History of Atomic Structure

Ancient Philosophy• Who: Aristotle, Democritus

• When: More than 2000 years ago

• Where: Greece

• What: Aristotle believed in 4 elements: Earth, Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atoms”.

• Why: Aristotle and Democritus used observation and inferrence to explain the existence of everything.

Aristotle

Democritus

Alchemists• Who: European Scientists

• When: 800 – 900 years ago

• Where: Europe

• What: Their work developed into what is now modern chemistry.

• Why: Trying to change ordinary materials into gold.

Alchemic Symbols

Particle Theory• Who: John Dalton

• When: 1808

• Where: England

• What: Described atoms as tiny particles that could not be divided. Thought each element was made of its own kind of atom.

• Why: Building on the ideas of Democritus in ancient Greece.

John Dalton

Solid, Tiny Particle

Discovery of Electrons• Who: J. J. Thompson

• When: 1897

• Where: England

• What: Thompson discovered that electrons were smaller particles of an atom and were negatively charged.

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

J. J. Thompson

Electron Charge and Mass• Who: Robert A. Millikan (1868-1953)• When:1910• Where: United States• What: Found the quantity of an electron charge in

his oil-drop experiment. He calculated the charge by suspending negatively charged oil droplets between two plates and measure the rate of fall when the voltage was changed.

• Why: Wanted to discover whether electrical charge occurred in a specific amount.

Robert Millikan

Atomic Structure I• Who: Ernest Rutherford

• When: 1911

• Where: England

• What: 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.

• Why: He knew that atoms had positive and negative particles, but could not decide how they were arranged.

Ernest Rutherford

Atomic Structure II• Who: Niels Bohr

• When: 1913

• Where: England

• What: Proposed that electrons traveled in fixed paths around the nucleus. Scientists still use the Bohr model to show the number of electrons in each orbit around the nucleus.

• Why: Bohr was trying to show why the negative electrons were not sucked into the nucleus of the atom.

Niels Bohr

HELIUM ATOM

+N

N

+-

-

proton

electron

neutron

Shell

What do these particles consist of?

Electron Cloud Model-1920s• Electrons travel around the nucleus in

random orbits.

• Scientists cannot predict where they will be at any given moment.

• Electrons travel so fast, they appear to form a “cloud” around the nucleus.

Electron Orbitals• Who: Erwin Schrödinger

• When: 1926

• Where: Austria

• What: Develops mathematical equations to describe the motion of electrons in atoms, which leads to the quantum mechanical model

• Why: Wanted to better understand the movement of the electrons in an atom

Erwin Schrödinger

Electron Cloud Model

Birth of Quantum Mechanics: • Who: Werner Heizenberg • When: 1925• Where: Germany• What: Proposed that electrons traveled in

atomic orbitials• Why: Heizenberg realized that the reason

Bohr’s model failed was that it was trying to predict none observable variables (position, speed)

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