History of Atomic Theory

Democritus (460 - 370 BC)
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Was the first person to come up with the idea of atom
Believed that all matter was composed of indivisible particles he called “ ATOMS ”
Which is derived from the Greek word
“ Atomos ” – meaning indivisible
He also believed that different atoms:
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Are different sizes
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Have different properties
Other philosophers of that time did not agree with his theories.

John Dalton (1766-1844)
• Dalton is the “ Father of
Atomic Theory ”
• Dalton ’ s ideas were so brilliant that they have remained essentially intact up to the present time and has only been slightly corrected.

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aka: 5 Postulates
1. All matter is composed of extremely small particles called atoms .
(I agree with Democritus!)
2. All atoms of a given element are identical, having the same :
- size
- mass
- chemical properties.
3. All atoms of different elements are different .

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aka: 5 Postulates
4. Atoms cannot be created, divided into
smaller particles, or destroyed.
**In a chemical reaction, atoms of different elements are separated, joined or rearranged. They are never changed into the atoms of another element.
We will learn more later**
5. Atoms combine in definite whole number ratios to make compounds
( you can ’ t have a ½ of a Carbon bonding with
Oxygen; it ’ s a whole atom or no atom )

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• Based on Dalton ’ s Atomic
Theory (5 postulates), most scientists in the 1800s believed that the atom was like a tiny solid ball that
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• Dalton was credited for the three Atomic Laws that were proven after his time .

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1. Law of Conservation of Mass
Matter cannot be created or
destroyed in any physical or chemical process, just transferred.
2.
Law of Constant Composition
When atoms combine to form molecules, the ratio of atoms is constant.
Example – H
2
O will always have 2 times as many Hydrogen atoms as Oxygen.

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3.
Law of Multiple Proportions – if two elements can combine to form more than one compound, then the ratio of the second element combined with a certain mass of the first element is always a ratio of small whole numbers.
Example:
CO vs. CO
2 Formula
Ratio of N:O

•When a chemical reaction occurs, mass is neither created nor destroyed but only changed.

JJ Thomson (1856-1940)
• Used cathode rays to prove that Dalton ’ s
Solid-ball model could be
•Thomson is credited with discovering electrons

• Cathode rays had been used for some time before Thompson ’ s experiments.
• A cathode ray is a tube that has a piece of metal, called an electrode, at each end. Each electrode is connected to a power source (battery).
• When the power is turned on, the electrodes become charged and produce a stream of charged particles . They travel from cathode, across the tube to the anode.

• Thomson put the tube in a magnetic field.
He predicted that the stream would travel in a straight path.
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• Instead, he found that the path curved away from a negatively charged plate and toward a positively charged plate
• Why?
•Like charges repel each other, and objects with unlike charges
attract each other, Thomson concluded that the stream of charged particles had electrons in them.

• Thompson Concluded :
•Cathode rays are made up of invisible, negatively charged particles called Electrons .
•These electrons had to come from the matter (atoms) of the negative electrode.
•Since the electrodes could be made from a variety of
metals, then all atoms must contain electrons!

•Thomson ’ s Plum Pudding model is a + charge sphere that has (- )charged electrons scattered inside, like “ raisins ” in
“ plum pudding ” .
•Overall, the atom is neutral atom because the atom had the same number of positive and negative charges.
•From Thomson ’ s experiments, scientists concluded that atoms were not just neutral spheres, but somehow were composed of electrically charged particles .
•The balance of positive and negative charge supports the neutral atom.

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Took Thomson ’ s Plum
Pudding Model and added to it
Used the “ Gold Foil
Experiment ” to discover the existence of:
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An atomic Nucleus
Protons
(in later experiments)
You must be able to explain the Gold Foil
Experiment…it will be on the CST

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Rutherford directed a narrow beam of alpha particles (+ charges ) at a thin piece of gold foil.
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Based on observations from other experiments involving alpha particles, he predicted that the (+) charges would go through the foil

•Rutherford found that every once and a while, a + particle was deflected bounced back.
(about 1% of the time)
•Why?
•Because the + charge hit a central mass of positive charge and was repelled.

The Gold Foil Experiment

The Nuclear Model of the Atom
•To explain the results of the experiment,
Rutherford ’ s team proposed a new model of the atom.

(memorize this!)
• The atom contains a positively charged
“ nucleus ”
•This nucleus contains almost all of the mass of the atom, but occupies a very small volume of the atom.
•The negatively charged electrons occupied most of the volume of the atom.
• The atom is mostly empty space.

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• To explain his observations,
Rutherford developed a new model
•The electrons orbit the nucleus like the planets revolve around the sun
.

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Worked in Rutherford ’ s lab
Wondered why – electrons are not attracted to the + nucleus and cluster around it
Disproved Rutherford ’ s
Planetary Model
Experimented with light and its interaction with matter to develop a new model.

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Energy Level Model: Electrons are arranged in circles around the nucleus. Each circle has a different energy.
•Electrons are in constant motion, traveling around the circle at the speed of light.
•Electrons can “ jump ” from one circle to the next
•But they can ’ t go to the nucleus they traveling too fast to be fully attracted .

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 He proposed the following:
1. Protons and neutrons are in the nucleus
2. Electrons can only be certain distances from the nucleus.
3. The electrons orbit the nucleus at fixed energy levels.
4. The electrons must absorb or emit a fixed amount of energy to travel between these energy levels

Who is the father of atomic theory?
Dalton
What was the first model of the atom?
Dalton ’ s Tiny Ball Model
What are Dalton ’ s 3 Laws?
Law of Conservation of Mass, Law of
Constant Composition, Law of Multiple
Porportion

How were Thomson ’ s and Dalton ’ s model different?
Dalton ’ s model was 1 sphere that cannot be divided, Thomson had the plum pudding where electrons are randomly spread throughout a positively charged sphere.
What did Thomson find out?
Atoms have electrons, they have a - charge

What were Rutherford ’ s conclusions from the
Gold Foil Experiment?
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Atom has a positively charged nucleus electrons are outside,
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 atoms are mostly empty
Nucleus contains most of the mass.