CHEMISTRY UNIT 2 STRUCTURE OF ATOM COMPLETE KNOWLEDGE CLASS 11th
UNIT 2
STRUCTURE OF ATOM
The existence of atoms has been proposed since the times of early Indian and Greek philosophers around 400B.C.Anlndian philosopher Maharishi Kanad postulated that it we go on dividing matter (padarth), we shall ultimately get smallest particle beyond which further division will not be possible. He ancient Greek named these particles as parmanu. Around the same era, ancient Greek philosophers Democritus and Leucippus suggested that if we go on dividing matter, a stage will come when particles obtained cannot be divided further. Democritus called these particles as atoms. The word 'atom' has been derived i from the Greek word 'a-tomio' meaning un cutable or non-divisible. However all these ideas were based on philosophical considerations and not much experimental work to validate these ideas could be done till the 18th century
The The first definite theory about the structure of matter was put forward by John Dalton, a British school teacher in1808. His theory called Dalton's atomic theory regarded atom as the ultimate particle of matter. According to his theory, all matter are composed of extremely small particles called atoms. The atoms were regarded to be structureless, hard spherical particles. The Dalton's atomic theory remained undisputed upto the end of 19th century. However, the discoveries towards the end of 19th and early 20th centuries showed that atom hasa complex structure and is not indivisible. These studies further revealed that atom consists of still smaller particles such as electron, proton.andneutron, into which it may be divided. These particles are regarded as fundamental particles because these are the main constituents of all atoms.
SUBATOMIC PARTICLES
The Dalton's atomic theory was able to successfully explain the laws Chemical combinations. However, many experimental observations de by scientists towards the end of nineteenth and beginning of made entieth century showed that atoms have an internal structure twentieth cei consisting Da of subatomic particles. Many different kinds of subatomic particles were biets Were discovered in the twentieth century. Let us briefly review history Cal development for the discovery of these fundamental particles
Discovery of Electron
The electrical nature of matter has been known from ancient times For example it was known that glass or ebonite rod when rubbed with silk or fun generate electricity. In 1830 Michael Faraday gave the first important clue about the electrical nature is of matter. He observed that when electricity is passed through a solution of the electrolyte chemical reaction occurred at the electrodes which resulted in the liberation and deposition of matter at the electrodes This phenomenon is called electrolysis. He formulated certain laws known as Faradays laws of electrolysis which you will study in class XII
The importance of Faraday's laws lies in the fact hat Faraday suggested the relationship between electricity and matter for the first time. Later on, 1n 1874, Stoney pointed out that like matter, electricity 1s composed of small discrete units of electricity. He proposed the name electron for these discrete units of electricity. However, the discovery of electron came as a result of experiments on conduction of electricity through gases as described below.
Studies of Cathode Rays Discharge tube experiments
The electron was discovered by J.J. Thomson at the end of 19th century during the studies of the passage of electricity through gases at extremely low pressures. These experiments were known as discharge tube experiments.Under ordinary conditions, gases are poor conductors of electricity. However, when a high voltage is applied to them at very low pressures, the gases become conductors and electricity begins to flow in the form of rays. These rays are called cathode rays. The existence of these rays was shown by scientists like Plucker, Crookes, etc., but the main credit goes to J.J. Thomson. He studied the properties of cathode rays in detail which led to the discovery of an electron.
The experiment in its simplest form consists oI a cylindrical hard glass tube (about 50 cm long) closed at both ends [Fig.1]. It is known as discharge tube or Crookes tube. It is fitted with two metallic electrodes. The tube is connected to a side tube, through which it can be evacuated to any desired pressure with the help of a vacuum pump. The discharge tube is filled with the gas under study and the two electrodes are connected to a source of high voltage. During these experiments,the following observation sare made:
i) when the discharge tube contains any gas at normal pressure, nothing is observed even by applying high voltage (5,000-10,000V) between the electrodes. The gas remains non-conducting.
ii) The pressure of the gas inside the tuhe in. decreased by pumping out the gas with the help of a vacuum pump. When the pressure or the gasi LO mm of i8 decreased to about 10-2 atm (about 1 mm to 10 mm of is emitted vacuum Hg), the gas becomes conducting and light is emitted Dy the residual gas in the tube. The colour of the light depends upon the nature of the gas taken.
iii) When the pressure of the gas in the discharge tube is further reduced, the glow becomes weak. At about 10-atm pressure (about 0.01 mm of Hg), the glow between the electrodes disappears but the gas continues to conduct electricity. Moreover, if a perforated anode is used, a faint greenish glow is observed on the glass wall behind the anode.
