Download structure-of-atom

Development of the Atomic
structure
Greek Model
“To understand the very large,
we must understand the very small.”
Democritus
• Greek philosopher
• Idea of ‘democracy’
• Idea of ‘atomos’
• Atomos = ‘indivisible’
• ‘Atom’ is derived
• No experiments to support idea
• Continuous vs. discontinuous
theory of matter
Democritus’s model of atom
No protons, electrons, or neutrons
Solid and INDESTRUCTABLE
Democritus
DEMOCRITUS (400 BC) – First Atomic Hypothesis
Atomos: Greek for “uncuttable”. Chop up a piece of matter until you reach the atomos.
Properties of atoms:
• indestructible.
• changeable, however, into different forms.
• an infinite number of kinds so there are an infinite number of elements.
• hard substances have rough, prickly atoms that stick together.
• liquids have round, smooth atoms that slide over one another.
• smell is caused by atoms interacting with the nose – rough atoms hurt.
• sleep is caused by atoms escaping the brain.
• death – too many escaped or didn’t return.
• the heart is the center of anger.
• the brain is the center of thought.
• the liver is the seat of desire.
“Nothing exists but atoms and space, all else is opinion”.
Dalton Model of the Atom
Late 1700’s - John Dalton- England
Teacher- summarized results of his experiments and
those of others
Combined ideas of elements with that of atoms in
Dalton’s Atomic Theory
The Atomic Theory of Matter
• In 1803, Dalton proposed that elements consist of individual
particles called atoms.
• His atomic theory of matter contains four hypotheses:
1. All matter is composed of tiny particles called atoms.
2. All atoms of an element are identical in mass and
fundamental chemical properties.
3. A chemical compound is a substance that always
contains the same atoms in the same ratio.
4. In chemical reactions, atoms from one or more
compounds or elements redistribute or rearrange in
relation to other atoms to form one or more new
compounds. Atoms themselves do not undergo a
change of identity in chemical reactions.
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
Foundations of Atomic Theory
Law of Conservation of Mass
Mass is neither destroyed nor created during ordinary chemical reactions.
Law of Definite Proportions
The fact that a chemical compound contains the same elements in
exactly the same proportions by mass regardless of the size of the
sample or source of the compound.
Law of Multiple Proportions
If two or more different compounds are composed of the same two
elements, then the ratio of the masses of the second element
combined with a certain mass of the first elements is always a ratio
of small whole numbers.
Conservation of Atoms
2 H2 + O 2
2 H2O
John Dalton
H
H
H2
O
H
O2
+
H2
H
O
H
H
O
O
H
H
4 atoms hydrogen
2 atoms oxygen
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204
H2O
4 atoms hydrogen
2 atoms oxygen
H2O
Legos are Similar to Atoms
H2
H
H
H
O
+
H2
H
H
O2
H
O
H2O
H
O
O
H
H2O
Lego's can be taken apart and built into many different things.
Atoms can be rearranged into different substances.
Conservation of Mass
High
voltage
electrodes
Before reaction
After reaction
glass
chamber
O2
High
voltage
H2O
O2
5.0 g H2
H2
0 g H2
80 g O2
45
? g H 2O
300 g (mass
of chamber)
+
385 g total
40 g O2
300 g (mass
of chamber)
+
385 g total
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204
Law of Definite Proportions
Joseph Louis Proust (1754 – 1826)
• Each compound has a specific ratio of elements
• It is a ratio by mass
• Water is always 8 grams of oxygen for every one
gram of hydrogen
Law of Definite Proportions
Whether synthesized in the laboratory or obtained from
various natural sources, copper carbonate always has
the same composition.
Analysis of this compound led Proust to formulate
the law of definite proportions.
+
103 g of
copper carbonate
Ralph A. Burns, Fundamentals of Chemistry 1999, page 90
53 g of
copper
+
40 g of oxygen
10 g of carbon
What?
• Water is 8 grams of oxygen per gram of hydrogen.
• Hydrogen peroxide is 16 grams of oxygen per gram of hydrogen.
• 16 g to 8 g is a 2:1 ratio
• True, because you have to add a whole atom, you can’t add a piece of
an atom.
