Download THE HISTORY OF THE ATOM Table of Contents Black Boxes

THE HISTORY OF THE ATOM
Table of Contents
 Black Boxes
 Democritus
 Dalton’s Theory
 Thomson’s Electron
 Rutherford’s Nucleus
 Goldstein’s Protons
 Chadwick’s Neutron
 Atomic Symbols
Black Boxes
Black Boxes are anything that you cannot
see inside
Like a cell phone…
Or a locked box…
Or an atom…
The Beginning of the Atom
The ancient Greeks (back around 500BC) believed there only four elements:
Earth
Air
Water
Fire
Democritus (460-370 B.C.)
 He also lived in ancient Greece but he believed differently He said
that all matter was made up of particles that could not be divided
into smaller particles
 He called these particles – Atoms
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Aristotle (384-322 B.C.)
 Lived at the same time as Democritus but was more popular.
 People agreed with Aristotle and not Democritus.
Alchemists (for over 1600 years)
 The oldest “Chemists”.
 Attempted to change common metals into gold.
 Created symbols for elements.
 Identified elements based on Democritus' theory.
The Atomic Theory
Democritus’ idea of the atom was largely ignored until an English schoolteacher did some
experiments over 2000 years later, he was…
John Dalton (1766-1804)
“The Father of Modern Chemistry”
Leading to his atomic theory…
Dalton’s Atomic Theory
 All matter is made up of atoms
 Atoms are indestructible and cannot be divided into smaller particles (Atoms are indivisible)
 All atoms of one element are exactly alike, but they are different from atoms of other
elements
 A given compound always has the same relative numbers and kinds of atoms. Atoms join in
whole number ratios.
 Atoms are neither created nor destroyed in any chemical reaction.
Dalton’s Model of the Atom
Bowling Ball Theory:
He believed the atom was a solid sphere
An analogy to his atom would be…
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Thomson’s Cathode Ray Tube Experiments and the Discovery of
the Electron
JJ Thomson (1856-1940) used the cathode ray tube to prove that the atom was
made up of electrons
 But, what was that green light?
 Was it a light?
 Was it a particle?
 To test this, he brought a magnet close to
the cathode ray tube to see what would
happen.
 So, the magnet caused the cathode ray to move. What does that tell us?
 Would a magnet affect a light from a flashlight? (you could try this at home)
 Probably not.
 Therefore, the cathode ray must be a particle!
Thomson also noticed… That the cathode ray was coming out of the negative end (cathode) of the
tube and going toward the positive end (anode)
Negative end
Positive end
Therefore, because opposites attract, he concluded that the cathode ray must be negative
He called these particles…
ELECTRONS!
The discovery would alter Dalton’s model of the atom because now there
is something inside it
But, in addition to the negatively charged electrons, there must be
something giving it a positive charge because the overall charge of the
atom is neutral (not negative)
Let’s look at Thomson’s model of the atom… Thomson’s Plum
Pudding Model of the Atom
He believed the atom was made of positively charged stuff with
negatively charged particles scattered throughout. Why the “plum
pudding” model? What is “plum pudding”?
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Plum Pudding is an English dish sort of like bread pudding with raisins in it.
Rutherford and the Nucleus
In 1908, Ernest Rutherford performed the Gold Foil Experiment.
In it, he shot alpha particles (very small, very dense, very fast particles) at a
thin layer of gold foil.
 He expected all of the alpha particles to go straight through
 It would be like if you were shooting bullets at a cake…all of the bullets (or alpha particles)
would go straight through the cake (or gold foil atoms)
gold foil
detector screen
alpha particles
Alpha particles source
Thomson’s Atom
Alpha particles
But, what he found was this…
or
Did you notice how most of the alpha particles went straight through but a very tiny amount were deflected at odd angles?
That could only happen if there was something very tiny in the atom that
was dense enough to deflect the alpha particles. Like this…
To recap the Gold Foil Experiment
Rutherford proved that an atom:

was mostly empty space

with a very small, very dense, positively charged nucleus in it.
Rutherford’s Planetary Model of the Atom
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He believed the atom was made of a tiny positively charged nucleus with negatively charged particles orbiting it
Or…
If the atom were Soldier Field, the nucleus would be a pea on the fifty – yard line
Other important discoveries…
In 1886, Goldstein discovered the proton (which is located in the nucleus)
In 1932, Chadwick discovered the neutron (also located in the nucleus)
Bohr’s Model of the Atom

Modern Theory of the Atom.

Orbits in quantum levels.
(1920)
Although the diagram suggests that electrons travel in circular orbits, this is a simplification and is not
actually the case.
Feeling overwhelmed?
"Teacher, may I be excused? My brain is full."
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
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