2 Discovery of Small Particles The GreeksJohn Dalton
3 Democritus 460 BC - 370 BC Coined the term ‘atomos’ All matter is composed of small, indivisible particles (atoms)No experimentation
4 AristotleAristotle emphasized that nature consisted of four elements: air, earth, fire, and water.
5 John Dalton’s Solid Sphere Model In 1803, John Dalton was the first to show laboratory evidence of the existence of atoms.Dalton’s model of the atom was a solid sphere with no subatomic particles…the atom was indivisible!Dalton also proposed the first atomic theory which stated that all things are made up of atoms and these atoms can combine in simple whole number ratios to form compounds.
6 John Dalton Dalton's Atomic TheoryAll matter is made of atoms. Atoms are indivisible and indestructible.2) All atoms of a given element are identical in mass and properties3) Compounds are formed by a combination of two or more different kinds of atoms.
7 Discovery of the Electron William CrookesJ.J ThomsonRobert Millikan
11 J.J. Thomson: Cathode Rays Cathode rays can be deflected by a magnetic field.
12 J.J. Thomson: Cathode Rays The beam was composed of negatively charged fast-moving particles called ‘electrons’.
13 What were these particles??? Thomson reasoned that since the beam came from the cathode (a negative electrode), the particles may have a negative charge.Studied the deflection of the cathode ray by an electric field to determine if this was the caseBy studying factors that effected the deflection of the beam, Thomson calculated the charge to mass ratio of the particle
14 Robert A. Millikan Established the charge of an electron and Determined the atomic structure of electricity
15 Millikan’s Oil Drop Experiment When he sprayed oil droplets into a chamber and bombarded them with X-rays to place a negative charge on them, the charged droplets were attracted to the positive plate. Changing the strength of the electrical field offset the attraction and allowed Millikan to determine the charge of the electrons.He measured the charge of the electrons (1.60 X coulombs) and later used this to calculate their extremely small mass (9.11 x grams).
20 Thomson’s Plum Pudding Model (1896ish) Thomson concluded that the beam was made up of negatively charged subatomic particles called electrons (e-)No matter what metal the electrodes were made of, the same charge to mass ratio was calculated.Since atoms are neutral - there must be some positively charged aspect as well…..ball of positive chargeelectrons
25 Earnest Rutherford’s Gold Foil Experiment His expected resultsHe shot alpha particles (+ charged) through the atoms of gold in a thin sheet of foil, and studied how they were scattered by the atoms. Oddly, some of the particles “bounced” back off the foil but most when straight through.The circles are atoms of gold, the dots are the nucleus and the arrows are the alpha particles (positively charged electromagnetic radiation).The actual results
27 Interpretation of the experimental results Most of the atom is empty space.The mass concentrated at the center of an atom called ‘nucleus’.The nucleus is positively charged.The positively charged particle is called ‘proton’.