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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.

Kaitlyn Thursby

on 6 December 2017

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Transcript of Atomic Theory Timeline

Atomic Theory Timeline
~400 BC
Current Advancements and Technology
Electrons are located in orbitals within the electron cloud, not circular orbits.
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
First proponents of an atomic theory
"Atom" - derived from the Greek word atomos, meaning indivisible
1. Matter is composed of atoms separated by empty space through which the atoms move. (discontinuous matter)
2. Atoms are solid, homogeneous, indivisible, and unchangeable.
3. All apparent changes in matter result from changes in the groupings of atoms.
4. There are different kinds of atoms that differ in size and shape.
5. The properties of matter reflect the properties of the atoms the matter contains.
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

Niels Bohr
Atom is mostly empty space
Has a small, dense, positively charged nucleus in the middle
The electrons are orbiting the nucleus on specific paths (like planets in the solar system).
The electrons can change orbits.
the Modern Theory
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.
Antoine Lavoisier
"Father of Modern Chemistry"
Law of Conservation of Mass
Well known, popular philosophers
Thought that matter was 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.
Joseph Proust
Law of Definite Proportions
Oil Drop Experiment
Determined the charge of an electron.
Robert Millikan
James Chadwick
Discovered the neutron
Create a Venn Diagram that compares and contrasts
Dalton, Democritus, and Aristotle.
"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?
How was atomic theory
developed over time?
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.

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.

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 = 2.000
0.8964 1

a ratio of small whole numbers!

Compound A: CuCl
Compound B: CuCl2
77. Describe Rutherford's "gold foil" experiment that led to the discovery of the nuclear atom.

Rutherford directed alpha-particles toward a thin piece of gold foil that was surrounded by a fluorescent screen. If the plum-pudding model of the atom were correct, the alpha particles would travel straight through the gold foil and hit the screen in one location. Instead, the alpha particles were deflected, sometimes at large angles. The gold foil experiment gave evidence for the nucleus.

76. Summarize how Dalton's theory helped to support the conservation of mass.

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
Nature, 2003
Black Box Activity
39.3% Na
60.7% Cl
British chemist and school teacher / performed experiments

Law of Multiple Proportions

First modern atomic theory
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.
Neutrons have not been discovered yet.
Solar system model
Electrons move in discrete orbits around the nucleus.
Heisenberg Uncertainty Principle -
you can never simultaneously know the exact position and the exact speed of an object.

Why not?
Because everything in the universe behaves like both a particle and a wave at the same time.
Schrodinger Wave Equation -
Calculations relating to the location of electrons in terms of probability.
"matrix mechanics"
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