Download PowerPoint - Models of the Atom

Models of the Atom
a Historical Perspective
In order to understand our
modern concept of the atom…
…we must first have a basic understanding of
how that concept evolved. It took 2500 years
to get to where we are now, but the pace has
really heated up in the last century!
Early Greek Theories
• 400 B.C. - Democritus thought matter
could not be divided indefinitely –
there must be a smallest piece.
Democritus
• This led to the idea of atoms in a void.
• He called his theoretical smallest possible
piece atomos, meaning “uncuttable” or
“can’t be divided.
Early Greek Theories
fire
earth
Aristotle
air
water
• , 350 B.C. – (50 years later) - Aristotle
modified an earlier theory that matter
was made of four “elements”: earth, fire,
water, air.
• Aristotle was wrong! However, his
theory persisted for over 2000 years!
John Dalton
• 1800 -Dalton proposed a modern atomic model
based on experimentation not on pure reason.
•
•
•
•
All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine
in constant ratios to form compounds.
• Atoms are rearranged in reactions.
• His ideas account for the law of conservation of
mass (atoms are neither created nor destroyed)
and the law of constant composition (elements
combine in fixed ratios).
Adding Electrons to the Model
1) Dalton’s “Billiard ball” model (1800-1900)
Atoms are solid and indivisible.
2) Thompson “Plum pudding” model (1900)
Negative electrons in a positive framework.
3) The Rutherford model (around 1910)
Atoms are mostly empty space.
Negative electrons orbit a positive nucleus.
Millikan’s Oil Drop Experiment
1909 – Robert Millikan
devised an experiment
that measured the
electric charge on tiny
oil droplets. Based on
the mass of the droplets
he was able to calculate
the charge on individual
electrons.
Ernest Rutherford’s
Gold Foil Experiment
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive
substance path of invisible
-particles
• Rutherford shot alpha () particles at
gold foil.
Most particles passed through. So, atoms
are mostly empty.
Some positive -particles deflected or
bounced back!
Thus, a “nucleus” is positive & holds most
of an atom’s mass.
Neils Bohr
and The Bohr Model
In 1913 , Danish physicist Neils Bohr, working for
Rutherford, came up with the theory that the electrons
orbit the nucleus in “shells”, or very specific energy
levels. This turned out to be a great leap forward in
our understanding of atomic structure.
This is the Bohr Diagram of Beryllium
Be
4 p+
5
n°
Lewis “electron dot” Diagrams
In 1916, Gilbert Lewis used a simplified version
of the Bohr diagram to draw and describe the
ways that atoms bond to form molecules.
This is the Lewis diagram of Beryllium
Be
Good News!
That’s all the history that we’re going to cover
right now. For the next few days, we’ll learn
and practice how to draw Bohr diagrams, and
then Lewis (electron dot) diagrams using the
information found on the periodic table of the
elements. After we’ve mastered that, we’ll be
ready to explore chemical bonding.