ATOMIC THEORY
The history and structure of the atom
ATOMS



Atoms are the fundamental building block of
ordinary matter.
They are smallest identifiable unit of an element.
They are so small you can’t even see them with a
microscope.
THE GREEKS

Democritus

Proposed elements consist of tiny solid particles that
cannot be subdivided.
THE HISTORY OF THE ATOM

Dalton’s Atomic Theory (1808)
Each element is composed of tiny, indestructible
particles called atoms.
 All atoms of a given element have the same mass and
other properties that distinguish them from the
atoms of other elements.
 Atoms combine in simple, whole-number ratios for
form compounds.
 Atoms of one element cannot change into atoms of
another element.


In a chemical reaction, atoms change the way that they are
bound together with other atoms to form a new substance.
THE HISTORY OF THE ATOM
Scientists set out to prove Dalton’s theory
 Late 1800’s J.J. Thompson’s Cathode Ray
Experiment discovered the electron.

The cathode ray tube is a tube with partially
vacuumed air.
 In the tube there are two plates


one with a positive charge the other with a negative charge
When high voltage wires are attached an electrical
charge is produced.
 Rays are produced and can be seen when they hit a
fluorescent material at the end of the tube.

THE HISTORY OF THE ATOM

THE HISTORY OF THE ATOM

What Thompson found
The streams of light traveled in straight lines
 They were independent of the cathode plate material
used
 They carried a negative electrical charge.



These negatively charged particles he called Electrons.
The mass of these particles was much less than that
of Hydrogen the lightest element.

This meant that there were smaller particles than the
atom.
THOMSON MODEL 1904

Plum pudding model
THE HISTORY OF THE ATOM

Millikan’s Oil Drop Experiment (1909)





Found the Charge of the electron
Fine droplets of oil were sprayed into a drum with
two electrical plates
The drops were allowed to fall through a small hole
into the lower portion of the drum
The air was then bombarded with energy producing
electrons
The negatively charged drops interacted with the
negatively charged plate at the bottom of the drum
As the charge on the plate was varied the drops slowed,
stopped, or reversed.
 By measuring the voltage on the plate when the drops were
stopped the charge of each electron would be found.

THE HISTORY OF THE ATOM
THE HISTORY OF THE ATOM

Rutherford’s Gold Foil Experiment (1909)
Rutherford shot alpha particles at a sheet of gold foil
thinking that the particles would just past through.
 He found that some of the particles were deflected .
 This lead him to propose that the majority of the
mass of the atom and the positive charge of the atom
were all in the center of the atom NOT spread
throughout.


There were large amounts of empty space in the atom.
THE HISTORY OF THE ATOM
RUTHERFORD MODEL, 1911
THE HISTORY OF THE ATOM

Nuclear Theory of the Atom
Most of the atom’s mass and all of its positive charge
are contained in a small core called the nucleus.
 Most of the volume of the atom is empty space,
throughout which tiny, negatively charged electrons
are dispersed.
 There are as many negatively charged electrons
outside the nucleus as there are positively charged
particles (named protons) within the nucleus, so that
the atom is electrically neutral.

THE HISTORY OF THE ATOM

There was still unaccounted mass from the atom
presented in Rutherford’s findings.

This mass was explained by James Chadwick as
neutrons.

Neutrons are electrically neutral particles that also reside
in the nucleus.
THE ATOM
ELECTRON CLOUD MODEL
BEYOND THE SUBATOMIC PARTICLES
Scientists next question was what makes up
protons, neutrons, and electrons?
 The answer- Quarks!

Found by smashing protons together to break them
apart.
 Six different types.
