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Introduction to the Atom
and Atomic Models
Atomic - Molecular
Theory of Matter
The Atomic - Molecular Theory of
Matter states that all matter is
composed of small, fast moving
particles called atoms. These
atoms can join together to form
molecules.
This theory is really thousands of
individual ideas and models that
provide evidence for the whole
theory.
Matter
Since the atom is
too small to be seen
even with the most
powerful
microscopes,
it or not this is a
scientists rely upon Believe
microscope. Even with the
models to help us to world’s best microscopes we
cannot see the structure or
behavior of the atom.
understand the
atom.
DEMOCRITUS
400
BC
Democritus believed all things
consisted of tiny indivisible units.
He called these tiny units he
called atomos. The Greek word
for “can not be cut” or
“indivisible”
Ancient philosopher: Father of the Atom
John Dalton
(1799)
 Developed what is considered to
be the 1st Atomic Theory
 Was born into a modest Quaker
family in England
 Began lecturing in public at the age
of 12
Dalton’s Model (1799)
• Dalton's model was that the atoms were tiny,
indivisible, indestructible particles and that each one
had a certain mass, size, and chemical behavior that
was determined by what kind of element they were.
Dalton’s Model
Dalton’s model of the
atom was similar to a
tiny billiard ball.
Dalton’s model of the
atom was solid and had
no internal structure.
Dalton’s Atomic Theory
• elements consisted of tiny particles called
atoms.
1. all atoms of an element are identical
2. atoms of each element are different from one
another; they have different masses.
3. compounds consisted of atoms of different
elements combined together.
4. chemical reactions involved the rearrangement of
combinations of those atoms.
Flaws in Dalton’s Model
• Dalton’s falsely believed that the atom
was the most fundamental particle.
– We now know the atom is made up of even
smaller particles we call the proton, neutron
and electron.
• Dalton’s theory could also not account for
the formation of ions (charged particles)
Daltons Atomic Model Summary
• Called: Billiard Ball Model
• Could account for
– Atoms of different element have different
atomic masses
– Elements consists of tiny particles called
atoms
• Could NOT account for
– Atom being the smallest particle
– Atoms having an internal structure
– Atoms having charged particles
John J. Thomson (1897)
• Discovered the electron using
the Cathod Ray Tube (CRT)
• Thomson found that the beam
of charge in the CRT was
attracted to the positive end of
a magnet and repelled by the
negative end.
Thomson’s Hypothesis
• Concluded that the cathode beam was a stream of
negative particles (electrons).
• He tested several cathode materials and found that all of
them produced the same result.
• He also found that the charge to mass ratio was the same
for all electrons regardless of the material used in the
cathode or the gas in the tube.
• Thomson concluded that electrons must be part of all
atoms.
Thomson’s atomic model
• Called the “plum-pudding” it was the most
popular and most wildly accepted model of
the time.
Thompsons atomic model could account
for…..
• the atom having an
internal structure
• Light given off by atoms
• Atom with different
atomic masses
Thompsons atomic model could NOT
account for…..
• Empty space (had atom
filled with positive pudding)
• Formation of ions
Gold Foil Experiment
• Conducted by students of Rutherford.
• Proved that all atoms had a tiny, positively
charged center.
• Confirmed that atom’s were mostly empty
space.
Rutherford ~ early 1900s
• α-particle interaction with matter
studied in gold foil experiment
Rutherford's Nuclear Model
• 1. The atom contains a tiny dense center
– the volume is about 1/10 trillionth the volume
of the atom
• 2. The nucleus is essentially the entire mass
of the atom
• 3. The nucleus is positively charged
– the amount of positive charge of the nucleus
balances the negative charge of the
electrons
• 4. The electrons move around in the empty
space of the atom surrounding the nucleus
Rutherford’s atomic model (1911)
• Could account for:
– Empty space
– Ions
– Internal structure
– Light given off when heated
to high temperature.
• Could not account for:
– Stability
Planetary model
• Planetary model used to explain
electrons moving around the tiny, but
dense nucleus
• Nucleus contains
– Protons- existence proposed in 1900s
– Neutrons- existence proposed in 1930s
Bohr
• Questioned ‘planetary model’ of atom
– Electrons located in specific levels from
nucleus (discontinuous model)
• Proposed electron cloud model based on
evidence collected with H emission
spectra
Bohr’s Atomic Model (1913)
• Bohr was a student of Rutherford.
