atomic theory very good

8/13/2019 Atomic Theory Very Good

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When do scientific theories change?


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Democritus develops the idea of atoms

460 B.C.

he pounded up materials until he had reduced

them to smaller and smaller particles which

he called

ATOMA

(greek for indivisible)

Were Democritus tiny particles really undividable?


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The Greeks believed

that all matter was made from 4 elements

Aristotle (384-322 BC)Fire is NOT matter! What form of energy is fire?

WRONG


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1803 John Dalton

EXPERIMENTAL OBSERVATIONS

Combined elements to form compounds

Observed that the ratio of elements in acompound was always the same

element + element = compound

Magnesium + oxygen = magnesium oxide

10 grams + 6.58 grams = 16.58 grams

1000 grams + 658 grams = 1,658 grams


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1803 John Dalton

Theory (model)

All matter is made of particles that

cant be divided ATOMS

An ATOM is like a solid sphere

Elements are made of 1 type of atom

Compounds contain more than 1 kind

of atom


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1897 JJ Thompson

EXPERIMENTAL OBSERVATIONS

Negatively charged beam attracted to a

positive plate (magnet)

ELECTRON

Sealed tube filled with gas


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JJ THOMSON

Theory (model)

Negative(-) particles are a part of the atom

Since the atom as a whole, is neutral, there must also be

positive (+) matter within the atom

Chocolate Chip IceCream Model

Chips are negativeparticles

Ice cream is positivematter

1897


8/22

1911 Ernest Rutherford

oversaw Geiger and Marsden carrying out his

famous gold foil experiment.

they fired Helium nuclei (also called alpha

particles) at a very, very thin piece of gold

foil(only a few atoms thick).

They predicted that the alpha particles

would pass right through the foil.


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Experimental Observations

They found that most

of the alpha particles

passed through the foil.

However, about 1 in10,000 were deflectedhttp://regentsprep.org/Regents/physics/phys05/catomodel/ruther.htm
http://regentsprep.org/Regents/physics/phys05/catomodel/ruther.htmhttp://regentsprep.org/Regents/physics/phys05/catomodel/ruther.htm


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Theory (model)

Alpha particle deflection

was caused by a dense

nucleus

the positive chargein an

atom are concentrated in

the nucleus

Predicted result

Actual result

What do the blue circles on the diagram represent?


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1913 Niels Bohr

studied under Rutherford at the

Victoria University in Manchester.

Bohr refined Rutherford's idea by

adding that the electrons were in

orbits.like planets orbiting the

sun.

Link to Niels Bohrs paper on Atoms and Molecules

http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Bohr/Bohr-1913a.html
http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Bohr/Bohr-1913a.htmlhttp://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Bohr/Bohr-1913a.htmlhttp://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Bohr/Bohr-1913a.htmlhttp://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Bohr/Bohr-1913a.htmlhttp://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Bohr/Bohr-1913a.htmlhttp://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Bohr/Bohr-1913a.html


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Bohr Model


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Modified

Bohr Model of a

helium atom

++

-

-

proton

electron neutron

shell

(energy level)

Electrons travel

around the nucleuslike planets aroundthe sun.

In 1932, James Chadwick confirms the existence of neutrons on in the nucleus of atoms.

What is different about this model compared to the previous slide?


14/22

1926 Erwin Schrodinger

Schrodingers Cat: a thoughtexperiment.

Use mathematics to describethe motion and locations ofelectrons.

Electron Cloud Model of theatom was developed as aresult of Schrodingers workwith mathematical probability.


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Electron Cloud Model

This model shows theprobability (likelihood) offinding an electron.

The cloud model represents

a sort of history of where anelectron has been and whereit is going.

Imagine if the electron left

a trace of where it was. Thecollection of traces wouldbegin to resemble a cloud.

Where are we most likely to find

an electron within this atom?


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Scientific Theory May Change

As scientists have made new observations of matter,their understanding of the atom changed.

Scientists must be open to new evidence. A scientific

theory does not represent truth but instead our bestunderstanding.

Does the statement above mean that the scientistswho have studied the atom were wrong?


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MORE ON ATOMS

Actual picture of atoms. Kinda looks like

solid spheres from this perspective.


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Energy LevelsElectrons that are closer to

the nucleus have less

energy.

When electrons are in their

lowest energy levelsthey

are in their ground state

When an electron absorbsenough energy, it may move

to a higher energy

levelexcited state

When an electronreturns to its ground

state

light may be released

from the atom


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Isotopes

The same atomswith different

numbers of

neutrons.

How can you

determine the

mass number of

an atom?


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ATOMIC STRUCTURE

What is the atomic number of

the atom shown above?

What is its mass number?

17

35


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ATOMIC STRUCTURE

Particle

proton

neutron

electron

Charge

+ charge

- charge

No charge

1

1

1/1837

Mass


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SUMMARY

1. The Atomic Numberof an atom =number of

protons in the nucleus.

2. The Mass Numberof an atom =number of

Protons + Neutrons in the nucleus.

3. The number of Protons =Number of Electrons.

4. Electrons orbit the nucleus in shells(levels).

5. Electrons can move between levels as energy isabsorbed or emitted.

atomic theory very good

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