atomic models 1

8/8/2019 Atomic Models 1

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How do you generate a

picture of something youcant see?

Models of the Atom


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Democritus Atomic Philosophy

460 370 BC

Believed that atoms wereindivisible andindestructible.

Theory based in reason.

Lacked experimentalsupport and laterchallenged by Plato and Aristotle


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Daltons Billiard Ball Atom

1766 1844

English chemist, teacher

Studied the ratios in whichelements combine

Theory: All elements are composed of tiny indivisible particles called atoms.

Atoms of the same element are identical. The atoms of any one element are different

from those of any other element and can be distinguished by their mass. Atoms of different elements can physically mix together or can chemically combine in

simple whole-number ratios to form compounds. (law)

Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of

one element, however, are never changed into atoms of another element as a result of

a chemical reaction.

Atoms cannot be created, destroyed (law), or divided into smaller particles.


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Thomsons Plum Pudding model

1897

English physicist

Cathode ray experiment

Atoms contain some sortof charged subatomicparticle Implies positive & negative

Theory: positively chargedmedium with negativecharges embedded


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Rutherfords Nuclear Atomic Model

1911, English baron

Shot a stream of alpha

particles at a piece ofgold foil


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(Rutherford contd)

People thought that the particles would passthrough with a little deflection.

Surprise! Almost all of the particles passedthrough without any deflection.

A very small fraction were deflected at largeangles and some even bounced straight back.

Nuclear Atom theory Atoms are mostly empty space

Matter is concentrated at the center of atoms thenucleus and is very, VERY small

The nucleus has a positive charge

Electrons are distributed around the nucleus


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Sketch the models for

John Daltons Billiard Ball atom

J.J. Thomsons Plum Pudding atom

Lord Ernest Rutherfords Nuclear atom

How is each model similar to and different to

the others?


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Spectral lines (emission spectra)

When hit by light, elements give off

different spectra of light

Where is this light coming from?


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Bohrs Solar System Model

1913, Danish Physicist

Light emitted by excited atoms

is energy, emitted when

electrons gain or lose energy Electrons gain energy to move

away from nucleus and lose

energy when falling back

Electrons must orbit nucleusin fixed paths, each farther

from the nucleus than the last,

like planets around the sun http://www.mhhe.com/physsci/astronomy/applets/Bohr/applet_files/Bohr.html


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Heisenberg Uncertainty Principle

In order to see something, you need to

shine light on it and see the reflected light.

If you shine a light on an electron, it gainsenergy, jumps to a different level, and its

momentum changes.

Therefore, it is impossible to know both

the exact position and momentum of an

electron at any given time.


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Electron Cloud model (today)

If we cannot know the position and momentum of an

electron, then electrons cannot travel in orbits!

The best we can do is describe the areas of space in

which electrons are most likelyto be found known asorbitals

Based in quantum mechanics, developed

by Schrodinger, de Broglie, and others

Also known as the wave-mechanical orquantum-mechanical model


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Homework!

Summary of the five major atomic models

(Dalton, Thomson, Rutherford, Bohr, Cloud

Electron)

Sketch each model and label them by name

Beneath each sketch, explain why it was

proposed/how it was discovered

Describe the similarities and differences between

this model and the model before it Describe at least one shortcoming

(limitation/problem) with each of the first four

models

atomic models 1

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