SCES1200 L02 Atom Structure Classical Theories Student01

8/14/2019 SCES1200 L02 Atom Structure Classical Theories Student01

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The Structure of The Atoms


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Quantum Theory: Thomson (1903)

Rutherford (1911), Bohr (1913),Schrdinger (1923), Chadwick (1932)

Quantum Dots,

Nanotechnology

Femtotechnology?

Atoms and the Atomic Theorem

Democritus (Greek) defines

atomos as smallest possible

individual particle

1600

Industrial revolution deepest

meaning of atom by advanced

method (Classical theorem)


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Dalton Atomic Theory (1800)

Three assumptions:

1. Each chemical element is composed of minute,

indestructible particles called atoms. Atoms can neither be

created nor destroyed during a chemical change.

2. All atoms of an element are alike in mass (weight) andother properties, but the atoms of one element are differentfrom those of all other elements.

3. In each of their compounds, different elements combine in

a simple numerical ratio.


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If two elements form more than one compound between

them, then the ratios of the masses of the second element

which combine with a fixed mass of the first element will be

ratios of small whole numbers.

Example:

Consider two of the carbon oxides: CO and CO2, 3.0 g of C

may react with 4.0 g of O to produce 7.0 g; and also with 8.0g of O to produce 11.0 g CO2.

Law of Multiple Proportions


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Law of Multiple Proportions


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Components of Atom

Electron

Sir Humphrey Davis (1778-1829):

Reported relationship between electrical and chemical

properties in various substances.

Michael Faraday (1791-1867):

Discovered that chemical changes occurred when electrical

current is passed through chemical solutions.

Invented cathode ray tube and discovered the cathode rays.


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Michael Faradays Cathode Ray


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Cathode Ray Tube

Properties of cathode ray

The ray travels in straight line

It cast a shadow

Its properties are independent of the cathode ray materials

It spin a fly-wheel

It heats up metallic foil It is deflected by electric or magnetic fields


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J. J. Thomsons Experiments


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More Results on Cathode Ray

Cathode ray deflected by magnetic field, is restraightened

by electric field.

Measured the mass-to-charge ratio, m/e for cathode ray.

Precise measurements yield a value of 5.6857 x 10-9 g per

coulomb (g C-1).

Suggested that cathode rays is negatively charged.

Cathode rays became known as electron, as proposed by

George Stoney (1874).


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Robert Millikans Oil Drop Experiment


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Explanation on Oil Drop Experiment

Atomized oil droplets pass through a hole

Droplets fall under gravity

Air molecules ionized by the x-ray

Electrons attached to the oil droplets making them

negatively charged

By manipulating the charges on the plate, enabled to

balance the gravity pull and attraction by the positive

plate on the charged droplets

Charge can be calculated


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Oil droplets have different charges

Charge q on a droplet is integral multiple of a

minimum value e; q = ne

The minimum value e is 1.6x10-19 C

Using m/e from Thomsons experiment, Millikan wasable to find weight of an electron

Results from Cathode Ray andOil Drop Experiments


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The Plum Pudding Atomic Model

J. J. Thomson suggested:

The plum pudding model of an atom has negative

charges (electrons) embedded in a larger structure of

positive charge


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The Plum Pudding Atomic Model

He atom model:

He atom would have a +2 cloud of

positive charge; and two electrons (-2)

If helium atom loses one electron it

becomes charged, called ion. Referred

as He+, has net charge 1+

If it loses both electrons, He2+ ionforms

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SCES1200 L02 Atom Structure Classical Theories Student01