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Chapter 2 Atoms and their structure. History of the atom Original idea of the atom: Ancient Greece (400 B.C.) n Greek philosophers Democritus and Leucippus.

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Presentation on theme: "Chapter 2 Atoms and their structure. History of the atom Original idea of the atom: Ancient Greece (400 B.C.) n Greek philosophers Democritus and Leucippus."— Presentation transcript:

1 Chapter 2 Atoms and their structure

2 History of the atom Original idea of the atom: Ancient Greece (400 B.C.) n Greek philosophers Democritus and Leucippus n Empty space and small particles

3 The Beginning of Atomic Theory Atomos - not to be cut, indivisible Atomos - not to be cut, indivisible Atoms - smallest particles of matter, different types of atoms exist for every type of matter

4 Lavoisier n French chemist ( ) n Observed chemical changes in sealed containers Mass of reactants = Mass of products Conservation of Matter

5 Whos Next? n John Dalton: A teacher in Late 1700s England n Summarized results of his experiments and those of others in: Daltons Atomic Theory Daltons Atomic Theory n Combined ideas of elements with that of atoms

6 Daltons Atomic Theory 1. All matter is made of tiny particles called atoms. 2. Atoms are indestructible and cannot be divided into smaller parts. 3. Atoms of the same element are identical, those of different atoms are different.

7 Theory or Law? n A scientific law is a fact of nature –Ex: the sun rises, conservation of matter n A theory explains the law –Daltons theory explains why matter in conserved

8 Parts of Atoms n J. J. Thomson - English physicist, 1897 n Cathode ray tube experiment. n Vacuum tube - all air has been pumped out.

9 Thomsons Experiment Voltage source +- Vacuum tube Metal Disks

10 Thomsons Experiment Voltage source +-

11 Thomsons Experiment Voltage source +-

12 Thomsons Experiment Voltage source +-

13 n Passing an electric current makes a beam appear to move from the negative to the positive end Thomsons Experiment Voltage source +-

14 n Passing an electric current makes a beam appear to move from the negative to the positive end Thomsons Experiment Voltage source +-

15 n Passing an electric current makes a beam appear to move from the negative to the positive end Thomsons Experiment Voltage source +-

16 n Passing an electric current makes a beam appear to move from the negative to the positive end Thomsons Experiment Voltage source +-

17 Thomsons Experiment n By adding an electric field

18 Voltage source Thomsons Experiment n By adding an electric field + -

19 Voltage source Thomsons Experiment n By adding an electric field + -

20 Voltage source Thomsons Experiment n By adding an electric field + -

21 Voltage source Thomsons Experiment n By adding an electric field + -

22 Voltage source Thomsons Experiment n By adding an electric field + -

23 Voltage source Thomsons Experiment n By adding an electric field he found that the moving pieces were negative + -

24 Thomsons Model n Found the electron n Couldnt find positive charge(for a while) n Said the atom was like plum pudding n A bunch of positive stuff, with the electrons able to be removed

25 Other Subatomic Particles n Proton - positively charged particles – 1,840 times heavier than the electron n Neutron - no charge, but the same mass as a proton. n Where are the particles?

26 Rutherfords Experiment n Ernest Rutherford - English physicist, (1910) n Used radioactivity n Alpha particles - positively charged He nuclei given off by polonium n Shot them at gold foil which can be made a few atoms thick

27 Radioactivity

28 Rutherfords experiment When the alpha particles hit a fluorescent screen, it glows. Americium 241, just over 1cm from the screen.

29 Lead block Uranium Gold Foil Florescent Screen

30 He Expected… …The alpha particles to pass through without changing direction very much n Because… –The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles

31 What he expected

32 Because…

33 Because he thought the mass was evenly distributed in the atom

34

35 What he got

36 How Rutherford explained it + n Atom is mostly empty n Small, dense, positive particles at center n Alpha particles are deflected by it if they get close enough (like repels like)

37 +

38 Subatomic Particles Electron Proton Neutron NameSymbolCharge Relative mass Actual mass (g) e-e- p+p+ n0n / x g 1.67 x g

39 Structure of the Atom There are two regions: 1. The nucleus - protons and neutrons - protons and neutrons - positive charge - almost all the mass 2. Electron cloud - Most of the volume of an atom - The region where the electron can be found

40 Counting the Pieces n Atomic Number = number of protons –# of protons determines kind of atom –the same as the number of electrons in the neutral atom n Mass Number = the number of protons PLUS the number of neutrons –Includes all the things with mass

41 Isotopes n Dalton was incorrect. n Atoms of the same element can have different numbers of neutrons! –different mass numbers –called isotopes

42 Periodic Table Info Contains the symbol of the element, the atomic mass and the atomic number –Symbol: letters in the middle –Atomic mass: decimal number (usually at bottom) –Atomic number: whole number (usually at top)

43 Periodic Table Info Label the following diagram:

44 Atomic Mass n How heavy is an atom of oxygen? n There are different kinds of oxygen atoms. n Look at Average atomic mass. n Based on abundance of each element in nature.

45 Atomic Mass Calculate the atomic mass of copper if copper has two isotopes. 69.1% of Cu atoms have a mass of amu and the rest have a mass of amu.

46 Bohrs Model n Why dont electrons fall into the nucleus? n Move like planets around the sun. – Circular orbits at different levels. n Amounts of energy separate one level from another.

47 Bohrs Model Nucleus Electron Orbit Energy Levels

48 Bohrs Model Increasing energy Nucleus First Second Third Fourth Fifth } n Further away from the nucleus means higher energy. n There is no in between energy n Energy Levels


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