ATOMIC THEORY
DO NOW:
3
4
1
2
 Which shows a a correct image of
what an atom looks like? How do
you know?
THE GREEK CONCEPT OF
ATOMOS: THE INDIVISIBLE ATOM
 Around 440 BC, Leucippus originated the atom concept.
 His pupil, Democritus (c460-371 BC) extended it
 There are five major points to their atomic idea.
 All matter is composed of atoms, which are too small to be seen.
These atoms CANNOT be further split into smaller portions.
 There is a void, which is empty space between atoms.
 Atoms are completely solid.
 Atoms are homogeneous, with no internal structure.
 Atoms can differ in size, shape, and weight
ARISTOTLE (384-322 BC)
è
Spoke openly against the concept
—
è
è
Only a few scholars gave it much thought.
The Catholic Church accepted Aristotle's position
—
è
è
the atom concept diminished
equated atomistic ideas with Godlessness
It was not until 1660 that Pierre Gassendi succeeded
in separating the two
not until 1803 that John Dalton put the atom on
a solid scientific basis.
JOHN DALTON’S ATOMIC THEORY
• In 1808, John Dalton
developed an atomic
theory.
• Dalton believed that a few
kinds of atoms made up
all matter.
• According to Dalton,
elements are composed
of only one kind of atom
and compounds are made
from two or more kinds of
atoms.
JOHN DALTON 1803-1807
MODERN ATOMIC THEORY
1.
2.
3.
4.
5.
All matter is composed of tiny
particles called atoms.
All atoms of a given element
have identical chemical
properties that are characteristic
of that element.
Atoms form chemical compounds
by combining in whole-number
ratios.
Atoms can change how they are
combined, but they are neither
created nor destroyed in
chemical reactions.
Atoms of a given element are
identical in their physical and
chemical properties.
John Dalton’s Atomic Theory
Almost right. A good start.
very small
Structure of the atom after Dalton
(ca. 1810)
LAW OF CONSERVATION OF ATOMS
& MASS
Atoms are neither created nor destroyed
during physical or chemical processes. In a
closed system
Video
LAW OF CONSERVATION OF ATOMS
& MASS
LAW OF CONSERVATION OF ATOMS
& MASS
Mass is neither created nor destroyed
during physical or chemical processes.
THE LAW OF DEFINITE PROPORTIONS
 States that two samples of a given compound are
made of the same elements in exactly the same
proportions by mass regardless of the sizes or
sources of the samples.
 So for example, every molecule of ethylene glycol is
made of the same number and types of atoms.
THE LAW OF DEFINITE PROPORTIONS
 A molecule of ethylene glycol has the formula
C 2 H 6 O 2 , so the law of definite proportions tells you
that all other molecules of ethylene glycol have the
same formula.
 So where we have 1mg or 1000kg of Ethylene glycol
the ratio is the same
51.56% oxygen, 38.70% carbon,
and 9.74% hydrogen
THE LAW OF DEFINITE PROPORTIONS
•The law of multiple proportions states that when two
elements combine to form two or more compounds,
the mass of one element that combines with a given
mass of the other is in the ratio of small whole
numbers.
LAW OF MULTIPLE PROPORTIONS
 If elements A and B react to form two compounds, the
dif ferent masses of B that combine with a fixed mass of A can
be expressed as a ratio of small whole numbers:
Example: Nitrogen Oxides I & II
Nitrogen Oxide I : 46.68% Nitrogen and 53.32% Oxygen
Nitrogen Oxide II : 30.45% Nitrogen and 69.55% Oxygen
Cmpd I
Cmpd II
in 100 g of each Compound: g O = 53.32 g & 69.55 g
g N = 46.68 g & 30.45 g
Cmpd II 2.284
Cmpd I 1.142
=
2
1
g O /g N = 1.142 & 2.284
J.J. Thomson
(1856 – 1940)
Image taken from:
www.wired.com/.../news/2008/04/d
ayintech_0430
 Proved that an atom can be divided
into smaller parts
 While experimenting with cathoderay tubes, discovered corpuscles,
which were later called electrons
 Stated that the atom is neutral
 In 1897, proposed the Plum
Pudding Model which states that
atoms mostly consist of positively
charged material with negatively
charged particles (electrons)
located throughout the positive
material
 Won a Nobel Prize
J.J. Thomson (1897):
Cathode Rays
Atoms subjected to high voltages
give off cathode rays.
J.J. Thomson: Cathode Rays
Cathode rays can be deflected by a magnetic field.
Cathode rays are negatively charged particles (electrons).
Electrons are in atoms.
