Download History of the Atom File

History of
the Atom
Physical Science
Mr. Willis
Elements
 Element
- substance made up of
only one kind of atom
cannot be broken down by a chemical
change
 Represented by symbols such as N
(nitrogen)

The Model of the Atom has Changed
 Atom
- smallest particle of an
element w/the properties of
that element
 All matter is composed of
atoms
Democritus – 420 B.C.
 Cheese
thought
experiment

Eventually, matter
cannot be divided
any further. This
unit is called the
atom
Aristotle – 360 B.C.
 Matter
is
continuous;
can always be
divided
John Dalton – (1766-1844)

Revived & added to
Democritus’ theory
Solid spheres

Composed atomic theory

(next slide)
The Atomic Theory
1. All elements are composed of atoms, which are
indivisible and indestructible.
2.The atoms of an element are identical in their
masses. (This was proven false with the
discovery of Isotopes)
3. Atoms of different elements have different
masses
4. Compounds are formed by joining the atoms of
two or more elements in definite whole-number
ratios (e.g. 1:1, 1:2, 3:2, ...)
JJ Thomson - 1897
Plum-pudding model Ball of + charge w/charges stuck inside it
 Used cathode-ray tube
to discover electrons
& create theory

Lord Ernest Rutherford - 1911

Gold foil
experiment
 Solar
system model
dense, positively charged center
called nucleus
 Most of the atom is
empty space with
electron particles
orbiting the nucleus

Neils Bohr - 1913
Thought his theory solved
light-emission problem
 Electrons orbit nucleus in
certain energy levels



Electricity excites electrons &
they jump to higher energy level.
They then return to original
energy level, giving off excess
energy as heat and/or light
Only works with Hydrogen
Quantum Mechanical Model – 1920s
Electrons behave not only like a particle,
but also like a wave.
 Electron energy levels are made up of
sub-orbital levels
 The speed and location of an electron in
an atom cannot both be precisely known
so electrons are in all places at all times


This is called the Heisenberg Uncertainty
Principle

Today’s model:
 positive nucleus made of protons &
neutrons surrounded by cloud of
negatively charged particles called
electrons
 nucleus contains nearly all of the mass
 nucleus is extremely small
This picture is quite distorted. If we drew the atom to scale and made protons and neutrons a
centimeter in diameter, then the electrons and quarks would be less than the diameter of a hair
and the entire atom's diameter would be greater than the length of thirty football fields!
99.999999999999% of an atom's volume is just empty space!