Download Development of Atomic Theory

Development of Atomic
Theory
Democritus
• GREECE around 400 b.c. Democritus said:
– 47 The material cause of all things that exist is the coming
together of atoms and void. Atoms are too small to be
perceived by the senses. They are eternal and have many
different shapes, and they can cluster together to create
things that are perceivable. Differences in shape,
arrangement, and position of atoms produce different things.
By aggregation they provide bulky objects that we can
perceive with our sight and other senses.
– 48 We see changes in things because of the rearrangement
of atoms, but atoms themselves are eternal. Words such as
‘nothing’, ‘the void’, and ‘the infinite’ describe space.
Individual atoms are describable as ‘not nothing’, ‘being’,
and ‘the compact’. There is no void in atoms, so they cannot
be divided. I hold the same view as Leucippus regarding
atoms and space: atoms are always in motion in space.
Adapted from The Presocratic Philosophers, by G. S. Kirk
and J. E. Raven. The University Press, Cambridge,
England, 1962. Selections from Early Greek Philosophy
by Milton C. Nahm.
War of the Minds
• Democritus said:
• If you break a piece of
matter in half, then in
half again, and again,
how many times do you
have to break it until it
can’t be broken
anymore?
• There is a stopping
point of matter, that is
the atom.
• Artistotle said:
• There are only four
elements: earth, wind,
fire, and water.
• NO ATOMS, Matter
cannot be separated.
• I WIN!!!!
Jump ahead to the 1800’s
• John Dalton (17641844)
– Hey, that Democritus
guy was on to
something.
Dalton’s Atomic Theory
Elements are composed of atoms
– tiny, hard, unbreakable, spheres
All atoms of a given element are identical
– all carbon atoms have the same chemical and
physical properties
Atoms of a given element are different from
those of any other element
– carbon atoms have different chemical and physical
properties than sulfur atoms
Dalton’s Atomic Theory Cont.
Atoms of one element combine with atoms of
other elements to form compounds.
– Law of Constant Composition
• all samples of a compound contain the same proportions
(by mass) of the elements
– Chemical Formulas
Atoms are indivisible in a chemical process.
– all atoms present at beginning are present at the
end
– atoms are not created or destroyed, just
rearranged
– atoms of one element cannot change into atoms
of another element
• cannot turn Lead into Gold by a chemical reaction
Unbreakable? Really?
•if atoms have pieces,
they must be breakable
•Thomson also found that
atoms of different
elements all produced
these same electrons
• J.J. Thomson
investigated a beam
called a cathode ray
• he determined that the
ray was made of tiny
negatively charged
particles we call
electrons
• electrons were smaller
than a hydrogen atom
• if electrons are smaller
than atoms, they must
be pieces of atoms
Cathode Ray
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mmm…pudding
Plum Pudding Model
 Atom breakable!!
 Atom has structure
 Electrons suspended in a
positively charged electric
field
– must have positive
charge to balance
negative charge of
electrons and make the
atom neutral
 mass of atom due to
electrons
 atom mostly “empty” space
– compared size of electron to
size of atom
OK, Prove it.
• How can you prove something is empty?
• put something through it
– use large target atoms
• use very thin sheets of target so do not absorb “bullet”
– use very small particle as bullet with very high
energy
• but not so small that electrons will affect it
• bullet = alpha particles, target atoms = gold
foil
–  particles have a mass of 4 amu & charge of +2
c.u.
– gold has a mass of 197 amu & is very malleable
The Gold Foil Experiment
• Experiment performed by Ernest Rutherford
• “It was a result similar to shooting a gun at a
piece of paper and have the bullet bounce back.”
What’s Happening
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The atom contains a tiny dense center called
the nucleus
– the volume is about 1/10 trillionth the
volume of the atom
The nucleus is essentially the entire mass of
the atom
The nucleus is positively charged
– the amount of positive charge of the
nucleus balances the negative charge of
the electrons
The electrons move around in the empty
space of the atom surrounding the nucleus
(responsible for chemical reactions)
The Nuclear Atom
All about the nucleus
• The nucleus was found to be composed of
two kinds of particles
• Some of these particles are called protons
– charge = +1
– mass is about the same as a hydrogen atom
• Since protons and electrons have the same
amount of charge, for the atom to be neutral
there must be equal numbers of protons and
electrons
• The other particle is called a neutron
– has no charge
– has a mass slightly more than a proton
That’s all for now…
• Modern Atomic Theory
• Big Questions Remain:
• How do the electrons
travel in the atom?
• Why do the electrons
stay out of the nucleus?
• If all atoms are just a
combination of protons,
neutrons, and electrons,
what makes them
different from each
other?