Transcript Chemistry You Need to Know

Chapter 4: Glow in the Dark
Introduction
List as many things as you can think of
that “glow”
What do you have to do to make these
“glowing” things “glow”?
This chapter will introduce the chemistry
needed to understand how glowing things
work
Section 4.1: Development of Atomic Theory
Objective:
Describe the development of modern
atomic theory
Ancient Greece
 Various philosophers considered the many
mysteries of life.
 Aristotle concluded that matter was composed of
4 elements - earth, air, fire, and water – and that it
could be divided endlessly into ever smaller
pieces.
 Democritus was the first person to propose the
idea that matter was not infinitely divisible. He
believed that matter was composed of atomos or
atoms; atoms were solid & indivisible.
John Dalton
• Aristotle's idea went
unchallenged for 2000 yrs.
• John Dalton (1766-1844),
an English schoolteacher
and chemist, revised
Democritus’ ideas based
on careful & accurate
scientific research that he
conducted himself.
Dalton’s Atomic Theory (1803)
 All matter is made of tiny particles “atoms”
 Atoms cannot be created, divided, destroyed or
changed into other types of atoms
 Atoms of the same element have identical
properties
 Atoms of different elements have different
properties
 Atoms of different elements combine in wholenumber ratios to form compounds
 Chemical changes join, separate or rearrange
atoms in compounds
(p. 124 in text)
J. J. Thomson
• Because of Dalton’s
atomic theory, most
scientists in the 1800s
believed that the atom
was like a tiny solid ball
that could not be broken
up into parts.
• In 1897, a British
physicist, J.J. Thomson,
discovered that this
solid-ball model was not
accurate.
Thomson’s Cathode Ray Tube
• Thomson’s experiments used a
vacuum tube (a tube that has had all the
gases pumped out of it) called the cathode
ray tube.
Cathode Ray Tubes
When connected to a battery, electrodes at
the end of the tube seemed to generate a “cathode
ray.” The cathode ray looked like a ray of light
traveling through the tube, from the cathode plate
to the anode plate.
Cathode ray
Metal plate (cathode)
releases stream
Metal plate (anode) to
which stream travels
Cathode Ray Tubes & Charge
Next Thomson put charged plates outside the tube.
He found that the rays bent towards a positively
charged plate and away from a negative one.
-
Positively charged plate
Negatively charged plate
+
Ray is deflected
away from
negative plate
and towards
positive plate
Thomson’s conclusions
 Since like charges repel, Thomson’s knew that
there was something negatively charged in the
cathode ray.
 Since there were no particles in the tube, these
negative particles had to come from the atoms of
the metal plates.
 Since all types of metal produced the same
result, the negative charge had to be in all types
of atoms.
 In 1897, Thomson announced that the rays were
electrons and they had a negative charge
Theories change
 Thomson’s evidence showed Dalton’s idea of
solid, uniform atoms was incorrect.
 In addition, since atoms themselves are not
negatively charged but neutral, scientists
believed there had to be other particles in the
atom, especially positively charged ones.
The Plum Pudding Model
 Eugene Goldstein
conducted experiments to
find the positive parts
(protons) and determined
they had the opposite
charge as the electron but
were 1837 times heavier!
 Thomson developed a
model of the atom called
the “plum pudding” model.
Also Called The Cookie Dough
Model
The “chips” are the
negative electrons.
The “dough” is the
positive portion
The “chips” are
stationary and don’t
move within the
“dough”
Gold Foil Experiment
In 1911, a team of scientists led by Ernest
Rutherford in England carried out the first
of several important experiments that
revealed an arrangement far different from
the plum pudding model of the atom.
This team included Geiger and Marsden.
Gold Foil Experiment
Geiger and Marsden (under Rutherford’s
direction) bombarded very thin gold foil
with radioactive particles (alpha particles
“”)
They were to observe the direction the
particles took as a result of passing
through the foil.
Gold Foil Experiment
Gold Foil Experiment
It was believed that if
the plum pudding model
was correct, the alpha
particles would pass
straight through the gold
atoms.
Instead, researchers
found that some alpha
particles were deflected
at very wide angles.
Observations & Conclusions
Most of the alpha particles passed straight
through the foil with no deflection
These particles did not run into anything; they
traveled through empty space.
Some particles had slight deflections
These particles ran into something much
smaller than themselves.
A few particles were deflected at wide
angles – some came straight back!
These particles ran into something very dense
Rutherford’s Nuclear Model
To explain the results, Rutherford proposed a new
model of the atom – the nuclear model.
In this model, atoms are nearly all empty space
There is a small area of the atom that contains
most of the mass. This area caused the wide
deflections seen.
This area is called the nucleus. Protons are
found here.
Electrons (the smaller particles), the cause of the
small deflections, are found in the space outside the
nucleus.
The Neutron
The protons (+) and electrons (-) could
explain the charges of the various parts of
the atom that were observed.
They could not explain the total mass of
atoms.
Neutrons were proposed in 1920’s but not
confirmed until 1932 by James Chadwick.
Neutrons had mass similar to protons and no
charge. They were located in the nucleus.
Revisions to the Nuclear Model
In 1913, Neils Bohr
(who was working for
Rutherford) believed
Rutherford’s model
needed improvement.
Bohr’s Atomic Model
Bohr performed experiments with hydrogen
atoms & light.
He determined that electrons are in levels
according to how much energy they have
(energy levels) and that only certain energy
amounts were allowed.
Bohr’s Atomic Model
Bohr proposed that, within energy
levels, electrons are found in specific
circular paths, or orbits, around the
nucleus.
Bohr’s model
came to be
known as the
planetary
model.
The Bohr Model
The orbit closest to the nucleus contains
the lowest energy electrons.
The first level can hold 2 electrons, then
the next two levels can each hold 8 and
then levels farther out can hold 18.
Pictures of the Bohr Models
Oxygen
Use of the Bohr Model now
We no longer believe electrons are in
circular orbits.
However, this is still a convenient way to
show energy levels on 2-dimensional
paper.
Modern Atomic Theory
Bohr’s research lead the way for the study
of quantum mechanics (the study of tiny
particles) in the 1920’s.
Quantum mechanics uses calculus
equations to show how the electrons act
as both particles and waves.
These equations show the most probable
location of electrons in the atom (known
as atomic orbitals).