Atomic Theory: A Timeline
• More than 2000 years ago
ancient philosophers had
theories on matter.
• 450-360 BC - Democritus
believed that matter was
made of small invisible
particles he named
“atoms”.
• Aristotle believed in 4
elements : Earth, Air, Fire,
and Water.
John Dalton 1803
• “Father of Modern
Atomic Theory” – he
proposed the first
useful atomic theory
of matter
John Dalton – Atomic Theory
• He believed that:
1.All Matter is made of small particles called
Atoms
2.The atoms of an element are identical in their
masses
3.Atoms of different elements have different
masses
4.Compounds are created when atoms of different
elements link together in definite proportions.
5.Atoms can neither be created or destroyed
J.J. Thompson - 1897
• Atoms were thought to be
the smallest division of
matter
• In 1897 Thompson
discovered the electron
through experiments using
cathode ray tubes.
• He concluded that electrons
are smaller than atoms, and
are one of the components
that make up atoms.
Two Videos of Thompson’s
Discovery
• http://www.youtube.com/watch?v=XU8nM
KkzbT8
• http://www.youtube.com/watch?v=IdTxGJj
A4Jw
Thompson Cont…
• His discovery led him to
propose the “plum pudding” or
“muffin model” of the atom,
• This states that an atom is
composed of negative charges
(electrons) evenly distributed
on positive matter that makes
up the rest of the atom.
Ernest Rutherford - 1911
• He discovered the
Nucleus of an Atom in
an experiment known
as the “gold foil alpha
particle experiment”.
• http://www.youtube.com/watch?v=5p
Zj0u_XMbc&feature=related
• http://www.youtube.com/watch?v=wz
ALbzTdnc8&feature=related
Rutherford Cont…
• Through this experiment he
discovered the nucleus.
• Because the Alpha particles
are positive, some went
through the atom (passing
the negative electrons).
• A small fraction either
bounced back, or bent as
they went past.
Rutherford
• He observed that this small dense area in the
centre of an Atom was positively charged and
called it the Nucleus
• The negative charges (electrons) exist around
the nucleus in a much larger space
• He also proposed the idea of a Neutron when he
noticed the weights of the atom of gold did not
equal its 79 protons and 79 electrons. There
was missing mass. He concluded this missing
mass must be neutral. James Chadwick would
be the first person to prove this in 1932.
Niels Bohr - 1913
• He proposed the
planetary model of the
atom.
Bohr
• Proposed that electrons traveled in fixed paths
around the nucleus
• The atom has a tiny dense central core called the
nucleus, where positive and neutral charges are
held
• The remainder of the atom is empty space
• Electrons travel in orbits around the nucleus.
There are up to seven different levels of orbit
within an atom.
• Electrons always travel in the lowest orbit in
which there is space
• The electrons furthest from the nucleus play a
role in chemical bonding.
Orbits and Electrons!
Level
1
2
3
4
5
6
7
How Many
Electrons
it can hold
2
8
8
32
32
18
8
Current Atomic Model
• In 1920s Werner Heisenberg and Erwin
Schrodinger argued that it is impossible to
know where an electron is at any given
time.
•The Electron Cloud Theory was developed
as a result of their discoveries in quantum
mechanics
•The electron cloud model says that we
can't know exactly where an electron is, but
the electrons are more likely to be in
specific areas of an atom.
Electron Cloud Model
• An electron tends to exist more frequently in
certain regions around the nucleus than others
depending on its energy level.
• These regions are referred as its atomic
orbital.
• Orbitals come in a
variety of shapes and
sizes: from the simple
sphere of the full helium
orbital, to the dumbbell
shape of the full neon
orbital.
Atomic Theory
http://videos.howstuffworks.com/discovery/2
9430-assignment-discovery-origin-of-theatom-theory-video.htm
• Atom – The basic building
block of all matter.
• Most of the mass is in the
central nucleus made of
protons and neutrons, and is
surrounded by a cloud of
electrons.
• An atom is classified
according to the number of
protons and neutrons in its
nucleus. For Example
Hydrogen has one proton
and has the atomic number
of 1 on the periodic table.
The Atom Song
http://www.youtube.com/watch?v=vUzTQWn-wfE
Now You TRY!
• Using Bohr’s Diagram, draw 8 models of the
atom from the first 18 elements (first three
periods) – showing how many protons are in
the nucleus, how many orbital rings it has,
and place the electrons on their respectful
rings.
• Remember – the Atomic Number equals the
number of protons and the number of
protons equals the number of electrons – EX:
Hydrogen has the Atomic Number 1 and has
1 proton and 1 electron.
• See page 562-563 for periodic table