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MODELS OF ATOMS ÞDemocritus(400 B.C): said that matter is composed of indestructible particles. He called them atoms, a Greek word which means indestructible.

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Presentation on theme: "MODELS OF ATOMS ÞDemocritus(400 B.C): said that matter is composed of indestructible particles. He called them atoms, a Greek word which means indestructible."— Presentation transcript:

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2 MODELS OF ATOMS ÞDemocritus(400 B.C): said that matter is composed of indestructible particles. He called them atoms, a Greek word which means indestructible particle.

3 The Early History of Chemistry 4 Before 16th Century – Alchemy: Attempts (scientific or otherwise) to change cheap metals into gold 4 17th Century –Robert Boyle: First chemist to perform quantitative experiments 4 18th Century –George Stahl: Phlogiston flows out of a burning material. –Joseph Priestley: Discovers oxygen gas, dephlogisticated air.

4 Law of Conservation of Mass 4 Discovered by Antoine Lavoisier 4 Mass is neither created nor destroyed

5 Law of Definite Proportion (Proust) 4 A given compound always contains exactly the same proportion of elements by mass. 4 Carbon tetrachloride is always 1 atom carbon per 4 atoms chlorine.

6 Law of Multiple Proportions (Dalton) 4 When two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always be reduced to small whole numbers. 4 The ratio of the masses of oxygen in H 2 O and H 2 O 2 will be a small whole number (2).

7 John Dalton-1808

8 Daltons Atomic Theory (1808) ÊEach element is made up of tiny particles called atoms. ËThe atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways.

9 Daltons Atomic Theory (continued) ÌChemical compounds are formed when atoms combine with each other. A given compound always has the same relative numbers and types of atoms. ÍChemical reactions involve reorganization of the atoms - changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.

10 Early Experiments to Characterize the Atom H J. J. Thomson ( ) - postulated the existence of electrons using cathode ray tubes.

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12 Deflection of Cathode Rays by an Applied Electric Field

13 Thompson found electrons were produced from Electrodes of various metals. Therefore, he concluded that all atoms must contain electrons. Since atoms are neutral, he concluded that atoms must contain positive charge.

14 THOMPSON

15 Robert Millikan ÞHis OIL DROP EXPERIMENT determined the charge and mass of the electron. ÞMass = amu or 1/1850 amu or x g ÞCharge = -1 or x Coulomb

16 Diagram of the Millikan Apparatus Millikan calculated the mass of the electron = 9.11 x kg.

17 The Nuclear Atom H Ernest Rutherford H Famous Gold Foil experiment in 1911.

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20 CONCLUSIONS The atom contains a dense center called the nucleus. The nucleus contains a positive charge and most of the mass of the atom. The nucleus is approximately 100,000 times smaller than the atom.

21 CONCLUSIONS ÞThe nucleus is positively charged. The amount of positive charge balances the negative charge of the electrons. ÞThe electrons move around in the empty space of the atom surrounding the nucleus.

22 Incomplete Model Þ Did not explain where the electrons are located in the space surrounding the nucleus.


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