It was discovered that the glowing of the tube is due to the bombardment of the glass by certain rays emitted from the cathode. These rays moved from the cathode towards the anode in the form of streaks of light. The flow of current from cathode to anode was further checked by making a hole in anode and coating the tube behind anode with fluorescent material zine sulphide. When these rays after passing through anode strike the zinc sulphide coating, a bright spot on the coating is developed (same thing happens in a television set). These rays were named cathode rays because they originate from the cathode.
NATURE OF CATHODE RAYS
The following experiments help in understanding the nature cathode rays:
1. The 1The cathode rays start from cathode and move towards anode. Whenever an object is placed inside the tube, it casts a shadow on the wall opposite to the the tube, it casts a shadow on the wall opposite to the cathode During this experiment, the gas fluoresces only in regions outside the shadow. This experiment showed that the cathode rays travel in straight lines. Further, since the shadow falls on the wall opposite to the cathode, it- shows that the rays travel from cathode towards the anode.
2. These rays themselves are not visible but their behaviour can be observed with the help of certain kinds of materials (fluorescent or phosphorescent)which glow when hit by them. It may be remembered that television tubes are also cathode tubes and television pictures result due to fluorescence on the television screen coated with certain fluorescent or phosphorescent materials.
3. Cathode rays produce mechanical effects. For example, when a smal paddle wheel is placed between the electrodes,it rotates [Fig. 3]. This indicates that the cathode rays consist of material particles.
4. In the absence of electrical or magnetic fields. these rays travel in straight lines.When electric anc T magnetic fields are applied to the cathode rays in the discharge tube, the rays are deflected, thus establishing that they consist of charged particles. The direction of the deflection shows that the charge was negative as show below
5. When the cathode rays are allowed to strike a thin metal foil, it gets heated up. Thus, the cathode rays possess heating effect.
6. The cathode rays produce X-rays when they strike against hard metals like tungsten, copper, etc.
7. The cathode rays upon striking glass or certain other materials cause them to glow (produce fluorescence)
8. The cathode rays penetrate through thin sheets of aluminium and other metals.
9. The cathode rays affect the photographic plates.
10. The characteristics of cathode rays do not depend upon the nature of electrodes and the nature of gas present in the cathode rays
i) Effect of electric field = The effect of electric field on the cathode rays was studied by j. j. Thomson in 1897. The discharge tube was modified by adding a pair of metal plates as shown in Fig. 4. When these plates are given opposite electric charges, the beams of cathode rays are deflected towards the positively charged plate. This shows that the particles in the cathode rays carry negative charge.
ii) Effect of magnetic field. = when the cathode rays are made to pass through a magnetic field, these are deflected in the direction corresponding to the presence of the negative charge on the particles.
Charge and mass of electron
(a) Determination of charge to mass ratio of electrons = in L897, J.J. Thomson determined the ratio of the charge (e) of the electron to its mass (m) by measur in BU he deflection under the simultaneous influence electric and magnetic fields, applied perpendicular w each other. The apparatus is shown in Fig. 6. A ug potential is maintained between the cathode and the anode. Electrons emitted from the cathode are a celerated bythe high voltage.The circular disc after the anode selects the beam moving in a straight line. The beam then passes through electric and magnetic fields which are perpendicular to each other and also to the direction of the motion. Thomson suggested that the amount of deviation of the particles from their pathh in the presence of electrical and magnetic fields depends upon the following
i) Magnitude of the negative charge on the particles = Greater the magnitude of the charge on the particles, greater is the interaction with the electric or magnetic field and therefore, greater is the deflection.
ii) Mass of the particles.= The extent of deviation depends upon the mass of the particles. Lighter the particles greater is the deflection.
iii) Strength of electrical and magnetic field.= The deflection of the particles from their or original path increases with the increase in the voltage across the electrodes or strength of the magnetic field.
