2.1 Evidence for the existence of the electron

Transcription

1 2. The Atom

2 2.1 Evidence for the existence of the electron The universe consists of energy & matter. Matter occupies space and has mass. Law of conservation of mass states matter is neither created nor destroyed in the course of a chemical reaction All matter composed of particles (atoms, molecules or ions) Evidence is shown by expt involving the diffusion of ammonia gas and HCl gas in a glass tube

3 Diffusion Diffusion: is the movement of gas from areas of high gas concentration to areas of low gas concentration Definition!

4 Diffusion of ammonia and HCl gases

5 2.1 Evidence for the existence of the electron Atoms very minute particles e.g. H atom in its normal state has radius of about 4 x m The atom is made of even smaller particles The atom is considered to be the basic unit from which all substances are formed

6 2.2 History of the atom

7 Greeks Some ancient Greeks noticed that substances such as iron, gold, lead, silver were 'pure' They could not be broken down into simpler substances by heat.

8 400 BC The Greek philosophers were the first to propose that matter is made up of tiny particles. They didn t do any experiments to back this up.

9 Greeks The Greek word for indivisible is atomos from where we get the word atom to describe these indivisible particles

10 A Brief History of Chemistry Over the next two millennia, major advances in chemistry were achieved by alchemists. Their major goal was to convert certain elements into others by a process called transmutation. Copyright 2007 Pearson Benjamin Cummings. All rights reserved.

11 The Greeks History of the Atom Not the history of atom, but the idea of the atom The Greeks tried to understand matter (chemicals) and broke them down into earth, wind, fire, and air. ~ Democritus and Leucippus Greek philosophers

12 Greek Model Democritus To understand the very large, we must understand the very small. Greek philosopher Idea of democracy Idea of atomos Atomos = indivisible Atom is derived No experiments to support idea Democritus s model of atom No protons, electrons, or neutrons Solid and INDESTRUCTABLE

13 Democritus DEMOCRITUS (400 BC) First Atomic Hypothesis Atomos: Greek for uncuttable. Chop up a piece of matter until you reach the atomos. Properties of atoms: indestructible. changeable, however, into different forms. an infinite number of kinds so there are an infinite number of elements. hard substances have rough, prickly atoms that stick together. liquids have round, smooth atoms that slide over one another. smell is caused by atoms interacting with the nose rough atoms hurt. sleep is caused by atoms escaping the brain. death too many escaped or didn t return. the heart is the center of anger. the brain is the center of thought. the liver is the seat of desire. Nothing exists but atoms and space, all else is opinion.

14 Some Early Ideas on Matter Anaxagoras (Greek, born 500 B.C.) Suggested every substance had its own kind of seeds that clustered together to make the substance, much as our atoms cluster to make molecules. Empedocles (Greek, born in Sicily, 490 B.C.) Suggested there were only four basic seeds earth, air, fire, and water. The elementary substances (atoms to us) combined in various ways to make everything. Democritus (Thracian, born 470 B.C.) Actually proposed the word atom (indivisible) because he believed that all matter consisted of such tiny units with voids between, an idea quite similar to our own beliefs. It was rejected by Aristotle and thus lost for 2000 years. Aristotle (Greek, born 384 B.C.) Added the idea of qualities heat, cold, dryness, moisture as basic elements which combined as shown in the diagram (previous page). Hot + dry made fire; hot + wet made air, and so on. O Connor Davis, MacNab, McClellan, CHEMISTRY Experiments and Principles 1982, page 26,

15 Who was Right? Greek society was slave based Beneath famous to work with hands did not experiment Greeks settled disagreements by argument Aristotle was more famous He won! His ideas carried through middle ages. Alchemists change lead to gold

16 John Dalton 1808 Proposed an Atomic Theory

17 John Dalton (Atomic theory).. the existence of atoms Developed a theory that matter is composed of tiny, indivisible particles called atoms that can neither be created nor destroyed i.e. no change in mass law of conservation of mass

18 Conservation of Atoms 2 H 2 + O 2 2 H 2 O John Dalton H H 2 H H + O O 2 H O H 2 O H O H H 2 H O H 2 O H 4 atoms hydrogen 2 atoms oxygen 4 atoms hydrogen 2 atoms oxygen Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 204

19 Legos are Similar to Atoms H 2 H 2 H H H H + O O O 2 H H H H O O H 2 O H 2 O Lego's can be taken apart and built into many different things. Atoms can be rearranged into different substances.

