The Development of Modern The Development of Modern Atomic TheoryAtomic Theory

Viewing the

Invisible

V. Anderson 2005—Mergenthaler Voc. Tech. High School

Early Ideas!!Early Ideas!!

Early man, including the philosophers of Greece, believed that all matter was made of 4 basic “elements”

– Earth

– Water

– Air

– Fire

New IdeasNew Ideas

As time progressed, a number of individuals contributed to the idea of the modern atom.

DemocritusDemocritus

Lived: ancient GreeceBelieved: “According to convention, there

is a sweet and a bitter, a hot and a cold, and according to convention there is color. In truth there are atoms and a void.”

Coined the term Atom

John DaltonJohn Dalton

Born: 1766, Cumberland, EnglandOccupation: TeacherBelieved: “We might as well attempt

to introduce a new planet into the solar system, or annihilate one already in existence, as to create or destroy a particle of Hydrogen.” --J. Dalton, A New System of

Chemical Philosophy, 1808

Key Points of Dalton’s Atomic Key Points of Dalton’s Atomic TheoryTheory

Dalton's atomic theory rests on the following postulates.

1. All matter consists of tiny particles.(atoms)

2. Atoms are indestructible and unchangeable.

3. Elements are characterized by the mass of their atoms. All atoms of the same element are identical and different from atoms of other elements.

JJ ThomsonJJ Thomson

Born: 1856Place: Manchester,

EnglandDiscovered ElectronsPlum pudding Model

– A unit made of multiple pieces

Plum Pudding ModelPlum Pudding Model“I see the atom as being made of electricity. I call my picture the “plum pudding” model of an atom. Note that in this model negatively charged particle called electrons(the plums) are embedded in a sphere of positive charge(the pudding).”

Thomson’s ExperimentThomson’s ExperimentThomson used a Vacuum tube (Cathode

Ray Tube) to demonstrate that the produced ray was made of charged particles.

The ray that was produced in the tube would bend toward a Positive Magnet and away from a Negative magnet, proving that the particles were Negatively charged.– ELECTRONS!!!!

Thomson’s Experiment - Thomson’s Experiment - DiagramDiagram

Cathode Ray TubeCathode Ray Tube

It may seem mundane and boring to us today, but to audiences at popular scientific lectures, the glow in this simple cathode tube was eerie, and to physicists it presented a challenging mystery.

Ernest Rutherford’s AtomErnest Rutherford’s Atom

“I suggest that the atom has the following characteristics:”

A small core, or nucleus, that contains most of the mass of the atom.

The nucleus is made of particles called protons

The protons are surrounded by negatively charged electrons, but most of the atom is actually empty space.

Ernest Rutherford and the Ernest Rutherford and the Structure of the AtomStructure of the Atom

Rutherford’s observationsRutherford’s observations

Most particles passed through the gold foil without any effect.

A few particles passed through but were deflected to the side.

A few particles bounced back toward the source

He described these “bouncers” as cannon balls deflected by tissue paper.

Neils BohrNeils Bohr

1913 Neils Bohr, a Danish scientists, developed a theory:– Electrons travel around

the nucleus in orbits like those of the planets around the sun

– “The Bohr Model”

Development of the Modern Development of the Modern Atomic TheoryAtomic Theory

Solidparticle

ElectronsIn planetary Orbit around Nucleus

Nucleus Surrounded byA cloud of electronsIn energy levels