Download History Atomic Theory

History of the Atomic Theory
Objectives
• Understand the “evolution” of
atomic theory.
• Discuss how old accepted
scientific thought gives way to
new ideas.
• New technology leads the way
to new ideas.
Democritus
• 400 BC Philosopher
• Matter could not be infinitely
divided.
• A smallest unit would be
discovered.
• This atomos would be the smallest
possible piece of matter.
300 BC Egyptian alchemists...
•believed all metals on earth sought to
become more and more perfect, thus
gradually
changing to
gold. This
really
caught on
by 100 AD
13th & 14th c. Alchemists sought
To transmute metals to gold
Atomic Theory
• John Dalton - Atomic Theory
– All matter is composed of indivisible atoms
– All atoms of a given element are identical
– Atoms of different elements physically mix,
or chemically combine in simple, whole
number ratios to form compounds
– Reactions occur when atoms combine,
separate, or rearrange
Sub-Atomic Particles
• It turns out, atoms are divisible.
• J.J. Thomson in 1897 investigating electricity with his
cathode ray tube, discovered negatively charged electrons.
• He tested many elements in his Cathode ray tube, and found
that all cathode rays had the same charge and mass. He
reasoned that all elements have these particles.
• Since atoms are normally neutral, Thomson proposed the
electrons where embedded in a positively charged ball
The Cathode Ray Experiment
Plum-Pudding Model
Electrons
Positive ball
This model didn’t last long…..
Characteristics of the electron
• Robert Millikan conducted his famous “Oil
Drop” experiment to determine the charge
to mass ratio of an electron.
• He found that the charge is equal and
opposite to a proton. Defined as 1• The mass is much smaller than a proton and
equal to 1/1840 of a proton.
Ernest Rutherford
• Experiments with radio active elements
such as radium and uranium showed that
these emit high energy particles called alpha
particles.
• Alpha particles were known to have a
positive charge.
• They could be used as projectiles to fire at
atoms to probe their composition.
Ernest Rutherford’s Experiment
Rutherford’s Model
Rutherford Concluded...
• There was a localized
positive charge (nucleus)
• Nucleus must have a large mass
• Lots of empty space
• Protons are + charged particle
Imagine a marble on the 50 yd line….If the entire
atom is the stadium, then the marble is the size of the
nucleus
Atomic Mass
• Although atoms are very small,
they have measurable mass.
• With Rutherford’s proton and
electron model, there was still
unaccounted for mass!?!
Irene Joliot-Curie & James Chadwick
accounted for the extra mass in a
particle called the neutron which did
not have a charge.
Neutrons are located
in the nucleus with
the protons.
What does an atom look like?
• It’s agreed that the protons and
neutrons make up a nucleus.
• Rutherford suggested electrons
orbit the nucleus like planets orbit
the sun.
• But orbits
would decay
Quantum Theory states...
• Electrons are in orbitals (cloudlike) which are where you can
expect to find electrons
The Atom, finally...
Nucleus
protons
neutrons
Electrons
in orbitals
(cloud like)
http://www.youtube.com/watch?v=vUzTQWn-wfEatom song