A Brief History of Atomic Theory

Vocabulary:

atom

Dalton’s atomic theory

Law of Definite Composition

Law of Multiple Proportions

electron

nucleus

nuclear model

proton

neutron

“Black Circle” (1913)

Kasimir Malevich

Russian

How do you know that atoms exist?

• After all, atoms are “only a theory?” (By the way, “only a theory” is perhaps my least favorite phrase ever!)

Theory

• A set of statements or principles devised to explain a group of facts or phenomena, that has been repeatedly tested and can be used to make predictions about natural phenomena.

• Think about this:
• Imagine that the line below is a piece of iron.

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• Could you cut it in half?

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• Could you cut the half in half?

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• And again?

Where did the idea originate?

Where did the idea originate?

• Is there a point that you could not cut the wire in half and still have the same substance, in this case iron?

Where did the idea originate?

Around 400 BC, Greek philosopher Democritus reasoned that there must be a point where matter can not be cut in half and still be the same substance; therefore, all matter must be made of indivisible particles that he called “atomos” meaning indivisible.

Do you think that Democritus’ idea is scientific? Explain.

The first scientific atomic theory

John Dalton, 1807

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Drew conclusions which became his atomic theory from repeatable observation and experimentation.

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Dalton’s Model

By observing that compounds always decompose to form fixed ratios of the elements in the compounds no matter how much of the compound you started with, Dalton concluded that there must be individual particles (atoms) combining in fixed ratios to form the compounds.

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Dalton’s Model

Dalton’s Atom

Dalton’s Atomic Theory

1. All matter is composed of atoms (still supported.)
2. Atoms of the same element are identical (not supported any more.)
3. Atoms can not be created or destroyed. (This is now called the law of conservation of mass.)
4. Atoms of different elements combine in simple whole-number ratios to form compounds.
5. In chemical reactions, atoms are combined, separated, or rearranged.

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Results

Law of Definite Composition: Compounds have a fixed ratio of component elements.

(CO₂ in the U.S. is the same as CO₂ in Russia is the same CO₂ you breathe out is the same CO₂ plants absorb)

Law of Multiple Proportions: Elements can combine in different ratios to make different compounds.

(C₃H₈O is rubbing alcohol and C₁₂H₁₁O₂₂ is sugar)

Some new findings

• Facts:
• The tube contains a vacuum.
• There is a positive electrode on one side of the tube and a negative electrode on the other side of the tube.
• The electrodes are hooked up to a 10,000V electrical source.
• A magnet is the object in the person’s hand.

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Take a look

• So what’s going on in there?

Take a look

J.J. Thomson (1897 / England)

• Thompson concluded that the beam contained “corpuscles” that were small particles inside atoms and were negatively charged. We now call them electrons

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• Thompson knew atoms were neutrally charged, but couldn’t find the positive particle.

Thomson’s Experiment

Cathode Ray Tube, CRT, demonstrated properties of electrons

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Thomson’s Atomic Model

• Determined that the negatively charged particles have same mass and amount of charge regardless of the type of atom.
• Developed the “Plum Pudding” model of the atom with negative electrons in positive sphere.

Robert Millikan (1909 / America)

• Used “Oil Drop” experiment to find the charge of the electron (q = 1.60x10^-19 C)
• He did this by carefully balancing the gravitational and electric forces on tiny charged droplets of oil suspended between two metal electrodes.

Ernest Rutherford (1911 / England)

• Used Gold Foil Experiment
• He knew that atoms had positive and negative particles, but could not decide how they were arranged.
• Conducted an experiment to isolate the positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core.

Gold Foil Experiment

Ernest Rutherford’s “Nuclear” �Atomic Model

Dense, positive nucleus surrounded by negative electrons

James Chadwick (1932/ England)

• Mass of atom was too high to contain only protons and electrons

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• Discovered neutrons
• neutral particles in the nucleus of an atom

James Chadwick’s Atomic Model

• Revision of Rutherford’s Nuclear Model