20 Conservation of Mass High voltage electrodes Before reaction glass chamber O 2 After reaction High voltage H 2 O O 2 H g H 2 80 g O g (mass of chamber) g total 0 g H 2 45? g H 2 O 40 g O g (mass of chamber) g total Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 204

21 John Dalton proposed his atomic theory. 1. All matter is made up of small particles called atoms. 2. Atoms are indivisible they cannot be broken down into smaller particles. False! 3. Atoms cannot be created or destroyed. False!

22 FACT We now know that there are particles within atoms subatomic particles e.g. protons, neutrons and electrons. Radioactive atoms can change into other atoms.

23 John Dalton 1808 Dalton s Symbols

24 Daltons Models of Atoms Carbon dioxide, CO 2 Water, H 2 O Methane, CH 4

25 Structure of Atoms Scientist began to wonder what an atom was like. Was it solid throughout with no internal structure or was it made up of smaller, subatomic particles? It was not until the late 1800 s that evidence became available that atoms were composed of smaller parts.

26 Discovery of the Electron In the 1800 s electricity was new, exciting and the subject of a lot of study.

27 William Crookes 1875 Crookes passed an electric current through a complete vacuum just to see what would happen.

28 Crookes experiment Electricity seemed to leap across the vacuum from the negative plate to the positive plate. The radiation given off from the negative cathode was called Cathode rays.

29 Crookes Tube Crookes tube (Cathode ray tube) William Crookes Cathode (-) Glow Anode (+) Mask holder

30 William Crookes A discharge tube A long glass tube fitted with a metal electrode at each end When the gas inside is at a very low pressure, passing electricity through the tube results in invisible rays travelling in straight lines from the cathode (-ve) to the anode (+ve) Small objects placed in the path of the rays cast a sharp shadow in the fluorescence at the end of the tube

31 Crooke s Vacuum tube Cathode (-) Maltese Cross High Voltage Anode (+) Vacuum tube means most of the air has been pumped out of it

32 The Effect of an Obstruction on Cathode Rays High voltage source of high voltage shadow cathode Anode yellow-green fluorescence Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 117

33 Crooke s Vacuum tube Cathode rays Maltese cross Cathode (-) Shadow of cross on glass Anode (+) An electric current passing through air causes the air to glow Rays coming from the ve electrode (cathode) cast a shadow at the far end of the tube

34 The cathode rays caused shadows and could spin light paddle wheels.

35 Crooke s experiment This proved that the cathode rays carried energy, and that they might be made of particles. This also indicates that the rays (particles) moved from the cathode to the anode.

36 William Crookes (cathode rays) (Hons) Showed some form of radiation passes from the -ve electrode (cathode) to the +ve electrode (anode) when a current passes through a vacuum tube -ve cathode +ve anode CNAP He called these cathode rays and was convinced they consisted of particles

37 J J Thomson Was very interested in the Crooke s discoveries He used a cathode ray tube that had a positive and negative plate between which the rays had to travel.

38 Background Information Cathode Rays Form when high voltage is applied across electrodes in a partially evacuated tube. Originate at the cathode (negative electrode) and move to the anode (positive electrode) Carry energy and can do work Travel in straight lines.

39 Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58 A Cathode Ray Tube

40 Cathode Ray Experiment 1897 Experimentation Using a cathode ray tube, Thomson was able to deflect cathode rays with an electrical field. The rays bent towards the positive pole, indicating that they are negatively charged.

