Development of Atomic Theory

Physical ScienceChapter 4 - Section1

The Beginnings

• Atomic theory developed slowly over 2000 years• 4th century B.C., Greek philosopher Democritus

suggested matter consists of indivisible units– Called these units “Atoms”– Atomos – Greek for indivisible

• Not possible to acquire experimental evidence– No tools available during these ancient times

Dalton’s Atomic Theory

• English schoolteacher John Dalton used experimental observations in 1808– Some portions are still true today

• Concluded that all atoms of a given element are identical, and atoms of different elements join to form compounds

Dalton’s Atomic Theory ( cont. )• Observed different substances combining in proportions ( in

consistent ways )

• Law of Definite Proportions• A given compound always has elements in exactly the

same proportions by weight or mass• Water as example ( 11% H and 89% O )• Atoms combine in whole number ratios ( H2O )

• Dalton’s theory did not fit all observations– With further experiments by others, some observations were

found to be inaccurate

J.J. Thompson’s Contribution• In 1897, J.J. Thompson accidently discovered

electrons by experiment

• Conducted experiments with Cathode Ray Tube – suggested cathode rays consist of negatively-charged particles that come from atoms– Cathode Ray Tube consists of two metal plates on either end of a

vacuum tube ( Cathode – negative ; Anode – positive )– Formed an electron beam ( by applying a voltage across two plates )

and observed that a magnet deflected ( bent ) the beam

• His results also suggested that atoms could be divided into smaller parts

J.J. Thompson’s Model• Suggested that atoms can be thought of as plum

pudding ( alternatively, a blueberry muffin )

• In this model, electrons are spread throughout an atom ( mass and positive charge are evenly distributed throughout the atom however )

• Imagine a muffin with blueberries dispersed throughout it.– Blueberries are electrons– “Muffin” is the entire atom with the “bread” being the

positive portion of the atom ( evenly distributed )

Earnest Rutherford• Another Englishmen during 19th-20th century

• Developed experiment to test Thompson’s model

• Found that Thompson’s model needed refining

• In CONTRAST to Thompson’s model, Rutherford proposed that most of an atom’s mass was contained in its center ( the nucleus )

• Rutherford discovered the nucleus in a manner not vastly different than Thompson ( experiment )

Rutherford’s Experiment• Beam of positively charged alpha particles aimed

at a very thin piece of gold foil• By Thompson’s Model the positive charge at any

location in an atom would be too small to affect the paths of incoming particles

• Rutherford predicted that most particles would remain straight and not be deflected– Again, assuming Thompson’s model is accurate– If the positive charge on an atom was concentrated,

alpha articles would be deflected

Rutherford’s Gold Foil Experiment• Results showed that the alpha particles WERE DEFLECTED

• Some particles passed through, some were deflected by a large amount, some bounced straight back ( what did these hit? )

• This indicated:– That the positively-charged portion of the atom is NOT

dispersed ( or IS concentrated in the atom )– That Thompson’s model was not totally accurate– There is something in the middle of the atom!!! ( nucleus )

• Figure 7 ( pg. 118 ) shows model of atom after Rutherford’s work ( protons, neutrons, electrons )