Development of the Atom

Atomic ModelsA model uses

familiar ideas to explain unfamiliar facts observed in nature.

A model can be changed as new information is collected.

Democritus• 400 BC (More than 2400

years ago).

• Philosopher (no experiments were performed)

• He named the smallest piece of matter “atomos,” meaning “not to be cut or indivisible.”

Democritus’ Model

No definite model

atoms were small, hard particles

made of the same material

but were different shapes and sizes.

This theory was ignored and forgotten for more than 2000 years!

John Dalton

• 1803

• Performed experiments (details unknown) that led to the acceptance of the idea of atoms.

Dalton’s Model“Bowling Ball”

• He deduced that all elements are composed of atoms. Atoms are indivisible and indestructible particles.

• Atoms of the same element are exactly alike.

• Atoms of different elements are different.

• Compounds are formed by the joining of atoms of two or more elements.

J.J. Thomson

• 1897

• Discovered “the electron”

Thomson’s Model “plum pudding”

• Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.

Thomson’s Experiment • Thomson studied

the passage of an electric current through a gas in a cathode ray tube.

• As the current passed through the gas, it gave off rays of negatively charged particles.

Source ofElectricalPotential

Metal Plate

Gas-filledglass tube Metal plate

Stream of negativeparticles (electrons)

Thomson

• This surprised Thomson, because the atoms of the gas were uncharged. Where had the negative charges come from?

Where did they come from?

Thomson concluded that the negative charges came from within the atom.

A particle smaller than an atom had to exist.

The atom was divisible!

Thomson called the negatively charged “corpuscles,” today known as electrons.

Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom.

But he could never find them.

Ernest Rutherford

• 1909

• Discovered the dense positive “nucleus”

• Discovered over 99.5% of the atom is empty space

The Rutherford Model

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57

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Rutherford’s Experiment

• Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)

• Most of the positively charged particles passed right through the gold atoms in the sheet of gold foil without changing course at all. This could only mean that the gold atoms in the sheet were mostly open space. Atoms were not a pudding filled with a positively charged material.

Rutherford’s Gold Foil Experiment

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

beam of alpha particles

radioactive substance

fluorescent screencircular - ZnS coated

gold foil

Some of the positively charged particles however, did bounce away from the gold sheet as if they had hit something solid.

Since positive charges repel positive charges. He knew he had discovered a positive center he called the nucleus.

Beams of

POSTIVE

alpha

particles .

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gold foil

• The nucleus is tiny compared to the atom as a whole.

• Image a marble is the nucleus. Place it on the 50 yard line in a stadium. Now place several electrons in random seats. The rest of the stadium/atom is empty!

Niels Bohr• 1913

• Placed each electron in a specific energy level around the nucleus.

Bohr’s Model“planetary model”

• Electrons move in definite orbits around the nucleus, much like planets circle the sun.

Bohr’s Experiment

• Studied the Hydrogen Atom and noticed that electrons changed energy levels when excited or de-excited. Light is emitted when the electron are de-excited.

Erwin Schrodinger

• 1926

• Discovered that electrons exist in regions of probability and do not travel in orbits

• Used math instead of experimentation

Heisenberg’s contribution

• Schrodinger used Heisenberg’s uncertainty principle to create his model.

• The principle states that it is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has.

Schrodinger’s Model“electron cloud”

James Chadwick

• 1932

• Discovered the neutron

• Thus having a nucleus that was made of neutral neutrons and positive protons

Chadwick’s Model

Chadwick’s Experiment

• When Beryllium was bombarded by alpha particles (from polonium) it emitted particles that were found to be neutral. 

• These neutrons had the same mass as a the positive proton that is also in the nucleus

Gellman/Zweig

• 1964

• Discovered quarks inside protons and neutrons

• There are 6 types of quarks that have been discovered so far: up, down, charm, strange, top (1977), bottom (1995)

• Top and bottom quarks were discovered at Fermilab here in the Chicago suburb of Batavia

• This discovery of the quark has not changed Schrodinger’s Model. It has just added to.

• Proton is made of 2 up quarks and 1 down held together by gluons

• Neutron is made of 1 up and 2 down quarks held together by gluons

Gellman/Zweig’s Model

Gellman/Zweig Experiment

• It was just a theory until the 1970’s when a particle accelerator was used to accelerate and smash atoms.

• Through this process they proved the existence of new subatomic particles called quarks.

The FUTURE Since atomic theory is just a theory, we will continue to challenge it and hopefully find more new and exciting things in our atom