J.J. Thomson – The Electron
“Plum pudding”
model: Negative
electrons are
embedded in a
positively charged
mass.
Unlike electrical
charges attract, and
that is what holds the
atom together.
Electrons (-)
Positively charged
mass
Structure of the atom after
Thomson (ca. 1900)
Ernest Rutherford
(1871 – 1937)
 In 1909, performed the Gold Foil
Experiment and suggested the
following characteristics of the atom:
o It consists of a small core, or
nucleus, that contains most of the
mass of the atom
o This nucleus is made up of particles
called protons, which have a positive
charge
o The protons are surrounded by
negatively charged electrons, but
most of the atom is actually empty
space
 Did extensive work on radioactivity
(alpha & beta particles, gamma
rays/waves) and was referred to as
the “Father of Nuclear Physics”
 Won a Nobel Prize
 Was a student of J.J. Thomson
 Was on the New Zealand $100 bill
Image taken from:
http://www.scientificweb.com/en/Physics/Biographies/Er
nestRutherford.html
Ernest Rutherford (1910)
Scattering experiment: firing alpha particles at a gold foil
The Nuclear Atom
Some alpha particles
bounce off the gold foil.
This means the mass of
the atom must be
concentrated in the
center and is positively
charged!
Thompson’s model
could not be correct.
Ernest Rutherford
The Nucleus and the Proton
The mass is not spread evenly throughout the atom, but is
concentrated in the center, the nucleus.
The positively
charged particles in
the nucleus are
protons.
Electrons (-) are now
outside the nucleus.
Structure of the atom after
Rutherford (1910)
Niels Bohr
(1885 – 1962)
Image taken from:
commons.wikimedia.org/wiki/File:Ni
els_Bohr.jpg
 In 1913, proposed the Bohr
Model, which suggests that
electrons travel around the
nucleus of an atom in orbits
or definite paths.
Additionally, the electrons
can jump from a path in one
level to a path in another
level (depending on their
energy)
 Won a Nobel Prize
 Worked with Ernest
Rutherford
Bohr - Rutherford diagrams
 Putting all this together, we get B -R diagrams
 To draw them you must know the # of protons, neutrons, and
electrons (2,8,8,2 filling order)
 Draw protons (p + ), (n 0 ) in circle (i.e. “nucleus”)
 Draw electrons around in shells
He
p+
2
2 n0
Li
Li shorthand
p+
3
4 n0
3 p+
4 n0
2e– 1e–
Draw Be, B, Al and shorthand diagrams for O, Na
Be
B
Al
4 p+
5 n°
5 p+
6 n°
O
13 p+
14 n°
Na
8 p+
8 n°
2e– 6e–
11 p+
12 n°
2e– 8e– 1e–
Erwin Schrodinger
(1887-1961)
 In 1926, he further explained
the nature of electrons in an
atom by stating that the
exact location of an electron
cannot be stated; therefore,
it is more accurate to view
the electrons in regions
called electron clouds;
electron clouds are places
where the electrons are
likely to be found
 Did extensive work on the
Wave formula  Schrodinger
equation
 Won a Nobel Prize
Image taken from:
nobelprize.org/.../1933/schrodinger
-bio.html
James Chadwick
(1891 – 1974)
Image taken from:
www.wired.com/.../news/2009/02/d
ayintech_0227
 Realized that the atomic
mass of most elements was
double the number of protons
 discovery of the neutron in
1932
 Worked on the Manhattan
Project
 Worked with Ernest
Rutherford
 Won a Nobel Prize
James Chadwick – The Neutron
In the nucleus with the protons are particles of similar mass
but no electrical charge called neutrons.
The positively
charged particles in
the nucleus are
protons.
+ nn
Electrons (-) are now
outside the nucleus
in quantized energy
states called
orbitals. (From Niels
Bohr and quantum
mechanics)
Structure of the atom after
Chadwick (1932)
Structure of the Atom
proton (+)
neutron
electrons responsible for the
volume and size of
the atom, negatively
charged
10-10 m
10-14
m
nucleus - responsible for the mass
of the atom, positively charged
Subatomic Particles
 Protons and electrons are the only particles that
have a charge.
 Protons and neutrons have essentially the same
mass.
 The mass of an electron is so small we ignore it.
Atomic Facts
Feature
Size
Mass
electron (-)
10-18 m ???
0.0006 amu
1 amu = 1 atomic mass
unit = 1.66054 x 10-24 g
+nn
proton (+)
10-15 m
1.0073 amu
neutron (0)
10-15 m
1.0087 amu
Electrons are outside
the nucleus in
quantized energy
states called orbitals.