1
History and Structure of
the Atom
From Democritus to...
History of Atomic Theory
2
Democritus (from about 440 BC)
 coined the term atom which means uncuttable
 He felt that if you kept cutting matter smaller and
smaller eventually you will no longer be able to
cut any further.

John Dalton (1766-1844)
3
Felt that an atom was indivisible
 spherical in shape
 Model: Sphere
 Analogy: Billiard ball

Dalton’s Postulates
4
Atoms are tiny, indivisible particles.
 Atoms of the same element are identical.
 Atoms of different elements can chemically
combine.
 Chemical reactions occur when atoms are
separated, joined or rearranged.
 Atoms of one element can never be changed to
atoms of another element.

Size of an atom
5

Pure copper coin the size of a penny = 2.4X1022 atoms

Earth’s population is about 6 X 109 people
JJ Thomson (1856-1940)
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Discovered electrons
 Felt that an atom was negatively charged
particles floating in a positive soup
 Model: Charges floating around
 Analogy: Raisin bun or Plum Pudding

Ernest Rutherford (1871-1937)
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Discovered the nucleus, the proton and
first split an atom
 Model: An atom was a small positively
charged nucleus surrounded by electrons
orbiting around it.
 Analogy: A beehive

Rutherfords Famous Experiment
8

Called the Gold Foil Experiment
Alpha
particles were fired at thin gold foil
A detector encircled the foil and lit up when hit with
alpha particles.
If the plum pudding model were true it was expected
most particles would go straight through the foil with
only slight deflection.
However, in the experiment, some particles were
deflected back at a sharp angle proving the existence
of a small, dense, and positively charged nucleus.
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Rutherfords results
Plum pudding expected
results. Particles go
through.
Text
Actual results. Some are
deflected back due to a
nucleus
Another view of the experiment
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Niels Bohr
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Worked out details of atomic structure.
Notably orbital layers. Solved problems
related to Rutherfords model.
 Model: Electrons orbit in rings at
different distances from the nucleus.
 Analogy: Planets orbiting the sun

Erwin Schroedinger (1887-1961)
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Major discoveries in quantum mechanics. Nature
of electrons in atoms.
 Model: Electrons exist in a probability distribution
around the atom. Kind of like a cloud.
 Analogy: A spinning fan blade.

Quick Quiz: Historical Atom
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1. What did Dalton say an atom was like?
1. A billiard ball, or a pool table ball
2. What did JJ Thomson say an atom was like?
2. Raisin bun
3. What did Rutherford say an atom was like?
3. A beehive
4. What are the 3 parts of an atom?
4. Proton, Neutron, Electron
Parts of the Atom
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The Nucleus
Electrons
Parts of the Atom
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Electrons
Mass=0
Charge=-1
Protons
Mass=1
Charge=+1
The Nucleus
Neutrons
Mass=1
Charge=0
Atomic Structure
An atom is considered electrically neutral.
Electrically neutral means the number of
protons (+) = the number of electrons (-)
4 red protons = 4 blue electrons
Atoms on the Periodic Table
17
Atomic # = Number of protons
3
Be
7.0
neutrons
+ protons
Atomic mass
Nuclear Symbol
You must know how to find:
A.
B.
# of protons = atomic number
mass # = # of n0 + # of p+ (atomic #)

C.
What’s in the nucleus of the atom
# of electrons = # of protons (in a neutral atom)
Boron
5
atomic number
B
(Not the same as the mass #)
10.811
atomic mass
16. What is the mass number?
The mass of the entire atom!
 The sum of the protons and neutrons in an atom.
 Electrons are not included (too small)

17. What is the atomic mass?

The weighted average of the masses of all
naturally occurring isotopes.
How to write a Nuclear Symbol
Element Symbol
Mass Number
= p+ + n0
Atomic Number
= p+
11
5
Charge if ion
-3
To Determine Other Numbers
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To find
Proton #
Do this
same as atomic number
Electron #
same as atomic number for
neutral atoms
Neutron #
=atomic mass-atomic number
Mass
=Prot # + Neut#
Nuclear Symbol Examples
35
17
Cl
Atomic Number
Mass Number
17
27
12
35
Mg
Atomic Number
12
Number of
Protons
17
Number of
Neutrons
18
Number of
Electrons
17
+2
Mass Number
27
Number of
Protons
12
Number of
Neutrons
15
Number of
Electrons
10
Nuclear Symbol Examples
40
20
+2
Ca
Atomic Number
Mass Number
20
17
8
O
Atomic Number
8
40
Number of
Protons
20
Number of
Neutrons
20
Number of
Electrons
18
-2
Mass Number
17
Number of
Protons
8
Number of
Neutrons
9
Number of
Electrons
10
Try This
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Atomic #
Element Symbol
Be
Na
Fe
I
Atomic Mass
Fill in the blanks
26
Element
Atomic #
e- #
Mass #
Neut #
16
8
12
P
15
Foldable
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Ions - an atom that has lost or gained
an electron(s)
The charge on an ion indicates an imbalance between protons
and electrons.
If the atom GAINS electrons it will have a negative charge.
(more e- than p+)
If the atom LOSES electrons it will have a positive charge.
(more p+ than e-)
Protons =
Protons = Electrons =
11
5
B
11
5
+2
B
Electrons =
Isotopes
Isotopes are atoms of the same element
+
that have the same # p , but different
mass numbers.
Isotopes have different masses because
they have a different number of
neutrons.
Boron
5
B
10.811
atomic number
(average) atomic mass
19. How are isotopes written?
The element name with its mass number after it:
 Carbon-12
 Carbon-13
 Carbon-14

Learning Check
Naturally occurring carbon consists of three isotopes,
C-12, C-13, and C-14. State the number of protons,
neutrons, and electrons in each of these carbon
atoms.
12C
6
13C
6
14C
6
# p _______
_______
_______
#n _______
_______
_______
#e _______
#
_______
_______
The average atomic mass - weighted average of
all the naturally occurring isotopes of an
element.
Example:
A sample of cesium is 75% 133Cs, 20% 132Cs and
5% 134Cs. What is the average atomic mass?
Answer:
.75 x 133 = 99.75
.20 x 132 = 26.4
.05 x 134 = 6.7
132.85 = average atomic mass