EARLY ATOMIC THEORIES

DEMOCRITUS

The Greek philosopher Democritus (460-370BC) was

the first person to propose the idea that matter was not infinitely

divisible.

He believed that matter was made up of tiny individual particles

called atomos, from which the English word atom is derived.

Democritus believed that atoms could not be created, destroyed, or

further divided.

Democritus also believed that matter is composed of empty space

through which atoms move.

ARISTOTLE

The Greek philosopher Aristotle (384-322BC) rejected

Democritus’ atomic “theory” because he did

not like the idea that atoms moved through empty space.

He did not believe that the “nothingness” of empty space

could exist.

Aristotle was able to gain wide acceptance for his ideas

and his denial of the existence of atoms went largely

unchallenged for 2000 years.

DALTON

The English schoolteacher John Dalton

(1766-1844) began the development

of modern atomic

theory.

Dalton revived and revised Democritus’

ideas based upon the results of his

scientific research.

DALTON’S ATOMIC THEORY

① Elements are made of tiny particles called atoms.

② All atoms of a given element are identical.

③ The atoms of a given element are different from those of any other

element.

④ Atoms of one element can combine with atoms of other elements to form

compounds. A given compound always has the same relative numbers

and types of atoms (law of constant composition).

⑤ Atoms are indivisible in chemical processes. That is, atoms are not

created nor destroyed in chemical reactions. A chemical reaction simply

changes the way the atoms are grouped together.

DALTON’S ATOMIC THEORY

Was Dalton’s atomic theory a huge step toward

our current atomic model of matter? YES

Was all of Dalton’s theory accurate? NO

Dalton’s theory had to be revised as additional

information was learned that could not be

explained by his theory.

DALTON’S ATOMIC THEORY

You will soon learn that Dalton was wrong about

atoms being indivisible (they are divisible into

several subatomic particles – the proton, neutron,

and electron).

all atoms of a given element are identical (atoms

of an element may have slightly different masses –

isotopes)

J.J. THOMSON

The English physicist J.J. Thomson (1856-1940)

discovered the first subatomic particle known as

the ELECTRON.

He did this by using a cathode ray tube and he concluded

that the mass of the charged particle (the electron) was much

less than that of a Hydrogen atom, the lightest known atom.

The conclusion was shocking because it meant there were

particles smaller than the atom.

J.J. THOMSON

J.J. Thomson proved Dalton’s Atomic Theory

incorrect.

Dalton said that atoms were indivisible

But, J.J. Thomson showed that atoms were divisible

into smaller subatomic particles.

ROBERT MILLIKAN

The American physicist Robert Millikan

(1868-1953) discovered the charge of the

electron.

So good was Millikan’s experimental setup and

technique that the charge

measured almost 100 years ago

is within 1% of the currently accepted

value.

J.J. THOMSON

After the electron’s charge was discovered, two

questions arose:

1)If electrons are part of all matter and they possess

a negative charge, how is it that all matter is

neutral?

2)If the mass of an electron is so small, what

accounts for the rest of the mass in an atom?

J.J. Thomson attempted to answer these

questions…

J.J. THOMSON

He proposed a model known as the Plum Pudding

Model of the atom, which showed a spherically

shaped atom that was positively charged with

electrons evenly distributed throughout, like raisins

distributed in a plum pudding.

The Plum Pudding was quickly proved incorrect.

ERNEST RUTHERFORD

The New Zealand born Ernest Rutherford (1871-1937) was a

student of J.J. Thomson at Cambridge University in England.

Rutherford came up with The Gold Foil Experiment.

The Gold Foil Experiment was

setup to shoot a beam of alpha

particles at a piece of gold foil that

was surrounded by a zinc sulfide

screen.

ERNEST RUTHERFORD

If the Plum Pudding Model were correct,

Rutherford would see only minor deflections as the

alpha particles passed through.

ERNEST RUTHERFORD

Rutherford saw some of the alpha particles go

straight through the foil without any deflection, but

many of the alpha particles hit the gold foil and

deflected off at large angles.

ERNEST RUTHERFORD

This is because the alpha particles, which are

positively charged, are hitting the dense positively

charged core in the middle of the atom and because

positive repels positive, they are being deflected.

The Plum Pudding Model was shown to be

incorrect.

ERNEST RUTHERFORD

Rutherford finalized his experiment concluding

that there was a tiny, dense region, which he called

the NUCLEUS, centrally located within the atom that

contained all of the atom’s positive charge and

almost all of the atom’s mass.

ERNEST RUTHERFORD

His new model also showed that most of an atom

consists of electrons moving rapidly through empty

space.

The electrons are held within the atom by their

attraction to the positively charged nucleus.

The volume of space through which the electrons

move is huge compared to the volume of the nucleus.

ERNEST RUTHERFORD

Later, Rutherford did further research to conclude

that the positively charged particle was to be called

a PROTON and that they have a charge equal to but

opposite of an electron.

JAMES CHADWICK

Rutherford’s student, an English physicist named

James Chadwick (1891-1974) discovered the final

subatomic particle, the NEUTRON, which is present

in the nucleus, has a neutral charge, and is about

equal in mass to the proton.