b) Determination of charge on the electron
The charge on the electron was measured by RA drop Millikan in 1909 by a method known as oil dron Small method. The apparatus used is shown in Fig 7. Smai drops of oil in the form of mist are formed by a sprayer and these are allowed to fall in between two metal plates, which could be electrically charged. A single drop between the plates is observed by means of a telescope equipped with a micro meter eye piece. The oil in the form of mist drop falls through the air under the influence of gravitational force. He then irradiated the space between the plates with X-rays. These knocked electrons out of some of the molecules of the air and some of these electrons, were caught by oil droplets which acquired electrical charge. By charging the upper plate positive and the lower plate negative, the oil drop experiences electric field in the upper direction. By adjusting the electrical field strength, the upward electrical field on the oil droplet was balanced against the downward gravitational force. Under these conditions, the drop remains stationary. From the amount of charge on the plates and the mass of the droplet, the charge on the droplet was determined. The mass of the droplet was determined from the rate of fall of droplet through the air when the plates were were uncharged
The Charge and Mass Of Particles Constituting
Anode Rays
By means of deflections in electric and magnetic field, charge to mass ratio (e/m) of the particles constituting anode rays was determined for different gases (in the same manner as described for cathode rays). It was found that the value of e /m for these rays is not constant and is considerably smaller than that for electrons. Unlike cathode rays, the charge to mass (elm) ratio of anode rays was found to depend upon the nature of the gas taken in the discharge tube. This is obvious because positively charged particles are produced by the loss of one or more electrons from the neutral atoms of the gas contained in the discharge tube. Therefore, the mass of the positively charged Darticles will depend upon the nature of the gas. In case of hydrogen, the charge to mass (elm) ratio was maximum. The value was found to be 9.58107 coulombs per kg. Its charge has been found t 1.6022 x 10-1 coulombs which is equal in magnitude but opposite in sign to tha ol an electron i.e., it has ona unit of positive charge.
Origin of Positive Rays
We have studied that in discharge tube, cathode rays are emitted from the cathode. These rays consist of stream of electrons which move towards anode with very high speeds. When these fast-moving electrons strike the atoms or molecules of the gas contained in the discharge tube, they knock off one or more electrons. The removal of electrons from neutral atoms or molecules of gas results in the formation of positively charged ions. These positively charged iens move towards perforated cathode and constitute the eam of positive rays or anode rays coming through the holes of the cathode. These charged ions carry various amounts of positive charge depending upon the number of electrons lost by the corresponding atoms or molecules.
ARRANGING ELECTRONS AND PROTONS IN AN ATOM
THOMSON'S MODEL OF ATOM = After the discovery of electron and proton, the scientists started thinking of arranging these particles in an atom. Different models were propOsed to explain the distribution of subatomic particles in an atom. The first simple model was proposed by J.J. Thomson known as Thomson's atomic model.
J.J. Thomson proposed that an atom consists of a uniform sphere in which positive charge is uniformly distributed. The electrons are embeded into it in such a way as to give the most stable electro static arrangement(Fig. 11) The radius of the sphere is of the order of 10-10 m, which is equal to the size of the atom. This model was much like pudding or cake (of positive charge) with raisins (electrons) embedded into it. Therefore, this model is also known as raisin pudding model. This model was also compared with water melon model of positive charge in which seeds (electrons) are embedded. Therefore, this model is given different names such as raisin pudding, plum pudding or watermelon model. An important feature of this model is that the mass of the atom is considered to be evenly spread over the atom. This model explains some of the known properties and electrical neutrality of atom. However, it was soon discarded, when Rutherford and his co-workers observed unusual scattering of a-particles by the thin metal foils. Since this model could not explain the experimental results, it was therefore, rejected.
RUTHERFORD SCATTERING EXPERIMENT RUTHERFORD MODEL OF AN ATOM
In order to understand the arrangement of electrons and protons in an atom, Rutherford and his student in 1909 performed a series of experiments known as Rutherford's scattering experiments. In their experiments, they bombarded a target of atoms by subatomic projectiles. These projectiles called alpha
a) particles= were obtained from a radioactive substance. Alpha particles are high energy positively charged helium ions having charge +2 and mass 4 u. They bombarded alpha («) particles emitted from a radioactive substance on a piece of thin foil of gold or some other heavy metals.
In this experiment, a piece of radioactive substance (radium) is placed in a lead block The block is constructed in such a way with slits that only a narrow beam of a-particles could escape. The beam of high energy oa-particles was directed at a thin gold foil (thickness about 100 nm). In order to detect the a-particles after scattering, a movable circular screen coated with zinc sulphide is placed around the gold foil.
When particles strike the zinc sulphide screen these produce flashes of light or scintillation which can be detected. By examining different portions of the screen, it was possible to determine the proportions of the particles which got deflected through various angles. The following observations were made from these experiments.
i) Most of the particles ( nearly 99%) passed through the gold foil Undeflected.
ii) A small fraction of a- particles got deflected through small angles.
iii) Very few (about one in 20,000) did not pass through the foil at all but suffered large deflections (more than 90°) or even came back suffering a deflection of 180°
Now according to Thomson's model, if the positive charge of the atom was uniformly distributed, then a by positively charged a-particles with a considerable mass (4 a.m.u.) would pass through weak electric field largely undeflected or slightly deflected. The expected deflections on the basis of Thomson's model are shown e y a in Fig. 13. However, he noticed that some of the O-particles experienced strong deflections. Even some particles returned back from the foil. Thus, Thomson's model could not provide answers for these observations r and therefore, was discarded.
