History of Atomic TheoryReview of In-Class Research

Tuesday, September 15th, 2015

Ancient Philosophy§ Who: Aristotle, Democritus, Leucippus§ When: More than 2000 years agoears 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 “atomos”.§ Why: Aristotle and Democritus used observation and inference to explain the existence of everything.

What did an Atom Look Like to the Greeks

(Democritus)?

This was Democritus’ atomic model. It was simply a round sphere with no electrons, protons, or neutrons. Democritus created the first atomic model. His contribution helped people to understand the idea of an atom, then other scientists looked further into the science of the atom and its general makeup.

Dalton’s Particle Theory of Matter

• 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.

What did an Atom look like to Dalton?

• Who: J. J. Thomson• When: 1897• Where: England• What: Thomson discovered that

electrons were smaller particles of an atom and were negatively charged.• Why: Thomson knew atoms were

neutrally charged, but couldn’t find the positive particle.

Discovery of Electrons

Thomson’s Cathode Ray Tube

Understanding Thomson’s Experiment

What did an Atom look like to Thomson?

Bunch of negative charged particles (raisins) in a sphere of positive charge (plum pudding)

Discovery of Nucleus• Who: Ernest Rutherford• When: 1911• Where: England• What: Conducted an experiment that resulted in discovery of atom’s nucleus. 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.

Rutherford’s Gold Foil Experiment

What did an Atom Look Like to Rutherford?

• Who: Niels Bohr• When: 1913• Where: England• What: Proposed that electrons traveled in

fixed paths around the nucleus. Scientists still use the Bohr model.

• Why: Bohr was trying to show why each element emitted its own unique frequency of energy as electrons jumped orbits.

Bohr’s Model of the Atom

Bohr’s Model of the Atom

Our Current Model - The Electron Cloud

- Bohr said electrons orbit the nucleus (like the planets around the Sun)- The electron cloud model says that we can't know exactly where an electron is at any given time, but theelectrons are more likelyto be in specific areas.

Our Current Model - The Electron Cloud

- Bohr assigned electrons to different shells- Using quantum mechanics, the electron cloud model assigns electrons to different atomic orbitals. ** These are not all spheres

For the quiz/test

• You should know the main scientists and their experiments/contributions.

• You should be able to draw their models of the atom and explain differences.