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Atomic Theory Timeline

The development of the atomic theory has changed over time as scientists, armed with new technologies, attempt to test the accepted understanding of the atom.
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Kaitlyn Thursby

on 3 December 2015

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Transcript of Atomic Theory Timeline

Atomic Theory Timeline
~400 BC
1802
1897
1909
1913
"Present"
Current Advancements and Technology
Electrons are located in orbitals, not spherical "clouds"
Theory: A unifying explanation for a broad range of hypotheses and observations that have been supported by testing.
Atom: the smallest particle into which an element can be divided and still be the same substance
Democritus
Proposed the "uncuttable" atom
Solid, homogeneous, indestructible, indivisible particle
Matter is discontinuous
Atom - derived from the Greek word atomos, meaning uncuttable
John Dalton
Postulates of Dalton's Atomic Theory

1. All substances are made of small particles called atoms.

2. Atoms cannot be created, divided, or destroyed.

3. Atoms of the same element are exactly alike in mass, size, and other properties; atoms of different elements are different in mass, size, and other properties.

4. Different atoms join in simple whole-number ratios to form compounds.

5. In a chemical reaction, atoms are combined, separated, or rearranged to form new substances.
J.J. Thomson
Cathode Ray Tube Experiment
Particle Beam
Negative Plate
Positive Plate
When the plates were charged, the particle beam was attracted to the positive plate.
This means that the beam was negative because....
Proved the existence of negatively charged particles - called them corpuscles. Later, these particles were called electrons.

Atoms are divisible!
Democritus and Dalton were wrong! Why?! Stay tuned to find out!
Ernest Rutherford
Discovered the nucleus

The atom is mostly empty space with a small, dense, positively charged nucleus in the center. The electrons are moving around the nucleus in no particular order or path.
Niels Bohr
Atom is mostly empty space
Has a small, dense, positively charged nucleus in the middle
BUT:
The electrons are orbiting the nucleus on specific paths (like planets in the solar system).
The electrons can change orbits.
the Modern Theory
Heisenberg
Schrodinger
Electrons are located in regions of space called "electron clouds" - exact locations can't be determined
Plum Pudding Model
The "solar system" model
Models are often used for things that are too small or too large to be observed or that are too difficult to be understood easily.
1789
Antoine Lavoisier
"Father of Modern Chemistry"
Law of Conservation of Mass
Aristotle
Plato
Well known, popular philosophers
Thought that matter was continuous
Continuous - could always get smaller and smaller
No such thing as empty space
Compare and Contrast
The Greek Philosophers

With your partner, describe their theories.
1779
Joseph Proust
Law of Definite Proportions
Oil Drop Experiment
Determined the charge of an electron.
Robert Millikan
James Chadwick
Discovered the neutron
1932
Create a Venn Diagram that compares and contrasts
Dalton with Democritus.
"IBM scientists discovered how to move and position individual atoms on a metal surface using a scanning tunneling microscope. The technique was demonstrated in April 1990 at IBM's Almaden Research Center in San Jose, Calif., where scientists created the world's first structure: the letters "I-B-M" -- assembled one atom at a time." - IBM
In 2009, IBM imaged the molecule pentacene using AFM (atomic force microscopy).
How do we know that atoms exist?
Can we see them?
If we can't see them through a microscope, how do we know they are there?
Agenda:
History of atomic theory

Objectives:

75. Contrast the early theories of matter from Democritus and Aristotle.
76. Describe the development of atomic theory leading to the current model of the atom based on the works of Dalton, Thomson, Rutherford, and Bohr.
77. Describe Rutherford's "gold foil" experiment that led to the discovery of the nuclear atom.
How was atomic theory
developed over time?
Objectives

75. Contrast the early theories of matter from Democritus and Aristotle.
Notes
75. Contrast the early theories of matter from Democritus and Aristotle.

Democritus was the first person to propose the existence of atoms (discontinuous matter).
According to Democritus, atoms are solid, homogeneous, and indivisible.
Aristotle did not believe in the existence of atoms. He thought matter was continuous.
Remember the three laws?

Law of Definite Proportions
Law of Conservation of Mass
Law of Multiple Proportions
Law of Definite Proportions

A compound is always composed of the same elements in the same relative amounts (ratio), no matter how large or small the sample.

The relative amounts are expressed as percent by mass, the ratio of the mass of each element to the total mass of the compound expressed as a percentage.
3.00 g salt contains:
1.18 g sodium (Na)
1.82 g chlorine (Cl)




How does this example demonstrate the Law of Definite Proportions?
5.50 g salt contains:
2.16 g sodium (Na)
3.34 g chlorine (Cl)
Law of Multiple Proportions

When different compounds are formed by a combination of the same elements, different masses of one element combine with the same relative mass of the other element in a ratio of small whole numbers.

For 2 different compounds made of elements X and Y:




This is where we developed chemical formulas, and how we know atoms exist!

water and hydrogen peroxide
H2O , H2O2

carbon monoxide and carbon dioxide
CO , CO2
Law of Conservation of Mass

Mass is conserved during a chemical reaction; it is neither created nor destroyed.

mass of reactants = mass of products
In a laboratory, 178.8 g of water is separated into hydrogen gas and oxygen gas. The hydrogen gas has a mass of 20.0 g. What is the mass of the oxygen gas produced?

178.8 g - 20.0 g = 158.8 g of oxygen gas


How does this example demonstrate the Law of Conservation of Mass?
39.3% Na
60.7% Cl
1.793 g Cu : 1 g Cl
0.8964 g Cu : 1 g Cl
1.793 / 0.8964 = 2.000
a whole number!

2:1 ratio

Compound A: CuCl
Compound B: CuCl2
Agenda:
History of atomic theory

Objectives:

76. Describe the development of atomic theory leading to the current model of the atom based on the works of Dalton, Thomson, Rutherford, and Bohr.
77. Describe Rutherford's "gold foil" experiment that led to the discovery of the nuclear atom.

Remember the three laws? Dalton put them together to make the first atomic theory.

Dalton said that all substances are made of atoms, which are small particles that cannot be created nor destroyed.
He also said that a chemical reaction is simply the rearrangement of atoms.
Expected Result
mass X / mass Y for compound 1
mass X / mass Y for compound 2
= a whole number ratio
mass ratio for compound 1
mass ratio for compound 2
=
Nature, 2003
Black Box Activity
39.3% Na
60.7% Cl
Notes
76. Describe the development of atomic theory leading to the current model of the atom based on the works of Dalton, Thomson, Rutherford, and Bohr.
77. Describe Rutherford's "gold foil" experiment that led to the discovery of the nuclear atom.

76: [See graphic organizer]

77: Rutherford shot alpha particles through a thin piece of gold foil and observed that some particles passed through and some particles deflected at extreme angles. He concluded that the atom was a lot of empty space with a very dense nucleus in the center.


British chemist and school teacher / performed experiments

Law of Multiple Proportions

First atomic theory
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