Rutherford explained these observations as follows :
i) Since most of the a-particles passed through the gold foil undeflected, it means that there must be very large empty space within the atom
ii) Alpha particles are positively charged and have considerable mass. They can be deflected only if they come close to some heavy positively charged mass due to enormous force of repulsion. Since some of the a-particles are deflected to certain angles, it means that there is a heavy positively charged mass present in the atom. Moreover, this mass must be occupying a very small space with in the atom because only a few a-particles suffered large deflections.
iii) The strong deflections or even bouncing back of o-particles from the foil, were explained to be the direct collision with the heavy positively charged mass.
The positively charged heavy mass which occupies only a small volume in am atom is called nucleus. It is supposed to be present in the centre of the atom.
All these types of deflections of a-particles from atoms are shown in Fig. 14. It is clear from the figure that the a-particles which pass at large distances from the nucleus do not suffer any deflections(marked a), those which pass close to the nucleus suffer small deflections (marked b), while very few which hit the nucleus are either deflected to large angles or they retraced their paths (marked c).
On the basis of the above experiments and observations, Rutherford proposed a model for the structure of the atom called Rutherford's nuclear model of atom.
Rutherford Nuclear Model of Atom
i) In atom the entire mass and the positive charge is concentrated in a very small region at the centre known as nucleus. It was observed that the volume occupied by the nucleus is negligibly as compared to the total volume of the atom.
ii) The positive charge of the nucleus is due to protons. The magnitude of the positive charge on the nucleus number of protons) is different for different atoms.
iii) The mass of the nucleus is due to protons and some other neutral particles each having mass nearly equal to the mass of the proton. This neutral particle was discovered later on by Chadwick in 1932. It was called neutron.
iv) The nucleus is surrounded by negatively charged electrons which balance the positive charge on the nucleus. Thus, the atom is electrically neutral.
v) The electrons are not stationary but are revolving around the nucleus at very high speeds like planets revolving around the sun. As a result, the electrons are also called planetary electrons.* Thus, the Rutherford's model of atom resembles the solar system in which the nucleus plays the role of the sun and revolving electrons play the role of planets.
vi) The electrons and the nucleus are held together by electrostatic forces of attraction.
vii) Most of the space in an atom between the nucleus and the revolving electrons is empty.
CONCEPT OF ATOMIC NUMBER, MASS NUMBER AND DISCOVERY OF NEUTRON
In 1913 Moseley determined the magnitude of the positive charge on the nucleus of an atom by studying the nature of X-rays emitted by different elements. The number of unit positive charges carried by the nucleus of an atom is termed as the atomic number. Since the positive charge on the nucleus is due to the presence of protons in it and each proton carries one unit positive charge, therefore, the atomic number is numerically equal to the number of protons present in the nucleus of an atom. For example, the number of protons in the hydrogen nucleus is 1 and therefore, atomic number of hydrogen is 1. Similarly, the atomic number of carbon is 6.
Moreover, the number of protons in an atom is equal to the number of electrons since atom on the whole is electrically neutral, Thus, atomic nucleus of an element is equal to the number of protons prese in the nucleus or the number her of electrons prese nt outside the nucleus. For example, number of elev in hydrogen atom and carbon atoms are l an respectively.It is generally denoted by the letter 2. Thus,
Atomic number (Z)= Nuclear charge or number of protons (P)
= number of electrons (e)
Discovery of Neutrons
The whole of the mass of an atom is due to the nucleus containing protons because the electrons revolving outside are of negligible mass. This means that the nucleus must contain protons equal to the mass of the atom (atomic mass).But, the number of protons is equal to atomic number. This means that atomic mass should be equal to the atomic number. However, it has been found that for all atoms except hydrogen, atomic mass is more than the atomic number. For example, the atomic number of carbon is 6 and thus,it should have 6 protons in the nucleus and six extra nuclear electrons. This means that carbon atom should be six times heavier than the hydrogen atom. However, its mass has been found to be twelve times heavier than that of the hydrogen atom (at. mass = 12 a.m.u.). To solve this problem, it was thought that the balance of the mass has to be provided by some neutral particles as the positive charge is already balanced by electrons. The neutral particle. called neutron, was discovered by Chadwick
Chadwick, in 1932 bombarded a thin sheet of beryllium element with a-particles and observed highly penetrating rays which consist of streams o neutral particles. The neutral particles were found to have mass of 1.675 x 10-27 kg which is nearly the same as that of hydrogen atom aand have no charge. These were named neutrons.
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