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Democritus (400 B.C.)
•
World is made of 2
things
empty space
“atoms”
Big Players in Atomic Theory
•
•
•
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The Greeks (staring Democritus)
John Dalton (1808)
J.J. Thompson (1898)
Rutherford (1909)
Milliken (1909)
More History
Ernie’s (Rutherford) Big Adventure
Thin Sheet of Gold
Atoms
Rutherford’s Conclusion
• Atoms are made of
mostly EMPTY SPACE!
– Actually, the ratio of the size
of the nucleus to the diameter
of the orbits of electrons can
be compared with placing a
marble in the middle of a
football stadium!
LAVOISIER
Found out that the
mass of the products
must equal the mass
of the reactants
(in a closed system)
PROUST
LAW OF DEFINITE
PROPORTIONS
Specific substances
always contain
elements in the same
ratio BY MASS
DALTON
LAW OF MULTIPLE
PROPORTIONS
The ratio of masses of 1
element that combine with a
constant mass of another
element can be expressed in
whole numbers
Two major parts of an atom
Nucleus
(not to scale)
Electron
Cloud
Three Major
Sub-Atomic Particles
• Protons
• Neutrons
• Electrons
PROTON
+
(p )
a single, relatively large
particle with a
positive charge that is
found in the nucleus
THE PROTON
• Fat
(heavy)
+
p
• Positive 
(charge)
• Doesn’t move
(lazy)
NEUTRON (N°)
a single, relatively large
particle with a
neutral charge that is
found in the nucleus
THE NEUTRON

°
N
• Fat
(heavy)
• Neutral 
(charge)
• Doesn’t move
(lazy)
ELECTRON
(e )
a single, very small
particle with a
negative charge that is
found in a “cloud”
around the nucleus
THE ELECTRON
• Skinny
(very light)
e
• Negative 
(charge)
• Moves a lot
(runs around)
Review: Subatomic Particles
+
p

°
N
e-
Please complete the following table
Protons
Neutrons Electrons
Where are
they found?
Nucleus
Nucleus
Electron
Cloud
Mass
Heavy
Heavy
Very Light
Charge
(attitude)
Positive 
Neutral 
Negative 
ATOMIC MASS #
(A)
The total mass of all of
the subatomic particles
in an atom
(but really # of protons and
neutrons)
ATOMIC NUMBER
(Z)
the number of protons
in an atom
(assuming the atom is
+
neutral, # of p = # of e )
Example: Sodium
Atomic Mass # =
+
p &
22.99
Na
11
Atomic # = # of protons
°
N
Another Notation
Atomic Mass # =
+
p &
Atomic # = # of protons
°
N
To calculate the number
of neutrons, subtract the
atomic number (smaller)
from the atomic mass
number (larger)
A – Z = # of neutrons
Ex: How many neutrons
does Sodium have?
Mass # - Atomic # = #N°
(You may need to round the atomic #)
22.99
Na
11
23 - 11 = 12
N°
ION
Atoms of the same
element that differ in
charge.
(They have the same # of
+
p , but different # of e )
Positive Ions Negative Ions
(cations)
(anions)
•
2+
• Ca (lost 2 e )
3+
• Al (lost 3 e )
4+
• Pb (lost 4 e )
+
• H (lost 1 e )
•
2• O (gain 2 e )
3• P (gain 3 e )
2• S (gain 2 e )
• OH (gain 1 e )
+
Na (lost 1 e )
Cl (gain 1 e )
If an atom GAINS
electrons, its overall charge
becomes more negative.
If it LOSES electrons, its
charge becomes more
positive
ISOTOPE
Atoms of the same
element that differ in
mass.
(They have the same # of
+
p , but different # of N°)
Isotopes are
CHEMICALLY the
SAME as atoms, but
DIFFER PHYSICALLY
because they have
different masses.
A few examples of isotopes…
Complete the following table
Protons
Na+
Br w/ mass
84
O2- with
mass 13
Neutrons
Electrons
So, why do the elements
on the PT have masses
with decimals???
AVERAGE ATOMIC
MASS
The average mass of all
of the isotopes of a
substance
Example using
exam scores!
To calculate the average
atomic mass:
1. Mass each isotope and
multiply by # present
2. Add these products
3. Divide this sum by the
total # present
To calculate the
average atomic mass:
1. Multiply the mass of each
isotope by the % of the
isotope
2. Add the products
Carbon has two isotopes.
99% of carbon has a mass of
12 amu, and 1% has a mass of
13 amu. Calculate the
average atomic mass.
12.01 amu
76% of chlorine has a mass of
35 amu. The other 24% has a
mass of 37 amu. Calculate
the average atomic mass of
chlorine.
35.48 amu
Sulfur has three main
isotopes. 95% is Sulfur-32,
4% is Sulfur-34, and 1% is
Sulfur- 33. Calculate the
average atomic mass of
sulfur.
32.09 amu