• Improved Rutherford’s model by proposing electrons are
found only in specific fixed orbits.
• These orbits have fixed levels of energy
• This explained how electrons could give off light (gain or
lose energy)
BOHR MODEL
• Electrons are placed in energy levels
surrounding the nucleus
8e8eNucleus
(p+ & n0)
2e-
Bohr’s Atomic Model
• Could account for
– Internal structure
– Atoms of different masses
– Atom being mostly empty space
– Light given off
– Formation of positive ions
• Flaws
– Only really worked for Hydrogen
Chadwick (1932)
• Discovered the neutron by bombarding Be with
beta radiation.
• Nuclear fission released a neutron.
chart
Review
• Describe each of the 6 different atomic
models. Give the
– Scientist Name
– Name of model
– What they could account for
– What they could not account for (flaws)
Subatomic particle summary
Particle
Discovery by
Year
Proton
Rutherford
1911 Gold Foil Experiment
Electron
Thompson
1897 The response of
cathode ray tube to a
magnetic and electric
fields
1932 Bombarded Be with
beta radiation and a
neutron was released
Neutrons Chadwich
experiment
Subatomic Particles
Name
Relative
Symbol Charge mass
Actual
mass (g)
Electron
e-
-1
1/1840 9.11 x 10-28
Proton
p+
+1
1
1.67 x 10-24
Neutron
n0
0
1
1.67 x 10-24
Subatomic Particles (cont.)
• All atoms of an element have the same # of
protons
protons identify an atom  atomic #
• Atoms are electrically neutral
#p = #e-
• Only neutrons and protons contribute to an
atoms mass
#n + #p = atomic mass
Subatomic Particles
ATOM
ATOM
NUCLEUS
NUCLEUS
ELECTRONS
ELECTRONS
PROTONS
PROTONS
NEUTRONS
NEUTRONS
POSITIVE
CHARGE
NEUTRAL
CHARGE
NEGATIVE
CHARGE
NEGATIVE CHARGE
equal in a
Atomic
Most Number
of the atom’s mass.
neutral atom
equals the # of...
QUARKS
ISOTOPES
= atoms with the same
number of protons
but DIFFERENT
numbers of neutrons
Atomic Number
Mass Number
Element Symbol
Ex.
Na-23 or Sodium-23
C-14 or Carbon-14
F-19 or
B. Isotopes
• Atoms of the same element with different
mass numbers.
 Nuclear symbol:
Mass #
Atomic #
12
6
 Hyphen notation: carbon-12
C. Johannesson
C
B. Isotopes
C. Johannesson
© Addison-Wesley Publishing Company, Inc.
B. Isotopes
• Chlorine-37
– atomic #:
– mass #:
– # of protons:
– # of electrons:
– # of neutrons:
17
37
17
17
20
C. Johannesson
37
17
Cl
C. Relative Atomic Mass
•
12C
atom = 1.992 × 10-23 g
 atomic mass unit (amu)
 1 amu = 1/12 the mass of a
12C
atom
 1 p = 1.007276 amu
1 n = 1.008665 amu
1 e- = 0.0005486 amu
C. Johannesson
© Addison-Wesley Publishing Company, Inc.
D. Average Atomic Mass
• weighted average of all isotopes
• on the Periodic Table
• round to 2 decimal places
Avg.
Atomic
Mass
(mass)(% )  (mass )(% )

100
C. Johannesson
D. Average Atomic Mass
• EX: Calculate the avg. atomic mass of oxygen if its
abundance in nature is 99.76% 16O, 0.04% 17O, and
0.20% 18O.
Avg.
(16)(99.76 )  (17)(0.04)  (18)(0.20)
 16.00
Atomic 
100
amu
Mass
D. Average Atomic Mass
• EX: Find chlorine’s average atomic mass if
approximately 8 of every 10 atoms are chlorine-35
and 2 are chlorine-37.
Avg.
Atomic
Mass
(35)(8)  (37)(2)

 35.40 amu
10
C. Johannesson