Transcript Ch_3___History_of_Modern_Atomic_Theory_2012

HISTORY OF THE MODERN
ATOMIC THEORY
Chapter 3 - Sections 3-1 and 3-2
Greek Philosophers
◦ Democritus (400 B.C. ) – All matter is
made of basic particles which are
indivisible and are called “atomos” .
Aristotle
(400 B.C.) – All matter is made
of earth, fire, wind, and water.
Aristotle’s theory of earth, wind, fire,
and water was believed until the early
1800’s.
Dalton’s Atomic Theory - 1808
1. All matter is composed of extremely small
particles called atoms.
2. Atoms of a given element are identical in
size, mass, and other properties.
3. Atoms cannot be subdivided, created,
or destroyed.
4. Atoms of different elements can combine in
simple, whole-number ratios to form
chemical compounds.
5. Chemical reactions change the
arrangement of atoms, but do not change
atoms of one element into atoms of
another element.
Dalton’s Atomic Theory
Not all of the points of Dalton’s theory are correct:
2 – Dalton did not know about
isotopes.
3 – Dalton did not know about
subatomic particles.
Discovery of the Electron
1897
J.J. Thomson (British) is credited with the
discovery of the electron
Used cathode ray tube and magnets
1907
R.A. Millikan (University of Chicago)
Mass of electron = 1/1837 mass of
proton
Prediction of Proton
◦ 1913 - J.J. Thompson found a negative particle and predicted
the discovery of the proton a positive particle to make the
atom neutral
◦ JJ Thompson- Plum pudding model
◦ Protons and electrons are arranged randomly in the atom.
Thompson’s Model of the Atom
Discovery of the Nucleus
◦1911
Lord
Ernst Rutherford
Gold
foil experiment
Found that most
of the atom was empty space
but there was a dense, positive area in the
middle that he called the nucleus.
Rutherford
shot alpha particles at a
piece of gold foil. Most of the
particles passed through the gold foil
without deflection and only a small
percentage bounced off the gold foil
at a very large angle
Discovery of the Proton
◦1919
Ernst Rutherford is credited with
the
discovery
Found he could make one element into another
by hitting the nucleus with a helium nucleus. A
hydrogen nucleus was emitted by the process
and this was found to be only a proton
Discovery of the Neutron
◦1932
Sir James Chadwick
Student of Ernst Rutherford
The atom was heavier than the total
mass of the proton and the electron.
The charge of the atom was neutral.
There must be a neutral particle about
the size of the proton.

Bohr Model
◦ Danish physicist Niels Bohr, 1913
◦ A student of Rutherford, developed a new atomic model.
◦ Added new discoveries about how the energy of an atom
changes when the atom absorbs or emits light.
◦ Proposed that an electron is found only in specific circular
paths, or orbits, around the nucleus
◦ Discovered a Hydrogen atom absorbed light when an electron
is excited from a low energy orbit into a higher energy orbit.
Atoms can give off light when an electron drops from the high
to the low energy orbit.
Bohr Hydrogen Experiment
Quantum Mechanical Model◦ Current model of the atom
◦ In the Bohr model of the atom, electrons
move in circular orbits around the
nucleus.
◦ Scientists now know electrons DO NOT
travel in orbits around the nucleus.
◦ However, in our currently accepted model
of the atom, we only know the probability
of finding an electron in an electron cloud,
which is a region of space around the
nucleus.
Aristotle400BC
Said all
matter is
made earth,
fire, wind,
and water
DemocritusJohn Dalton400BC
1808
Said all
Came up
matter is
with 5
made of
postulates to
“atomos”
the atomic
theory.
1897- JJ
Thomson
discovered
electron
using
cathode ray
tube
1900- JJ
Thomson’s
plum
pudding
model.
Protons and
electrons are
arranged
randomly
1911Rutherford
discovered the
dense (+)
charged
nucleus- Gold
Foil Experiment
1919Rutherford
discovered
the proton
1922Niels
Bohr
Model
of atom
1932
Chadwick
discovered
the neutron
1940’s–
Quantum
Mechanic
Model of
atom
EXIT TICKET
List the following scientists in order of their
contributions to the structure of an atom.
