Early Ideas About

Matter

Chapter 4

Greek Philosophers • Matter composed of four

elements:

- earth

- air

- water

- fire

• Matter endlessly divided into smaller pieces

Democritus • Proposed idea that matter was

not infinitely divisible

• Tiny individual particles called atomos

• Cannot be created, destroyed, or divided

• Different kinds of atoms have different sizes and shapes

Democritus

Aristotle • Rejected Democritus’

ideas

• Did not believe empty space existed

• Denied the existence of atoms

• 4 elements (earth, fire, air, water)

Democritus v. Aristotle

• Democritus - unable to support his ideas with experiments

–Could not answer what held atoms together

• Aristotle was more influential

–He rejected Democritus’ ideas

–His ideas went unchallenged for nearly 2,000 years

John Dalton

• Revived concept of atom during late 18th century

• Work marks beginning of modern atomic theory

• Conducted experiments allowing him to refine and support his hypotheses

Dalton’s Atomic Theory

• Proposed in 1803

• Matter is composed of extremely small particles called atoms

• Atoms are indivisible & indestructible

• Different types of atoms

–Each type has its own size, mass, and chemical properties

• Atoms are separated, combined, or rearranged during chemical reactions

Dalton’s Elements

From Dalton’s book A New

System of Chemical

Philosophy

Problems with Dalton’s Theory

• Atoms are indivisible & indestructible BUT (we know) atoms are divisible into subatomic particles

• Atoms of the same element can have slightly different masses

Sir William Crookes • Cathode ray tubes led to an accidental discovery:

–Negative particles called electrons are found in all forms of matter

J. J. Thompson

• Studied negatively charged particles of matter

• Determined the mass of the charged particle (electron) was less than the mass of a hydrogen atom

– Shocking discovery & hard to

accept that there were particles

smaller than atoms

Robert Millikan

• Determined charge of an electron using an oil-drop apparatus

• Electron has a single unit of negative charge (-1)

• Calculated mass of electron (9.1 x 10-28 g)

J. J. Thompson & Plum Pudding Model

• Atoms are spherical

• Composed of a uniformly distributed positive charge with electrons residing inside

Ernest Rutherford - 1911

• Concluded that plum pudding model was incorrect

• Atoms consist mostly of empty space through which electrons move

• Positive charge & all mass contained in dense, center region known as nucleus

Ernest Rutherford

• Electrons are held in atom by positive charge of nucleus

• Called Nuclear Atomic Model

• Problem – model did not account for ALL mass in an atom

Thompson v. Rutherford

Ernest Rutherford

• Redefined nucleus in 1920

• Contained protons with a positive charge

–Proton has equal BUT opposite charge of electron

James Chadwick

• Showed nucleus contained another subatomic particle called neutron

• Neutron has

no charge

and mass

nearly equal

to proton

Atoms through time…

Modern Atomic Theory • All atoms composed of 3 parts:

–Neutrons

–Protons

–Electrons

• Small, dense nucleus with positive charge that contains 99.97% of atom’s mass

Modern Atomic Theory

• Held together by positive/negative attraction

• Electrons are fast moving & travel through empty space

• Current model: Electron Cloud Model