41 CATHODE RAYS IN ELECTRIC FIELDS PROVIDES ELECTRICAL FIELD - Thompson s cathode ray tube

42 The Effect of an Electric Field on Cathode Rays High voltage source of high voltage _ negative plate cathode + positive plate anode Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 117

43 Thomson s Experiment - voltage source + vacuum tube metal disks

44 Thomson s Experiment - ON OFF voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end

45 Thomson s Experiment - ON OFF voltage source By adding an electric field he found that the moving pieces were negative.

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47 Thomson He found the cathode rays were attracted towards the positive plate. The rays had to be negative (as opposites attract).

48 Thomson 1897 Discovered the electron 1897 Thomson announced that cathode rays consisted of negative charged particles. Thomson had discovered the ELECTRON! (He called them corpuscles)

49 Proposed that the smallest particle that has a negative charge should be called an electron. So Thomson's corpuscles were called electrons. George Stoney 1891 UCG An Irish scientist

50 Electrons Cathode rays are streams of negatively charged particles called electrons

51 But where had these tiny particles come from? Since they were so small, Thomson suggested that they could only have come from inside atoms. So Dalton's idea of the indestructible atom had to be revised.

52 1808 AD John Dalton proposed his atomic theory. 1. All matter is made up of small particles called atoms. 2. Atoms are indivisible they cannot be broken down into smaller particles. False! 3. Atoms cannot be created or destroyed.

53 This discovery implied that Dalton was wrong and the atom is not the smallest particle of matter. It looked like the atom could be broken down into smaller pieces electrons.

54 JJ Thomson He did another experiment. He placed an electro magnet around the vacuum tube. He changed the strength of the magnetic field to match the force of the electric field. Using the magnetic field he was able to bring the beam of electrons back down to its original position.

55 Cathode Ray Experiment

56 JJ Thomson Electromagnet

57 JJ Thomson (Hons) From the results of his expt: He was able to calculate the ratio of the charge of an electron to its mass. This is the e m of the electron.

58 JJ Thomsom (hons) But he couldn t determine the individual value of either number He found that these vely charged particles were about 2000 times lighter than H atoms and were to be found in all matter

59 JJ Thomsom Also proposed the plum pudding model of the atom He visualised an atom as a sphere of +ve charge into which ve electrons were embedded at random

60 Thomson Model of the Atom William Thomson proposed that atoms consist of small, negative electrons embedded in a massive, positive sphere. The electrons were like currants in a plum pudding. This is called the plum pudding model of the atom. - - electrons

61 Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56 Plum-Pudding Model

62 Robert Millikan (Hons) Oil Drop Experiment Determine the mass & charge of the electron Actual apparatus used

63 Robert Millikan 1911, Millikan used the Oil Drop expt to measure accurately the charge on the electron (see book) This then allowed its mass to be calculated The mass is much smaller than that of any atom Confirming that the atom is made up of smaller particles

64 Millikan s oil drop expt

65 Millikan s oil drop expt He sprayed tiny drops of oil between two charged plates. He used x-rays to cause the molecules in the air to lose electrons. As the oil droplets passed through, they picked up electrons from the air. i.e. the oil droplets were now vely charged Molecules in air Oil droplets - Electrons -

66 Millikan s oil drop expt

67 Oil Drop Experiment Charged plate oil droplets oil atomizer X-rays Robert Millikan Small hole Telescope - Charged plate oil droplet under observation

68 Millikan s oil drop expt By attaching a battery to the plates, he created an electric field between the plates that would act on the charged oil drops. i.e. the ve oil droplets were attracted to the +ve charged plate

69 Millikan s oil drop expt He adjusted the voltage till the electric field force would just balance the force of gravity on an oil drop, and the oil drop would hang suspended in mid-air.

70 When the oil droplet was stationary, the weight of the oil droplet due to gravity must have been equal to the electrical force pulling the droplet upwards

71 Robert Millikan Using this information, He was able to calculate: 1. The size of the charge on the electron 2. The mass of the electron.

