1. 1
DemocritusDemocritus
 Around 410 BC,Around 410 BC,
originated the idea of theoriginated the idea of the
atom.atom.
 400 B.C. -400 B.C. - Everything inEverything in
the world is made fromthe world is made from
tiny particles that cannottiny particles that cannot
be cut.be cut.
 atoms are indestructible.atoms are indestructible.

2. 2
John DaltonJohn Dalton
 1803, Proposed1803, Proposed
New AtomicNew Atomic
TheoryTheory

3. 3
Dalton’s Atomic TheoryDalton’s Atomic Theory
 1.1. All matter consists of tiny particles.All matter consists of tiny particles.
 2.2. Atoms are indestructible andAtoms are indestructible and
unchangeable.unchangeable.
 Atoms of an element cannot be created, destroyed, broken intoAtoms of an element cannot be created, destroyed, broken into
smaller parts or transformed into atoms of another element.smaller parts or transformed into atoms of another element.
 3.3. Elements are characterized by theElements are characterized by the
mass of their atoms.mass of their atoms.
 All atoms of the same element have identical weights, DaltonAll atoms of the same element have identical weights, Dalton
asserted. Atoms of different elements have different weights.asserted. Atoms of different elements have different weights.
 With the discovery of isotopes, however, the postulate wasWith the discovery of isotopes, however, the postulate was
amended to read, "Elements are characterized by their atomicamended to read, "Elements are characterized by their atomic
number".number".
 4.4. When elements react, their atomsWhen elements react, their atoms
combine in simple,combine in simple, whole-number ratios.whole-number ratios.
 effectively explained both the law of definite proportions and theeffectively explained both the law of definite proportions and the
law of multiple proportions.law of multiple proportions.

4. John Dalton (1800)John Dalton (1800)
Link
Law of definite Proportions: chemical
compound contains fixed and
constant proportions (by weight) of its
constituent elements.

5. 5
John Joseph ThomsonJohn Joseph Thomson
 In 1897, discovered theIn 1897, discovered the
electronelectron
 Experiment: high-vacuumExperiment: high-vacuum
cathode-ray tubecathode-ray tube
 1904, Calculated Charge1904, Calculated Charge
to Mass Ratioto Mass Ratio

6. 6
Cathode Ray-TubeCathode Ray-Tube
 C: Cathode (Rays introduced)C: Cathode (Rays introduced)
 A:A: AnodeAnode
 Deflected: When passed through electrical potentialDeflected: When passed through electrical potential

7. 7
Plum Pudding Model

8. Ernest RutherfordErnest Rutherford
1. Most particles passed through
• atoms mostly empty!
2. Some positive α-particles deflected or
bounced back!
• Thus, a nucleus holds most of an atom’s
mass.
 Rutherford shot alpha (Rutherford shot alpha (αα) particles at gold foil.) particles at gold foil.
 FOUND THEFOUND THE NUCLEUSNUCLEUS!!

10. 10
Rutherford’s Results:Rutherford’s Results:
 Observations:Observations:
 99% went through gold foil99% went through gold foil
 1% were deflected straight back1% were deflected straight back
 Rutherford: “It was . . . As if you had firedRutherford: “It was . . . As if you had fired
a 15-inch shell at a piece of tissue papera 15-inch shell at a piece of tissue paper
and it came back and hit you”and it came back and hit you”
 Atoms made of:Atoms made of:
 Dense, Positive NucleusDense, Positive Nucleus
 A lot of SpaceA lot of Space

11. 11
James ChadwickJames Chadwick
 Born in England:Born in England:
(1891 – 1974)(1891 – 1974)
 1932, discovered the1932, discovered the
neutronneutron
 Many Honors:Many Honors:
 1935: Nobel Prize for1935: Nobel Prize for
Physics.Physics.
 1945: Knighted1945: Knighted