Development of the Atom A brief review of the history…

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The History of Atomic Theory

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Presentation transcript:

Development of the Atom A brief review of the history…

John Dalton ( ) English school teacher who began teaching at the age of 12 years old. He combined previous knowledge from other scientists with experimental observations to formulate a set of empirical laws known as the Atomic Theory of Matter.

Dalton’s Atomic Theory All matter is made of tiny indivisible particles called atoms.

Thomson Model of the Atom J. J. Thomson - English physicist Experimented using a piece of equipment called a cathode ray tube (CRT), following Crookes’ design.

Thomson’s Experiment + - voltage source OFF ON Passing an electric current makes a beam appear to move from the negative to the positive end

Thomson’s Experiment + - voltage source OFF ON + - By adding an electric field… he found that the moving pieces were negative.

J.J. Thomson He proved that atoms of many elements 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 balancing the negative charge. J.J. Thomson

Plum-Pudding Model (aka Raisin Bun) Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 56 In 1910, Thomson proposed the Plum Pudding model –Negative electrons were embedded into a positively charged spherical cloud.

Ernest Rutherford ( ) Noticed that ‘alpha’ particles were sometimes deflected by something in the air. Awarded the Nobel Prize in Chemistry in 1908 for his Gold-Foil Experiment

Rutherford’s Apparatus beam of alpha particles radioactive substance gold foil circular ZnS - coated fluorescent screen Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 120

What he expected… California WEB

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

What he got…

Explanation of Alpha-Scattering Results Raisin Bun atom Alpha particles Nuclear atom Nucleus Thomson’s modelRutherford’s model

Actual Results of Gold-Leaf Experiment Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 57

Atom’s Nucleus Since most of the particles went through, the atom was mostly empty. Because the alpha rays were deflected so much, the positive pieces it was striking were heavy. This small dense positive area is the nucleus. California WEB

Bohr’s Model Bohr says:  Electrons orbit the nucleus.  Only certain orbits are allowed.  Each orbit has a specific energy. Nucleus Possible electron orbits e

Electron energy The energy of electrons is quantized. Electons must possess specific amounts of energy at each energy level.

When the electron absorbs light that has a specific amount of energy, it jumps to a higher orbit. It is said to be in an excited state.

When the electron falls to a lower position, it releases a specific amount of energy. When the electron is in the lowest possible energy level, it is said to be in ground state.

Light is emitted when electrons return to ground state from an excited state. The colour of that light depends on the amount of energy that is being released.

Formation of Light Nucleus Nucleus e e Lithium Atom + Ground State e e Excited State e Electron Returns to Ground State Light is given off e Ion is formed Li  e + Li 1+ hv n = 1 n = 2 n = 3 n = 4 n = 5 n = 6 n = 7

An Excited Lithium Atom Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 326 Photon of red light emitted Li atom in lower energy state Excited Li atom Energy

Excitation of Hydrogen Atoms Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 328

Return to Ground State

Bohr’s Experiment A spectroscope separates light from a source into its component colours.

1 nm = 1 x m = “a billionth of a meter” 410 nm434 nm486 nm656 nm These distinct lines exist because the electron is restricted to certain energy levels. The energy of the electron is quantized.

Summary… When an electric current passes through the gas, it excites or energizes the atoms.

Summary… As atoms return to a lower energy state, the gas releases this energy in the form of light.

Summary… When we pass this light through a prism or spectrometer, we see a series of individual lines called the element’s “atomic spectrum”. 1 nm = 1 x m = “a billionth of a meter” 410 nm434 nm486 nm656 nm