sealed at both ends. - These metal plates are called electrodes. These electrodes are connected to positive and negative terminals of a battery. - The electrode connected to the positive terminal is known as anode and the electrode connected to the negative terminal is known as cathode. ### Figure 3.8 The Crooke's Discharge Tube or Cathode Ray Tube - The electrodes are connected to high voltage for the current to flow. - The high voltage provides energy for the atoms of a gas to further split or break up. - When both electrodes are connected to high voltage, current starts flowing. - At high pressure no electricity flows through the air in the discharge tube, so low pressure is used. - Low pressure helps in conduction of electricity. - At high voltage of (10,000 - 20,000 volts) and at normal atmospheric pressure there is no effect. - But keeping the same voltage if pressure is reduced to 0.0001 mm of Hg, a greenish glow was observed at anode. - The rays are emitted from the direction of the cathode, and are called cathode rays. - A good example of a cathode ray would be a fluorescent bulb in your home. - These bulbs give white light where as those bulbs with tungsten filament give yellowish light due to the glowing of the tungsten filament when electric current passes through them. ### Figure 3.9 Compact fluorescent lamps ## The electron was one of the fundamental subatomic particle that was discovered by the British physicist, J.J. Thomson, in 1897 - In the discovery of electrons J. J. Thomson performed several experiments which are presented below. ### Thomson's Experiment on the Path of Cathode Rays - J. J. Thomson conducted some experiments with a discharge tube for studying the properties of cathode rays. - The first experiment he has done is studying how cathode rays travel in the discharge tube by placing a small object between the cathode and the anode. - The formation of a shadow of the object on the opposite side of the cathode revealed that the cathode rays travel in straight lines. - The cathode rays travel towards the node because they are attracted by the positively charged anode. - When an object is placed opposite to the direction of the cathode rays, a sharp shadow having the shape of the object is formed on the surface of the discharge tube glass. - This concludes that cathode rays travel in straight lines. ### Figure 3.10 Cathode rays creating a shadow of small object on the cathode ray tube screen. ### Thomson's Experiment on the Particle Nature of the Cathode Rays - J. J. Thomson's second experiment was performed by placing a light paddle wheel between cathode and anode to study the particulate nature of the cathode rays. - He expected to see if the cathode rays could move the paddle wheel. - If they do so, then they have a particle nature. - He observed the rotation of the light paddle wheel which revealed that the cathode rays are small particles having mass and kinetic energy. ### Figure 3.11 Cathode rays rotating light paddle wheel between cathode and anode ### Thomson's Experiment on the Charge of the Cathode Ray - William Crookes experimented with cathode rays and magnets for the first time. - His observations on the deflection of the rays by magnetic fields led him to conclude that they were composed of negatively charged molecules. - Years later J. J. Thomson determined the molecules hypothesized by Crookes were actually negatively charged subatomic particles that he called corpuscles, but which were eventually named electrons. - J. J. Thomson performed the third experiment to investigate the charge of the cathode ray by passing cathode rays through electric and magnetic fields. - Upon passing through an electric field, the cathode rays bent towards the positive plate. - This proved that cathode rays are negatively charged particles. - Thomson concluded that the particles had a net negative charge; these particles are now called electrons. - We know from our knowledge of electricity, in physics, when an electric field is applied perpendicularly to the path of a negatively charged species, they deflect towards positive plate. - As opposite charges attract, the cathode ray particles are negatively charged. - Today we know that the only negatively charged fundamental particle of an atom is the electron. - JJ Thomson's contribution is immense in this regard. ### Figure 3.12 Cathode rays bend passing through electric field. - Passing cathode rays through magnetic field applied perpendicular to the path of the cathode rays. - This resulted in the deflection of the cathode rays perpendicular to the applied magnetic field. - From our electromagnetism lessons, in physics, we know that a moving electric charge generates a magnetic field. - A magnetic field induces electric charge movement, producing an electric current. - In an electromagnetic wave, the electric field and magnetic field are perpendicular to one another. - When the magnetic field is applied perpendicularly to the path of cathode rays, they get deflected towards the north pole of the magnet which is expected of negatively charged particles. - This further confirms that cathode rays are negatively charged. ### Figure 3.13 Cathode rays bend passing through a magnetic field. - Thomson carried out the above experiments with different gases, at low pressure to decrease the interaction between gas molecules, in the discharge tube. - No change in the properties of the cathode rays was