Development of the Atomic Theory

Chapter 6 Section 1

The Beginning of Atomic Theory

• An atom is the smallest particle an element can be divided into and still have the properties of that element.

Ancient Greece• Around 440 B.C., Democritus

proposed that matter is made up of “uncuttable” particles which are always moving.

• He called the particle an atom.

Dalton’s Atomic Theory• All substances are made of

atoms. Atoms are small particles that cannot be created, divided, or destroyed.

• Atoms of the same elements are exactly alike, but different from other elements.

• Atoms join with other atoms to make new substances.

Thomson’s Discovery of Electrons

• He discovered there are small particles inside the atom and that atoms can be divided.

• He discovered that some particles had a negative charge called electrons.

• He didn’t know how they were arranged, but thought they might look like chocolate-chip ice cream.

Cathode Ray Tube

Rutherford’s Atomic

“Shooting Gallery”• He aimed a beam

of positively charged particles into gold foil.

• Most passed through, but some bounced back.

The Nucleus & the Electrons

• Rutherford discovered that atoms are mostly empty space.

• He said atoms have a tightly packed center called a nucleus.

• He developed a new model of the atom.

Bohr’s Electron Levels

•He studied the way atoms react to light.

•He suggested that electrons travel around the nucleus in definite paths.

The Modern Atomic Theory• Schrodinger and Heisenberg showed

that electrons do not travel in definite paths.

• The exact path can not be predicted.

ENERGY LEVELS• Each electron cloud

exists at a certain energy level.

• The energy of each electron keeps it in motion around the nucleus of the atom.SILVER

The Size of an Atom

• Atoms are very small, yet the atom was discovered without seeing a single atom.

• There are 20 thousand billion, billion atoms in a penny.

OBSERVING ATOMS• Atoms are so small

that light waves are too large to be used to observe them.

• The tools that scientist now use to observe atoms include the electron microscope.

End of Section 1

Electron Microscope image of

Silicon Nitrate

Video