Chapter 4.1
Atomic Models
Atomic Structure
A. Studying the structure of atoms
poses a problem due to the fact that
they are extremely small. Even with a
microscope scientists cannot see the
structure of an atom. Since the
structure of atoms cannot be seen
indirect evidence must be used to
develop models of atoms.
Ancient Greek Models
B. Ancient Greek Models of Atoms
A. Democritus believed that all matter
consisted of extremely small particles.
1. He called these particles atoms.
2. He though they could not be divided.
3. He thought there were different types of
atoms with specific sets of properties.
Ex. Atoms in liquids were round and smooth,
but the atoms in solids were rough and prickly.
Aristotle
B. Aristotle
1. Believed there was no
limit to the number of
times matter could be
divided.
2. All substances were
built up from four elements:
Earth, air, fire, and water.
3. These elements had
four qualities: hot, cold, dry,
and wet.
John Dalton
C. John Dalton
A. John Dalton was born in England in 1766. He became a
teacher and spent his spare time doing experiments.
B. Correctly concluded that a gas consists of individual
particles.
C. Gathered evidence of the existence of atoms by measuring
the masses of elements that combine when compounds form.
D. The ratio of the masses of the elements in the compound is
always the same.
E. Compounds have a fixed composition.
Ex. When magnesium burns it combines with oxygen. 100g of
magnesium combines with 65.8g of oxygen. 10g of
magnesium combines with 6.58g of oxygen.
John Dalton
D. Dalton’s Atomic Theory
A. All matter is made up of individual particles called
atoms, which cannot be divided.
1. All elements are composed of atoms.
2. All atoms of the same element have the same
mass, and atoms of different elements have
different masses.
3. Compounds contain atoms of more than one
element.
4. In a particular compound, atoms of different
elements always combine in the same way.
Dalton’s Atomic Model
B. While this theory was not
perfect it was not discarded but
revised to account for new
discoveries.
C. What did Dalton notice that all
compounds have in common?
• The ratio of masses of
elements in a compound is
always the same.
Pic: Dalton represented elements
as solid spheres. Each type of
atom is represented by tiny, solid
spheres with a different mass.
E. Joseph John Thomson (1856-1940), J.J. Thomson,
used an electric current to learn more about atoms.
A. The device used to create an electric current
was a glass tube with most of the air vacuumed out
sealed with a metal disk on each end. When the electric
current is turned on one disk becomes positively
charged while the other disk becomes negatively
charged. This created a glowing beam in the glass tube.
Thomson’s Model
B. Thomson hypothesized that the beam was a
stream of charged particles that interacted
with the air in the tube and caused the air to
glow.
C. Thomson found, in an experiment, that the
beam of charged particles was attracted to a
positively charged plate.
D. He hypothesized that the particles came
from inside atoms.
Thomson’s Model
E. Thomson’s experiments provided
the first evidence that atoms are
made of even smaller particles.
F. This changed how scientists
thought about atoms.
G. Thomson revised Dalton’s model
to account for these subatomic
particles.
Thomson’s Model
F. Thomson’s Model
A. An atom is neutral, meaning it has neither a
negative nor a positive charge. However,
Thomson knew from his experiments that
atoms contained negatively charged particles.
B. How can an atom contain negative particles
and still be neutral?
Thomson’s Model
C. Plum Pudding Model
(Chocolate Chip Ice Cream)
1. In Thomson’s model
the negative charges were
evenly scattered
throughout an atom filled
with a positively charged
mass of matter.
Thomson’s Model
2. Think of chocolate chips as negative particles
and the ice cream as a positively charged
mass of matter. With the chocolate chips
spread evenly throughout the ice cream the
charges of the chocolate chips and ice cream
balance each other out and create an overall
neutral charge.
D. What evidence did Thompson’s experiments
provide?
Atoms are made of even smaller particles.
Ernest Rutherford
G. Ernest Rutherford (1871-1937)
A. Surprised by unexpected results that lead
to important discoveries.
B. Discovered that uranium emits fast moving
particles that have a positive charge. He
named these particles alpha particles.
C. Rutherford’s experiment consisted of
sending a beam of alpha particles through a
thin sheet of gold foil.
Ernest Rutherford
E. He hypothesized, based on Thomson’s
model, that the mass and charge of the
alpha particles traveling through the gold
foil would be too small to change the
path of the particles. The particles would
travel straight through the gold foil and
strike a screen that would light up when
hit.
Ernest Rutherford
F. Some of the particles that hit the
gold foil were deflected. He found
that this hypothesis was false.
G. This led Rutherford to believe
that the positive charge of an
atom is not evenly spread
throughout the atom.
Ernest Rutherford
H. The positive charge is concentrated
in a very small, central area called
the nucleus.
I. The particles that were deflected
came close to a nucleus, while the
ones that traveled straight through it
did not.
Ernest Rutherford
J. Rutherford’s
model showed that
all of the positively
charged particles
of an atom are
concentrated in its
nucleus not spread
evenly out like
Thomson’s model.
Atomic Model Review
Scientist Evidence
Model
Dalton
Ratio of masses
Indivisible Solid
Spheres
Thomson
Deflected Beam
of charged
particles
Deflection of
alpha particles
Negative and
Positive charges
Rutherford
Nucleus