Evolution of the Atom 1 Evolution of Atomic Theory

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Evolution of the Atom

Evolution of Atomic Theory

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The Early History of Atomic Theory

Democritus/Leucippus (450 BC)Matter is made up of indivisible particlesCalled these particles “atomos”=indivisible

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Aristotle’s Composition Theory (354 BC)

The universe is made of matter and all matter is composed of four elements.

Earth

Air

Water

Fire

The structure of DNA was determined in 1954. How many years after Aristotle's idea is that?

Early thinkers looked for laws in science.

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What is a “Law” in science?

A “Law” in science is a concise statement or mathematical equation about a basic and repeatable relationship in nature.

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Mass is neither created nor destroyed (but this did not always agree with experimental observations).

2mcE =

What happen in American history in 1776?

Antoine Lavoisier (1776) Law of Conservation of Mass

Later, Einstein showed that there is a connection between mass and energy such that matter can be converted to energy (and vise versa). Einstein’s idea is expressed in the equation:

Where c is the speed of light in meters/second.

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Law of Conservation of Mass Example

Before a bulb burns out, the bulb contains some inert gas molecules and a metal filament.

After a bulb burns out, the bulb has a broken filament, small amount of metal atoms deposited on the glass inside the bulb, and the same amount of inert gas molecules it had before.

The total mass of the system is not different.

Light bulb

Antoine Lavoisier (1776)

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Evolution of the AtomAntoine Lavoisier (1776)

CuCO3 Cu + C + O2

Copper carbonate can ultimately be decomposed into its elements. This illustrates the mass conservation law.


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Evolution of the AtomAntoine Lavoisier (1776)

Before the reaction, you have 103 grams of copper carbonate.

After the complete reaction: you still have 103 grams of material but all of the copper carbonate has disappeared.

copper carbonate decomposition reaction


The total mass of the system is unchanged:

mass before reaction = mass after reaction

103.0 g = 40.2 g+ 52.5 g + 10.3 gCuCO3 Cu + C + O2

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Dalton’s Atomic Theory (1808) The first atomic theory

Was Thomas Jefferson, James Monroe, or James Madison president of the United States in 1808?

•Each element is made up of tiny, indivisible particles called atoms.

•Atoms of the same element are identical.

•(If the atoms are not the same, they are not from the same element.)

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A given compound always has the same relative numbers and types of atoms.

Is this a law?

Chemical compounds are formed when atoms combine with each other.

Dalton’s 1808 Atomic Theory (continued)

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Atoms are not changed in chemical reactions.

Is this a law?

Chemical reactions involve reorganization of atoms.


How are the different atoms held together?

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Laws of chemical combination led to a theory of the atom as a solid indivisible sphere (i.e., a marble).


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The atom is divisible

Thomson postulated the existence of electrons based on experiments using cathode ray tubes.

The atom is a ball of diffuse positive (+) charge with small negative (-) bits throughout its volume.

What was the name of the U.S. battle ship that sunk in 1898?

J. J. Thomson’s “Plum Pudding” Model (1898)

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Thomson’s Experiment: Deflection of “Cathode

Rays” by an Applied Electric Field

Electron beam

Electrical signal detected only when electrons collide with this positive plate

Negatively charged end cap

Electromagnet connected to power supply

Electromagnet connected to power supply

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Ernest Rutherford’s Model (1909)

Was New Mexico a state in the United States in 1909?• He postulated that an atom:

• has a dense positive (+) core that was called the atom’s nucleus.

• has several negative (-) particles called electrons traveling around the nucleus at a large distance from the nucleus.

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Rutherford’s Experiment: Deflection of “Alpha” Particles by a Thin Piece of Gold Foil

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Rutherford’s Experiment: Deflection of “Alpha” Particles by a Thin Piece of Gold Foil

Some of the alpha particles were reflected almost straight back toward the emitter. Thomson’s model of the atom could not explain why the alpha particles would not simply pass through the foil all the time.

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Alpha particles emitted from a radioactive atom source

Rutherford’s Experiment (continued)

This alpha particle missed the nucleus and did pass through the gold foil.

This alpha particle struck the nucleus and was reflected to the side. It did not pass through the gold foil.

This alpha particle reflects back.

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Dalton’s atomic theory (1808)

J. J. Thomson’s “Plum Pudding” model (1898)

Ernest Rutherford’s nucleus model (1909)

Aristotle’s composition theory (354 BC)How did earth, air, water, and fire get changed into trees, rocks, and living things?

Thinking people tried to develop theories to help explain what they observed. However, each new theory always left questions unanswered.

Why do some of these indivisible particles (atoms) join together and some do not?

Why do some of the alpha particles pass through a material when some of the particles do not?

Why don’t the electrons eventually lose their energy for motion and crash into the atom’s positive nucleus?

Democritus/Leucippus atomos theory (450 BC)

How do different things get made and big enough to see?

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As the models get better and better, more observations are explained and the questions to be answered get more complicated.

Thinking people tried to develop theories to help explain what they observed. However, each new theory always left questions unanswered.

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Evolution of the Atom 1 Evolution of Atomic Theory