Download Lesson 27 History of Atomic Therory

Lesson 15 History of Atomic
Theory
Objectives:
* The student will list and explain the five points in
Dalton’s atomic theory.
* The student will describe and reenact
Rutherford’s Gold Foil Experiment, and explain his
theory of an atom.
* The student will describe Neils Bohr’s atomic
theory.
History of Atomic Theory Virtual
Lab 1 video
I. Dalton’s Atomic Theory
a.
b.
John Dalton looked over knowledge that had been accumulated before
his time, and write five basic principles dealing with matter.
Democritius (460-370 BC) was the first to propose the idea of “atoms” .
This theory was rejected by Aristotle (384-322) and lay dormant for
2000 years
i. All matter is made of indivisible and
indestructible atoms
ii. Atoms of a given element are identical in their
physical and chemical properties
iii. Atoms of different elements have different
physical and chemical properties
iv. Atoms of different elements combine in
simple, whole-number ratios to form
compounds – also known as
the Law of Multiple Proportions
v. Atoms cannot be subdivided, created, or destroyed when they are
combined, separated, or rearranged in chemical reactions –
also known as the Law of Conservation of Mass
c. Dalton’s Atomic Theory Stated:
i.
All matter is made of indivisible and
indestructible atoms
ii. Atoms of a given element are identical in their
physical
and chemical properties
iii. Atoms of different elements have different
physical and chemical properties
iv. Atoms of different elements combine in
simple, whole-number ratios to form
compounds – also known as
the Law of Multiple Proportions
v. Atoms cannot be subdivided, created, or
destroyed when they are combined,
separated, or rearranged in chemical reactions
– also known as the Law of Conservation of
Mass
d. Dalton’s theory of the atom makes it out
to be something similar to a marble.
II. Finding the Structure of the Atom
a.
Electrons are negatively charged particles that have a
small mass
i. Cathode ray tubes (CRT) are used in
televisions and other devices.
ii. Small particles travel from the cathode to
the anode.
1. Cathode – a negative electrode through which
current flows
2. Anode – a positive electrode through which
current flows
iii. J.J. Thompson discovered that electrically
charged plates could deflect these beams of
particles. He determined they were
negatively charged.
iv. He was able to determine the mass of what
is now called the electron, identifying the
first subatomic particle, disproving
Dalton’s Theory
v. Robert Millikan was then able to determine
the exact charge of this electron.
Thompson's Experiment Virtual Lab
1 video
Cathode Ray Tube Drawing:
V.
His model of the atom was called the Plum
Pudding model.
1. He is given credit for the discovery of these
negatively charged electrons embedded in a
ball of positive charge.
2. Resembles “plum pudding”, a bread with fruit
pieces embedded in it.
3. Draw the “Plum Pudding Model” in this space:
b.
Each atom has a positively charged inner core
i. Ernest Rutherford did an experiment known as the
gold foil experiment.
1.
He fired tiny positively charged particles (alpha
particles) at a very thin piece of gold foil, and
determined where the electrons ended up going.
2.
He found that most passed straight through, but
some were deflected anywhere from a little to almost
straight back
3.
He determined from this that the atoms were
mostly empty space and had a positively charged
“core”, and it was named the nucleus.
4.
Draw the setup of the Gold Foil Experiment in this space:
The majority of the particles passed
through with an unaltered path.
But a few alpha particles had their
pathway drastically changed
Rutherford’s Experiment Virtual
Lab 1 video
electrons
i. His model of the atom is similar to that of the solar system, with
traveling around the nucleus in well-defined paths.
ii.
Draw a Rutherford “solar system model” in this space:
LAB ACTIVITY
• SUBATOMIC BOWLING!
Group
TOTALS
Total Trials
Number of HITS
Conclusions from Rutherford’s
Experiment
•
•
•
•
Most of the atom is empty space
Atoms have a solid core called the nucleus
Nucleus is positively charged
They measured the approximate size of the
nucleus
• ADD THIS!
c.
Electrons occupy energy levels within an atom
i.
Neils Bohr came up with the idea that electrons would be found only
in specific energy levels, similar to the rungs on a ladder
ii.
The energy levels closer to the nucleus have lower energy than those
farther away.
iii.
The difference in energy between any one level and the next is
called a quantum.
iv.
Electrons can only be found in those energy levels, never in between.
They are “quantized”
v.
Bohr’s model of the atom can be compared to a stepladder
d.
Neutrons add mass to the nucleus
i. It was found that the entire mass of the nucleus couldn’t be
accounted for with just protons
ii. Neutrons were discovered by James Chadwick in 1932, and
some of his research was based on discoveries by Rutherford and
Irène and Frédéric Joliot-Curie.
iii. This particle was very difficult to discover, because of the fact
that it does not possess a charge.
iv. Originally, the neutron was thought to be a combination of a
proton and an electron, but later it was determined that it was a
unique particle.