observed despite the use of different gases in the discharge tube. - J. J. Thomson proved that atoms were not the most basic form of matter. - He demonstrated that cathode rays could move a paddle wheel placed between two electrodes, could be deflected, or bent, by magnetic or electric fields, which indicated that cathode rays consist of charged particles. - More important, by measuring the extent of the deflection of the cathode rays in magnetic or electric fields of various strengths, Thomson was able to calculate the mass-to-charge ratio of the particles to be 1.76 x 10° C/g. - These particles were emitted by the negatively charged cathode and repelled by the negative terminal of an electric field. - Because like charges repel each other and opposite charges attract, Thomson concluded that the particles had a net negative charge; these particles are now called electrons. ### Summary of the properties of cathode rays: 1. They travel in a straight line from the cathode and cast shadows of metallic objects placed in their path. 2. They cause mechanical motion of small puddle-wheel placed in their path; they possess kinetic energy and must be material particles. 3. Their properties are independent of the electrodes, and the gas present in the cathode ray tube. 4. The charge/mass ratio of the rays is constant. 5. Upon passing through electric field, the cathode rays bend towards the positive plate showing that they are negatively charged. 6. Cathode rays are negatively charged and affected by magnetic field. 7. Cathode rays affect ZnS screen. When cathode rays are allowed to strike ZnS screen, they produce a faint greenish fluorescence. 8. Cathode rays ionises the gases. When cathode rays are allowed to pass through gases, different glows are seen in the tube. These different glows are due to the ionisation of gases. 9. have good penetrating power. 10. Cathode rays produce X-rays. When cathode rays are allowed to fall on metals such as tungsten, copper, X-rays are observed. ## 3.4.3 J. J. Thomson’s Atomic Model ### Activity 3.9 - Students, form a group and discuss the following questions. - Present your discussion points to the class. 1. What is your understanding about atomic model? 2. According to Democritus, what would an atom look like? 3. What do you expect Dalton's Atomic Model to be like, based on his experimental findings? - Following the discovery of the electron, J.J. Thomson developed what became known as the 'plum pudding' model in 1904. - Plum pudding is an English dessert similar to a blueberry muffin. - In Thomson's plum pudding model of the atom, the electrons were embedded in a uniform sphere of positive charge like blueberries stuck into a muffin. - The positive matter was thought to be jelly-like or similar to a thick soup. - The electrons were somewhat mobile. - As they got closer to the outer portion of the atom, the positive charge in the region was greater than the neighbouring negative charges, and the electron would be pulled back more toward the centre region of the atom. - The best example of locally available material for Thomson's Model is watermelon. - The seeds of the watermelon would mimic the electrons, and the watery soft reddish part is the positively charged matter. ### Figure 3.14 J. J. Thomson's atomic model (left); the "plum pudding" model (centre); watermelon (right). ### Validity of Thomson's Atomic Model - Thomson's Atomic Model could successfully explain the electrical neutrality of an atom. - It, however, failed to explain how the positively charged particles are shielded from the negatively charged electrons without getting neutralized. - Today we know that the positively charged protons exist in the nucleus of an atom which of course is at the centre of the atomic structure and the electrons are revolving around them. - Therefore, Thomson's prediction is not negligible although it lacks some knowledge. ### Millikan's Oil Drop Experiment - Subsequently, an American scientist, Robert Millikan (1909) carried out a series of experiments using electrically charged oil droplets. - In this experiment some fine oil droplets were allowed to be sprayed into the chamber by an atomizer. - The air in the chamber is subjected to ionization by X-rays. - The electrons produced by the ionization of air attach themselves to the oil drops. - When a sufficient amount of electric field is applied, which can just balance the gravitational force acting on an oil drop, the drop remains suspended in the air. - From this experiment, Millikan observed that the smallest charge found on the cathode rays was approximately 1.59 x 10 coulombs, and the charge on each drop was always an integral multiple of that value. - Based on this observation, he concluded that 1.59 x 10 coulomb is the smallest possible charge, and considered that value as the charge of the electron. - With this information and Thomson's charge-to-mass ratio (1.76 x 10°C/g), Millikan determined the mass of an electron. ### Figure 3.15 The apparatus used by Millikan to determine the charge of an electron. - From our understanding of electricity today, it is obvious that the force on any electric charge in an electric field is equal to the product of the charge and the electric field. - Millikan was able to measure both the amount of electric force and magnitude of electric field on the tiny charge of an isolated oil droplet. - From this data he determined the magnitude of the charge itself.