Transcript Ch L15 History of Atomic Therory

Lesson 15 History of Atomic
Theory
Learning Targets:
* I can list and explain the five points in
Dalton’s atomic theory.
* I can describe and reenact Rutherford’s Gold Foil
Experiment, and explain his theory of an atom.
* I can describe Neils Bohr’s atomic theory.
• Dalton’s atomic theory
– 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
– Aristotle (c. 384-322 BCE) disagreed with Democritus
because he did not believe empty space could exist.
– Aristotle’s views went unchallenged for 2,000 years
until science developed methods to test the validity of
his ideas.
– John Dalton looked over knowledge that had been
accumulated before his time, and wrote five basic
principles dealing with matter.
I. Dalton’s Atomic Theory
a. 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
b. 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. Millikan was then able to determine the exact charge of this electron.
Cathode Ray Tube Drawing:
Thompson's Experiment
Virtual Lab 1 video
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:
Robert A. Millikan
1868-1953
1909 – Using Thomson’s
work, he was able to
determine the exact charge
and mass of an electron.
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
i. His model of the atom is similar to that of the solar system, with
electrons traveling around the nucleus in well-defined paths.
ii.
Draw a Rutherford “solar system model” in this space:
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
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
Rutherford’s Model vs. Bohr’s Model
Unstable Atom
electrons continuously emit
energy and eventually fall into
the nucleus
Stable Atom
electrons continuously absorb
and emit energy by changing
orbitals
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.
Neutron Experiment
1930
James Chadwick used this experiment to mathematically
prove the existence of the neutron.
another scientist
Irène Joliot-Curie and Frédéric Joliot
(husband)
Atomic Theory Summary
• All atoms are made of three fundamental
subatomic particles: the electron, the proton,
and the neutron.
• Atoms are spherically shaped.
• Atoms are mostly empty space, and electrons
travel around the nucleus held by an attraction
to the positively charged nucleus.
Atomic Theory Summary
Relative Sizes
If a hydrogen atom was enlarged to the size of a golf
ball, then an actual golf ball would have to be enlarged
to the size of the earth to keep the scale correct.
Relative Sizes
If the nucleus was enlarged to the size of a 0.75 cm pea,
then the atom would have to be as large as a baseball
field.
Relative Sizes
If you imagine a proton to be the same mass as a
honeydew melon (1200 grams), then an electron would
be as massive as a pit (0.6g).