Development of Atomic Theory Unit 2 Chapter 4, Section 1

source Democritus vs. Aristotle

Democritus 400 B.C. Democritus 400 B.C. Matter cannot be divided forever. Matter cannot be divided forever. Matter is composed of atoms - solid, homogeneous, indivisible particles. Matter is composed of atoms - solid, homogeneous, indivisible particles. Different kinds of atoms have different shapes. Different kinds of atoms have different shapes. Changes in matter result from changes in groupings of atoms. Changes in matter result from changes in groupings of atoms. Atoms move through empty space. Atoms move through empty space. Matter can be subdivided into smaller and smaller particles forever. Matter can be subdivided into smaller and smaller particles forever. Matter is composed of combinations of air, earth, fire, and water. Matter is composed of combinations of air, earth, fire, and water. No such thing as empty space. Had the “ether.” No such thing as empty space. Had the “ether.” Aristotle 350 B.C.

For 2000 years most people believed that the 4 elements were air, earth, water, and fire. And there was no such thing as empty space! Aristotle won that debate!

Antoine LaVoisier Demonstrated the importance of measurements in chemistry.Demonstrated the importance of measurements in chemistry. Very interested in combustion.Very interested in combustion. - Corrosion of metals - Respiration Disproved the phlogiston theory.Disproved the phlogiston theory. First stated Law of Conservation of Mass.First stated Law of Conservation of Mass.

LaVoisier was the father of modern chemistry, but he lost his head in 1794!

Phlogiston Theory wood  calx (ash) + phlogiston (to the air) wood  calx (ash) + phlogiston (to the air) iron  calx (rust) + phlogiston (to the air) iron  calx (rust) + phlogiston (to the air) They weren’t too clear on the difference between heat (a form of energy) and matter. They weren’t too clear on the difference between heat (a form of energy) and matter.

J. Proust – 1797 Law of Definite Proportions In a pure compound, the elements combine in definite proportions to each other. In a pure compound, the elements combine in definite proportions to each other.

source John Dalton & Atomic Theory 1803 source

Dalton’s Atomic Theory Matter is composed of very small particles – atoms. Matter is composed of very small particles – atoms. All atoms of a given element are identical.* All atoms of a given element are identical.* Atoms of different elements are different. Atoms of different elements are different. Atoms are indivisible and cannot be created or destroyed.* Atoms are indivisible and cannot be created or destroyed.* Different atoms combine in whole number ratios to form compounds. Different atoms combine in whole number ratios to form compounds. In a reaction, atoms are rearranged. In a reaction, atoms are rearranged.

So Dalton’s atoms are kind of like billiard balls. Atoms combine in whole-number ratios.

In a reaction, atoms are rearranged. +  + +

So how big is an atom? Most atoms are 1-2 angstroms across. Most atoms are 1-2 angstroms across. An angstrom = 1 X m. An angstrom = 1 X m. 6 Billion Cu atoms in a line – less than 1 meter! 6 Billion Cu atoms in a line – less than 1 meter! Can we “see” an atom? Can we “see” an atom?

28 Mn on Ag