72 JJ Thomson Since atom is neutral There must be a positive charge to neutralise the negative electrons.

73 JJ Thomson He developed the idea that atoms are made of negative electrons embedded in a gel of positive charge (a "plum pudding" model).

74 Other pieces Proton - positively charged pieces 1840 times heavier than the electron Neutron - no charge but the same mass as a proton. How were these pieces discovered? Where are the pieces?

75 1909 Scientists began to question the simple view of atom.

76 Ernest Rutherford (The modern view of the atom was developed by Ernest Rutherford) Discovered the nucleus & proton Worked in JJ Thomson s lab overturned Thomson's atom model in 1911 with his gold foil experiment in which he demonstrated that the atom has a nucleus.

77 Ernest Rutherford Two of Ernest Rutherford's students, Hans Geiger and Ernest Marsden, were doing an experiment at Manchester University with radiation.

78 Ernest Rutherford They were using the dense, positively charged particles (called alpha particles) as 'bullets' to fire at a very thin piece of gold foil.

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80 Rutherford Scattering Lead collimator Gold foil a particle source q

81 GOLD ATOM They expected the particles to pass straight through the gold atoms.

82 Experiment method Bombarded a thin leaf of gold foil with alpha particles (+vely charged) Used a zinc sulfide screen to detect scattered alpha particles

83 Rutherford s Goil Foil Experiment

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85 Experiment Results Gold atom

86 Ruherford s experiment Most alpha particles were observed to pass straight through the gold foil. They had passed through basically empty space!

87 Rutherford s experiment A few, were scattered at small and large angles (90 ), and some even bounced back toward the source.

88 Interpreting the Observed Deflections... beam of alpha particles undeflected particles deflected particle gold foil Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 120

89 Density and the Atom Since most of the particles went through, the atom was mostly empty. Because the alpha rays were deflected so much, the positive pieces it was striking were heavy. Small volume and big mass = big density This small dense positive area is the nucleus California WEB

90 Discovery of Nucleus Only a positively charged and relatively heavy particle, such as the proposed nucleus, could account for such strong repulsion.

91 New model of Atom Tiny, dense, positively charged core called a nucleus, around which the light, negative electrons, circulate at some distance, much like planets revolving around the Sun.

92 Top: Expected results of Rutherford's gold foil experiment: alpha particles passing through the plum pudding model of the atom undisturbed. Bottom: Observed results: Some of the particles were deflected, and some by very large angles. Rutherford concluded that the positive charge of the atom must be concentrated into a very small location: the atomic nucleus.

93 Rutherford s Gold-Leaf Experiment Conclusions: Atom is mostly empty space Nucleus has (+) charge Electrons float around nucleus Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 120

94 Discovery of Proton Rutherford continued to bombard various elements with Alpha particles.

95 Ernest Rutherford (Hons) With light atoms e.g. Oxygen and Nitrogen The alpha particles broke up the nucleus to release positive charged particles.

96 Rutherford had discovered the proton.

97 James Chadwick Discovered the Neutron Bombarded a sample of Beryllium with alpha particles.

98 Discovery of Neutron (Hons)

99 James Chadwick Some type of radiation with no charge came from the Beryllium.

100 James Chadwick The particles had the same mass as protons but no charge. He called them neutrons

101 Models of the Atom "In science, a wrong theory can be valuable and better than no theory at all." - Sir William L. Bragg e e + e + e + e + e + + e + e e + e Dalton s Greek model (400 (1803) B.C.) Thomson s plum-pudding model (1897) Rutherford s model (1909)

102 2.3 Properties of sub-atomic Particle Relative charge particles Relative mass Location Proton +1 1 Nucleus Neutron 0 1 Nucleus Electron -1 1/1838 Shells

103 Recheck Evidence for the existence of small particles History of the atom Discovery of the electron Thomsom s plum pudding model of the atom Millikan s oil drop expt Discovery of the nucleus Discovery of the proton Discovery of te neutron Properties of sub-atomic particles