Daltons Atomic Theory
• Dalton stated that elements
consisted of tiny particles
called atoms
• He also called the elements
pure substances because all
atoms of an element were
identical and that in
particular they had the same
mass.
Dalton’s Atomic Theory
.1. All matter is made of tiny indivisible particles called atoms.
“atoms”.
2.
Atoms of one element can neither be subdivided nor changed into
atoms of any other element.
3.
Atoms can neither be created nor destroyed.
4. All atoms of the same element are identical in mass, size, and other
properties.
5.
Atoms of one element differ in mass and other properties from atoms
of other elements.
6.
In compounds, atoms of different elements combine in simple, whole
number ratios.
The Atomic Theory of Matter
• Dalton’s atomic theory is essentially correct, with four minor
modifications:
1. Not all atoms of an element must have precisely the same mass.
2. Atoms of one element can be transformed into another through
reactions.
3. The composition of many solid compounds are somewhat
variable.
4. Under certain circumstances, some atoms can be divided
(split into smaller particles: i.e. nuclear fission).
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
nuclear
Thomson Model of the
Atom
J. J. Thomson - English physicist. 1897
Made a piece of equipment called a cathode ray
tube.
It is a vacuum tube - all the air has been pumped
out.
A Cathode Ray Tube
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58
The Effect of an Electric Field on Cathode Rays
source of
high voltage
High
voltage
cathode
negative
plate
_
+
anode
positive
plate
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117
Thomson’s Experiment
-
voltage
source
+
vacuum tube
metal disks
Thomson’s Experiment
ON
-
OFF
voltage
source
+
Passing an electric current makes a beam appear
to move from the negative to the positive end
Thomson’s Experiment
ON
-
OFF
voltage
source
+
Thomson’s Experiment
ON
-
OFF
voltage
source
+
+
By adding an electric field…
he found that the moving pieces were negative.
Video 1
Video 2
J.J. Thomson
• He proved that atoms of any element can be made to emit tiny
negative particles.
• He conducted the experiment with different gases and got the
same result
• From this he concluded that ALL atoms must contain these
negative particles.
• He named these particles ELECTRONS.
Thomson Model of the Atom
• J.J. Thomson discovered the electron and knew that electrons
could be emitted from matter (1897)
• He explained that the electrons are li
• He knew that atoms did not have a net negative charge and so
there must be balancing the negative charge.
• William Thomson proposed that atoms consist of small, negative
electrons embedded in a massive, positive sphere.
• The electrons were like currants in a plum pudding.
• This is called the ‘plum pudding’ model of the atom.
-
-
electrons
-
-
-
-
Goldstein - 1886
• Used modified type of discharge tube with a perforated cathode
Rutherford’s
Apparatus
Rutherford received the 1908 Nobel Prize in Chemistry for his pioneering work in nuclear chemis
beam of alpha particles
radioactive
substance
circular ZnS - coated
fluorescent screen
gold foil
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
What he expected…
Becausehe thought the mass was evenly
distributed in the atom.
-
-
-
-
Because, he thought the
mass was evenly
distributed in the atom
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What he got…
richocheting
alpha particles
What he got…
richocheting
alpha particles
The Predicted Result:
expected
path
expected
marks on screen
Observed Result:
mark on
screen
likely alpha
particle path
Explanation of Alpha-Scattering Results
Alpha particles
Nucleus
+
-
- +
+
+
-
+
-
-
+
-
+
-
+
-
Plum-pudding atom
Nuclear atom
Thomson’s model
Rutherford’s model
Bohr’s Model
Nucleus
Electron
Orbit
Energy Levels
Bohr Model of Atom
Increasing energy
of orbits
n=3
e-
n=2
e-
n=1
ee-
e-
e-
e-
e-
e-
e-
eA photon is emitted
with energy E = hf
The Bohr model of the atom, like many ideas in
the history of science, was at first prompted by
and later partially disproved by experimentation.
http://en.wikipedia.org/wiki/Category:Chemistry
An unsatisfactory model
for the hydrogen atom
According to classical physics, light
should be emitted as the electron
circles the nucleus. A loss of energy
would cause the electron to be drawn
closer to the nucleus and eventually
spiral into it.
Hill, Petrucci, General Chemistry An Integrated Approach 2nd Edition, page 294