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A History of the Atom A long time to study a little thing.

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1 A History of the Atom A long time to study a little thing

2 Introduction

3 The Philosophers Ancient Greeks Ancient Greeks Formed ideas of nature based on their experiences Formed ideas of nature based on their experiences Did no experiments, just thought Did no experiments, just thought

4 Everything made of : Everything made of : EarthAir Fire Water Aristotle (384–322 B.C.)

5 Democritus (460 – 370 B.C.) Greek philosopher (not scientist) Idea of the atom (smallest particle) Named the atom Different kinds of atoms have different sizes and shapes. Different kinds of atoms have different sizes and shapes. Apparent changes in matter result from changes in the groupings of atoms and not from changes in the atoms themselves. Apparent changes in matter result from changes in the groupings of atoms and not from changes in the atoms themselves.

6 Democritus and Aristotle

7 The Alchemists Rearrange the basic elements Rearrange the basic elements Wanted to turn lead in gold Wanted to turn lead in gold

8 Elements We now know that there are 92 basic elements We now know that there are 92 basic elements The type of element is determined by the number of protons in the atom The type of element is determined by the number of protons in the atom

9 Modern elemental theory

10 John Dalton ( ) First true atomic theory All matter is composed of atoms. All atoms of a given element are identical and different from those of any other element. Atoms combine in simple whole number ratios to form compounds. In a chemical reaction, atoms are separated, combined, or rearranged.

11 Dalton’s Particle Theory

12 By the late 1800s… Cathode ray tube (aka Crook’s tube) invented Cathode ray tube (aka Crook’s tube) invented

13 JJ Thomson (1897) Cathode rays made of particles smaller than atoms Cathode rays made of particles smaller than atoms First subatomic particles First subatomic particles Discovered electron Plum pudding model of the atom

14 J. J. Thomson

15 Plum pudding model Atom is a sphere Atom is a sphere Positive charge is evenly distrubuted Positive charge is evenly distrubuted Negatively charged electrons spread randomly through the sphere Negatively charged electrons spread randomly through the sphere

16 Questions raised by Thomson If electrons are particles smaller than atoms, are there other particles? If electrons are particles smaller than atoms, are there other particles? What makes up most of the mass of the atom? What makes up most of the mass of the atom? If electrons are negatively charged, why are atoms neutral? If electrons are negatively charged, why are atoms neutral?

17 The proton/electron atom

18 Ernest Rutherford (1911) Aimed a beam of “alpha particles” at a thin gold foil Aimed a beam of “alpha particles” at a thin gold foil The beam made a bright spot on a fluorescent screen The beam made a bright spot on a fluorescent screen

19 Lead block Uranium Gold Foil Florescent Screen

20 What Rutherford expected

21 α α α α α α

22 What Rutherford saw

23 + α α α α α α α α α

24 Rutherford concluded There had to be a heavy central core to the atom: the nucleus Nucleus contains the protons

25 Rutherford and the Structure of Atoms

26 James Chadwick (1932) Experimenting with radiation sources Experimenting with radiation sources Discovered a new particle with no charge : the neutron

27 Niels Bohr (1913) Student of Rutherford and Thomson Student of Rutherford and Thomson Electrons travel in orbitals around the nucleus Electrons travel in orbitals around the nucleus Electron can move from one orbital to another with input or release of energy Electron can move from one orbital to another with input or release of energy More on this later… More on this later…

28 Bohr’s atomic model

29 Structure of the atom 99.97% of mass in nucleus 99.97% of mass in nucleus Most of the volume is empty space Most of the volume is empty space Electrons in cloud Electrons in cloud

30 How big is an atom? Simulate the size of a hydrogen atom: Simulate the size of a hydrogen atom: Nucleus : place a baseball on the 50 yard line of Reliant Stadium Nucleus : place a baseball on the 50 yard line of Reliant Stadium Electron : put a grain of sand on the back row of the highest section Electron : put a grain of sand on the back row of the highest section

31 Size of Atoms

32 Atomic Number

33 Atomic Mass

34 Isotopes

35 Atomic Number, Mass, & Isotopes


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