____ Niels Bohr
____ James Chadwick
____ John Dalton
____ Democritus
____ Ernst Rutherford
____ J. J. Thompson
Day 2
The Atom
◦Nucleus
◦ Proton- positively charged
◦ Neutron- neutral
◦Outside the nucleus- Clouds
◦ Electron- negatively charged
Subatomic particles
Particle
Charge
Symbol
Electron
-1
e
Proton
+1
p+
1/2000 the Outside
mass of
nucleus
proton
Same mass In nucleus
as neutron
Neutron
0
n
Same mass In nucleus
as proton
-
Mass
Location
Important Definitions
◦ Atomic Number (Z) – The number of protons in the
nucleus
◦ Mass #= protons + neutrons
◦ Electrons= atomic #
◦ Neutrons= mass # - atomic #
Ions- Loss or gain of electrons
Cation= + charged particle, lost an eAnion= - charged particle, gained an e-
Element
Symb
ol
Calcium ion
267
P3-
15
31
Ca+2
20
40
33
75
82
50
19
39
26
56
16
32
Arsenic
Tin
Potassium
ion
K+1
Iron
Sulfur ion
# of
# of
Mass
Electrons Neutrons Number
104
Rutherfordium
Phosphorus
ion
Atomic Atomi # of
numbe c mass Protons
r
S-2
Important Definitions
◦ Atomic Mass – The weighted average of the mass
numbers of all of the naturally occurring isotopes of an
element.
Important Definitions
◦ Mass Number – The total number of protons and
neutrons in the nucleus of an isotope.
# p+
Isotope
Mass
number
Protium
Deuterium
Tritium
1
1
1
1
1
1
# e-
#n
0
1
2
1
2
3
Important Definitions
◦ Isotope – Two or more atoms of the same element that have the same number of
protons and electrons but a different number of neutrons.
Nuclide
– A general term for an isotope of any
element.
Every element has
at least one isotope and
some elements have many isotopes
General Format of Isotopes
◦ Hydrogen -3
◦ Oxygen-17
◦ Lithim-5
◦ Carbon-12
Hyphen Notation
◦
Write the general and hyphen
notation
General
Format
Atomic #= 2
Mass # = 4
Atomic #= 16
Mass # = 32
Atomic #= 17
Mass # = 37
Hyphen
Notation
Isotop
e
Zn-64
Zn-66
Zn-67
Gener Number Number Number
al
of
of
of
format protons electron neutrons
s
Mass
Number
Which of the following pairs are
isotopes?
A.
C.
B.
D.
40
X
20
42
X
20
Day 3
Isotopes of chlorine
◦ Chorine-35 accounts for about 75 percent of the naturally
occurring chlorine atoms; chlorine-37 accounts for only about 25
percent.
Natural percent abundance
Natural Percent Abundance of Stable Isotopes of Some Elements
Important Definitions
◦ Atomic Mass Unit – Exactly 1/12 the mass of a carbon-12 atom.
Symbol = amu
Examples:
Mg = 24.31 amu
N = 14.01 amu
Round off to two places past the decimal
Atomic Mass Practice
Copper consists of 69.17% Cu-63 and 30.83% Cu-65. Determine the
atomic mass of Cu.
63 x .6917 = 43.58 amu
65 x .3083 = 20.04 amu
Atomic Mass
= 63.62 amu
It’s your turn to practice
◦Neon is composed of three
naturally occurring isotopes:
◦90.92% is Neon-20,
◦0 .257% is Neon-21 and
◦8.82% is Neon-22. Calculate
the atomic mass of neon.
Solving the Problem
◦ 0.9092 x 20 = 18.184
◦ .00257 x 21 = .05397
◦ .0882 x 22 = 1.9404
◦ Sum
◦ Answer
= 20.1783
= 20.18 amu
THAT’S ALL FOR
TODAY