Download The Atom

The Atom
The Development of the Atomic Theory
Democritus
Greek philosopher
First to use atomos (not being able to divide)
Believed indivisible, indestructible, unchangeable
Atoms- different shapes and sizes
Determined physical properties of material
Ex: atoms of liquids were thought to be smooth, which
would allow the atoms to slide over each other.
Aristotle
Disagreed with Democritus
Believed you never end up with a single particle EVER!
Four Elements: earth, water, air, fire
So who was right?!
John Dalton
Late 1700s it was known:
Elements combine in certain proportions based on
mass to form compounds.
Dalton experiment with different gases to show why.
Experiments provided results!
John Dalton
1803
Five main points of Dalton's atomic theory:
All elements are made of atoms.
Atoms of a given element are identical in size and other
properties; atoms of different elements differ in all these
properties.
Atoms cannot be subdivided, created, or destroyed.
Atoms of different elements combine in simple wholenumber ratios to form chemical compounds.
In chemical reactions, atoms are combined separated, or
rearranged.
What do we know to be wrong with this theory today?
J.J. Thomson
1897
Discovered small particles inside the atom:
ELECTRONS! (negatively charged particles)
Experimented with a cathode-ray tube.
Showed streams of negatively charged particles
Model was known as the “plum pudding” model
Electrons located throughout an atom.
Ernest Rutherford
1909
Student of Thomson
Created an experiment to study the parts of an atom
Aimed a beam of positively charged particles at a thin sheet
of gold that had a special coating behind it.
Coating glowed when positively charged particles hit it.
Ernest Rutherford
1911
Revision of the atomic theory.
Atoms mostly dead space, with a tiny part of highly
dense matter.
Center of the atom contained a positively charged part
(proton) and neutrons (neutral charged particles)
NUCLEUS (atom’s central region made up of protons and
neutrons)
100,000 times smaller than the diameter of the atom
Electrons surround the nucleus at a distance.
Niels Bohr
1913
Worked with Rutherford
Studied atoms reaction to light
Proposed electrons move around nucleus in certain paths,
AKA energy levels.
Electrons could jump from a path in one level to another
level.
Modern Atomic Theory
Electrons do not travel in definite paths
Can not be predicted!
Regions inside the atom where electrons are likely to
be found – the electron cloud!
Nucleus contains both protons (+) and neutrons
(neutral charge)