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Atomic Theory Development What is Today’s Model? Dense, Positively Charged Nucleus Mostly Empty Space Negatively Charged Electron Cloud Most Probable.

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Presentation on theme: "Atomic Theory Development What is Today’s Model? Dense, Positively Charged Nucleus Mostly Empty Space Negatively Charged Electron Cloud Most Probable."— Presentation transcript:


2 Atomic Theory Development

3 What is Today’s Model? Dense, Positively Charged Nucleus Mostly Empty Space Negatively Charged Electron Cloud Most Probable Location of the Electrons Composed of Protons, Neutrons, and Electrons

4 Timeline of Development of Current Atomic Model BC Democritus proposed the idea of atomos Beginning of Modern Atomic Theory 1897 Discovery of the Electron 1911 Discovery of the Nucleus The Idea of Energy Levels for Electrons was Proposed Introduction of the wave mechanical model Discovery of the Proton Discovery of the Neutron

5 Early Greeks  Matter is made of indestructible particles called “atomos” Democritus (400 BC)

6 Summary for Dalton’s Atomic Theory (Father of the Modern Atomic Theory)  All atoms of a single element have the same mass  Atoms of different elements are different.  Atoms can’t be divided, created or destroyed.  Atoms of different elements combine in simple whole-number ratios to form compounds.

7 Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle. Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. Cathode Ray Crookes Tube

8 J.J. Thomson He proved that atoms of any element can be made to emit tiny negative particles. From this he concluded that ALL atoms must contain these negative particles. He knew that atoms did not have a net negative charge and so there must be something positive that balances the negative charge. J.J. Thomson

9 William Thomson’s (Sir Kelvin) Atomic Model (1910) Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

10 Ernest Rutherford’s ( )  electrons embedded in a positive pudding. Where exactly are those electrons? Thomson’s Theory: “Plum Pudding”  Shoot something at them to see where they are. Rutherford’s idea:

11 Rutherford’s has an idea… What if I shoot alpha radiation at gold atoms in gold foil? Discovery of the nucleus

12 Lead block Uranium Gold Foil Flourescent Screen

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

14 What he expected

15 Because

16 Because, he thought the mass was evenly distributed in the atom


18 What he got

19 How he explained it + Atom is mostly empty. Small dense, positive piece at center. Alpha particles are deflected by it if they get close enough.

20 +

21 Rutherford’s Conclusion (1911)…  Small, dense, positive nucleus.  Equal amounts of (-) electrons at large distances outside the nucleus.

22 Neils Bohr’s Atomic model (1913)  Small, dense, positive nucleus.  Equal amounts of (-) electrons at specific orbits around the nucleus. This incorrect version of the atom is often used to represented atoms because it shows energy levels for electrons.

23 And now we know of many other subatomic particles: Chadwick ** James Chadwick discovered neutrons in n 0 have no charge and are hard to detect purpose of n 0 = stability of nucleus quarks, muons, positrons, neutrinos, pions, etc. photo from liquid H 2 bubble chamber

24 Quantum Mechanical Model -electron cloud model- -charge cloud model- Schroedinger, Pauli, Heisenberg, Dirac (up to 1940): According to the QMM, we never know for certain where the e – are in an atom, but the equations of the QMM tell us the probability that we will find an electron at a certain distance from the nucleus.

25 Quantum Mechanical Model Modern atomic theory describes the electronic structure of the atom as the probability of finding electrons within certain regions of space (orbitals).

26 Modern Atomic Theory  Atoms of the same element are chemically alike with a characteristic average mass which is unique to that element.  Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!  All matter is composed of atoms.  Atoms of any one element differ in properties from atoms of another element  The exact path of electrons are unknown and e - ’s are found in the electron cloud.

27 The Atomic Scale  Most of the mass of the atom is in the nucleus (protons and neutrons)  Electrons are found outside of the nucleus (the electron cloud)  Most of the volume of the atom is empty space “q” is a particle called a “quark”

28 About Quarks… Protons and neutrons are NOT fundamental particles. Protons are made of two “up” quarks and one “down” quark. Neutrons are made of one “up” quark and two “down” quarks. Quarks are held together by “gluons”

29 Size of an atom Atoms are incredibly tiny. Measured in picometers ( meters) – Hydrogen atom, 32 pm radius Nucleus tiny compared to atom – Radius of the nucleus near m. – Density near g/cm 3 IF the atom was the size of a stadium, the nucleus would be the size of a marble. California WEB

30 Models of the Atom Dalton’s model (1803) Thomson’s plum-pudding model (1897) Rutherford’s model (1909) Bohr’s model (1913) Charge-cloud model (present) Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 125 Greek model (400 B.C.) John Dalton pictures atoms as tiny, indestructible particles, with no internal structure J.J. Thomson, a British scientist, discovers the electron, leading to his "plum-pudding" model. He pictures electrons embedded in a sphere of positive electric charge Hantaro Nagaoka, a Japanese physicist, suggests that an atom has a central nucleus. Electrons move in orbits like the rings around Saturn New Zealander Ernest Rutherford states that an atom has a dense, positively charged nucleus. Electrons move randomly in the space around the nucleus In Niels Bohr's model, the electrons move in spherical orbits at fixed distances from the nucleus Frenchman Louis de Broglie proposes that moving particles like electrons have some properties of waves. Within a few years evidence is collected to support his idea Erwin Schrödinger develops mathematical equations to describe the motion of electrons in atoms. His work leads to the electron cloud model James Chadwick, a British physicist, confirms the existence of neutrons, which have no charge. Atomic nuclei contain neutrons and positively charged protons.

31 Match The Models Billiard Ball Plum Pudding Nucleus Neutrons Energy Levels

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