Download Structure of the Atom - Models

Structure of the Atom Models
Chapter 19 Section 1 – Part 2
Pages 580-583
Models – Tools for Scientists
Models are used to represent things that
are difficult to visualize / see, or picture in
the mind.
Models must accurately represent all
information known about what is being
modeled.
The Changing Atomic Model
Figure 5, page 582
- Models of the atom have changed through time.
1. Democritus (~400 B.C.)  Greek philosopher
 Three main ideas about matter
a. Universe made of invisible units, called them “atomos”
 Atomos – means uncuttable
b. Movements of these atoms caused the
observed changes in matter.
c. Theory explains some of his observations
- Lacked evidence
The Changing Atomic Model
Figure 5, page 582
2. In the 1700’s emphasis was placed on
careful and repeated measurements.
The Changing Atomic Model
Figure 5, page 582
3. John Dalton (1808) –
English school teacher
- Widely accepted atomic theory
due to supporting evidence.
- 3 main points
a. Every element is made of
tiny unique particles
(atoms) that cannot be
divided.
b. Atoms of the same
element are exactly alike.
c. Atoms of different
elements can join to form
molecules.
The Changing Atomic Model
Figure 5, page 582
4. Joseph John Thomson (1904) – English physicist
- Inferred by experimentation that atoms
contain negatively charged particles
 electrons
- Thought electrons were
evenly embedded in
positively charged sphere.
 Like chocolate chip
cookie dough
The Changing Atomic Model
Figure 5, page 582
5. Ernest Rutherford (1911) – British physicist
- Proposed most of an atom’s mass (all the
positive charge)
is in the nucleus
- Nucleus surrounded
by electrons
- “Gold Foil” experiment
The Changing Atomic Model
Figure 5, page 582
6. Neils Bohr (1913) – Danish
physicist
- Proposed that
electrons orbit
nucleus in fixed
paths
- Like planets
orbiting the sun
The Changing Atomic Model
Figure 5, page 582
7. James Chadwick (1913) – Rutherford’s student
- Nucleus contains neutral neutrons and positive
protons
The Changing Atomic Model
Figure 5, page 582
8. Electron Cloud model – (1926) the current model
- Electrons do not follow fixed orbits
- Tend to occur more frequently in certain
areas around nucleus
 Looks like a fan blade spinning
- This model basically describes a cloud-like region
where the electron is likely to be found.
Electron Cloud Model / Modern Atomic Model
- Impossible to accurately locate an
electron at any given time.