CHAPTER 11 SECTION 1

The Development of Atomic Theory

Objectives

Describe some of the experiments that led to the current atomic theory

Compare the different models of the atom

Explain how the atomic theory has changed as scientists have discovered new information about the atom

Consider this quote…

“Color exists by convention, sweet by convention, bitter by convention; in reality

nothing exists but atoms and the void.”

What do you think this statement means?

Who wrote it?

The Beginning of the Atomic Theory

Ideas about atoms have changed over time

Democritus (Greek philosopher 460-370BCE): All phenomena in nature could be understood in terms of the movement of particles called atoms.

Greek word atomos meaning “not able to be divided”

Atoms are small, hard particles, made of a single material formed into different shapes & sizes

Democritus

Ideas sharply contrasted those of Aristotle who thought that all matter could only be reduced to a combination of: earth, water, air, and fire

Only fragments of Democritus’ writings survived

http://faculty.washington.edu/smcohen/320/Democritus.jpg

The Atom

The smallest particle into which an element can be divided and still be the same substance

http://www.scienceclarified.com/images/uesc_02_img0063.jpg

Dalton’s Atomic Theory

Late 1700’s: elements combine in certain proportions based on mass to form compounds

Dalton experimented with different substances

In 1803, Dalton published his atomic theory All substances are made of atoms Atoms of the same element are exactly alike Atoms join with other atoms to make new substances

*The modern atomic theory began to develop from here…

Thomson’s Discovery of Electrons

In 1897, Thomson pointed out errors in Dalton’s theory…

There are small particles inside an atom

Experimented with a cathode-ray tube Positively charged plate attracted the beam Negatively charged particles called electrons are

present in every kind of atom

Thomson’s Cathode-Ray Tube

http://www.hcc.mnscu.edu/chem/abomb/Thomson_ExpBoth.jpg

Thomson’s Plum Pudding Model

New model for atoms containing electrons

“Like plums in pudding”

https://reich-chemistry.wikispaces.com/file/view/Thomson1.jpg

New ideas: Rutherford’s Atomic “Shooting Gallery”

In 1909, Rutherford tested Thomson’s theory

He designed an experiment to study the parts of an atom

Gold-foil experiment Aimed a beam of positively charged particles at a gold

foil A special coating behind the foil glowed Particles were deflected unexpectedly

Rutherford’s Gold Foil Experiment

http://www.wardfam.com/~marshall/Chapter%203/Chapter%203_files/image004.jpg

Where are the electrons?

In 1911, Rutherford revised the atomic theory

The center of the atom is tiny, very dense, positively charged: nucleus

He calculated that the diameter of the nucleus was about 100,000 times smaller than the diameter of the gold atom

http://faculty.virginia.edu/consciousness/images/rutherford%20model.gif

Bohr’s Electron Levels

In 1913, Niels Bohr studied the way that atoms react to light

Electrons move around the nucleus in certain paths or energy levels

http://www.humanthermodynamics.com/Atom_diagram.jpg

Cartoon humor…

http://www.cs.utah.edu/~draperg/cartoons/jb/bohring.gif

Modern Atomic Theory

20th century scientists: Schrödinger & Heisenberg

Further explained the nature of electrons in the atom

Electrons do not travel in definite paths as Bohr suggested

Path of an electron cannot be predicted “electron clouds”

Electron-Cloud Model

http://www.csmate.colostate.edu/cltw/cohortpages/viney_off/atom.jpg

Quick Quiz

What error did Thompson find in Dalton’s atomic theory?

What is the name for Thompson’s model of the atom?

What is the current model of the atom called?