Transcript Models of the Atom a Historical Perspective

Models of the Atom
a Historical Perspective
Early Greek Theories
Democritus
• 400 B.C. - Democritus thought matter
could not be divided indefinitely.
• This led to the idea of atoms in a void.
fire
earth
air
water
• 350 B.C - Aristotle modified an earlier
theory that matter was made of four
“elements”: earth, fire, water, air.
Aristotle
Atomic Theory
The Beginning
Ancient Greece – 440 BCE
Who was right??
• Democritus
– atomos
• “not able to be divided”
– Thought that atoms
were different shapes
and sizes of a single
substance.
• Aristotle
– Disagreed with
Democritus
– Believed matter can be
infinitely divided
– Had great influence
•Aristotle was wrong. However, his theory
persisted for 2000 years.
Atomic Theory
Late 1700’s - England
• John Dalton
– Wanted to know WHY!
– 1803 – Dalton’s Atomic
Theory suggested that
elements are made up
of single atoms.
Dalton’s Atomic Theory
1803 - England
• Three main ideas of Dalton’s Atomic Theory
– All substances are made of atoms which cannot be
created, divided, or destroyed.
– Atoms of the same element are exactly alike and
atoms of different elements are different.
– Atoms join with other atoms to make new
substances.
One of the main ideas of Dalton’s Atomic
Theory stated that atoms join with other
atoms to create new substances.
What do we now call these substances?
A.
B.
C.
D.
Elements
Mixtures
Compounds
Solutions
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).
Atomic Theory
Science never rests!
• While most scientists agreed that Dalton’s
theory was mostly correct, new information
was being discovered which forced atomic
theory to change.
– Four major discoveries
•
•
•
•
J.J. Thomson
Ernest Rutherford
Niels Bohr
Erwin Schrodinger and Werner Heisenberg
Atomic Theory
• What’s missing???
Adding Electrons to the Model
Materials, when rubbed, can develop a charge
difference. This electricity is called “cathode rays”
when passed through an evacuated tube (demos).
These rays have a small mass and are negative.
Thompson noted that these negative subatomic
particles were a fundamental part of all atoms.
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.
Atomic Theory
1897 - England
• J.J. Thomson
– Discovered that there were
small particles inside the
atom.
• Importance: Meant that
atoms were divisible
– How did he discover this?
Atomic Theory
J.J. Thomson’s Discovery of the Electron
• Cathode-ray tube experiment
Atomic Theory
J.J. Thomson’s Discovery of the Electron
• The Plum Pudding Model
Atomic Theory
• J.J. Thomson’s discoveries caused a change in
the view of the atom proposed by Dalton.
– What was the change?
• All substances are made of atoms which
cannot be created, divided, or destroyed.
• Atoms of the same element are exactly alike
and atoms of different elements are different.
• Atoms join with other atoms to make new
substances.
J.J. Thomson’s experiment caused one of
the main ideas of Dalton’s Atomic Theory
to be changed.
Which idea was no longer valid?
A. All substances are made of atoms which
could not be created, divided, or destroyed.
B. Atoms of the same element are exactly alike
and atoms of different elements are different.
C. Atoms join with other atoms to make
new substances.
Atomic Theory
Ernest Rutherford – 1909 England
• Student of Thomson’s
– Designed an experiment
to test Thomson’s Plum
Pudding Model
– Made a surprising
discovery.
Ernest Rutherford (movie: 10 min.)
• Rutherford shot alpha () particles at gold foil.
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive
substance
path of invisible
-particles
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.
Rutherford’s experiment changed the view
of the atom once again.
Which change to the atom did Rutherford’s experiment force?
A. Most of the mass of an atom is contained in
the electrons.
B. Atoms contain very little empty space.
C. Most of the mass of the atom is
contained in the dense nucleus.
Atomic Theory
Niels Bohr – 1913 Denmark
• Associate of Rutherford
• Developed new model
of the atom by studying
how atoms react to
light.
Atomic Theory
Niels Bohr – 1913 Denmark
• Like rungs of a ladder.
• Electrons can only exist
at specific energy levels
and not in between.
• Electrons can jump from
one level to another.
Bohr’s model
• Electrons orbit the nucleus in “shells”
• Electrons can be bumped up to a higher
shell if hit by an electron or a photon of light.
There are 2 types of spectra: continuous spectra &
line spectra. It’s when electrons fall back down that
they release a photon. These jumps down from
“shell” to “shell” account for the line spectra seen in
gas discharge tubes (through spectroscopes).
Atomic numbers, Mass numbers
• There are 3 types of subatomic particles. We
already know about electrons (e–) & protons (p+).
• Neutrons (n0) were also shown to exist (1930s).
• They have: no charge, a mass similar to protons
• Elements are often symbolized with their mass
number and atomic number
E.g. Oxygen:
O
8
16
These values are given on the periodic table.
For now, round the mass # to a whole number.
These numbers tell you a lot about atoms.
# of protons = # of electrons = atomic number
# of neutrons = mass number – atomic number
Calculate # of e–, n0, p+ for Ca, Ar, and Br.
Atomic
Mass
p+
n0
e–
Ca
20
40
20
20
20
Ar
18
40
18
22
18
Br
35
80
35
45
35
Modern Atomic Theory
• Erwin Schrödinger
– Austrian physicist
• Werner Heisenberg
– German physicist
Modern Atomic Theory
• Electron clouds: regions in an atom where
electrons are likely to be found.
Atomic Theory Summary
• 440 BCE: Democritus – first describes atoms
• 1803: Dalton – proposes first model of the atom and
three principles
• 1897: Thomson – cathode ray tube experiment
discovers electrons
• 1909: Rutherford – gold foil experiment discovers
positively charged, dense nucleus of atom
• 1911: Rutherford – offers revised model of the atom
• 1913: Bohr – proposed the idea of energy levels
• 1927: Schrödinger/Heisenberg – modern cloud
model