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The Atom and
Elements
Democritus (460-370 BC)
Proposed that matter was made of small particles
he called atoms. In Greek this means indivisible
or cannot be divided. He believed different atoms
would vary in size and would be in constant motion.
Extremely small
particles in motion
John Dalton (1766-1844)
Published the atomic theory:
1. Elements were composed of atoms.
2. Atoms of a given element are identical.
3. Atoms of different elements have different properties.
4. Atoms don’t change, but can be combined and
rearranged with other atoms.
5. Compounds are formed when atoms of more than one
element combine.
Joseph John Thomson (1856-1940)
Identified the electron, which carries a negative
charge. He thought that electrons were embedded
in the atom like raisins in raisin bread.
Atom as
solid object
Atom as
solid object with
a positive center and
electrons embedded
in the atom
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Ernest Rutherford (1871-1937)
Used high speed lightweight atoms called
alpha particles to bombard very thin gold foil.
Most of these alpha particles passed through
the gold foil. The fact that these particles went
through the foil lead to his theory that atoms
have mostly empty space.
Alpha particles
went through
spaces.
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Neils Bohr (1885-1962)
Described the electrons moving around the nucleus in
fixed orbits. Each orbit has a set amount of energy. We
use this model for a basic understanding of the atom’s
structure.
However, more recent research has shown that
electrons move around the nucleus in waves rather
than elliptical orbits. Electrons are better represented
as an “electron cloud.”
James Chadwick (1891-1974)
Solved the problem of “missing mass” in the atom by
discovering the neutron.
Electrons move so fast
that scientists prefer the
atom model where
electrons are represented
by a cloud.
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Basic Structure of an Atom
Proton
Positive Charge
Neutron
++
1 AMU
1 AMU
1 AMU
1 AMU
No Charge
Electron
Negative Charge
AMU is an atomic mass unit.
Electrons do not
have much mass
compared to protons
and neutrons.
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5
What Do You Notice?
1
H
1.01
3
4
2
He
4.0
5
6
7
8
9
10
Ne
Li
Be
B
C
N
O
F
6.94
9.01
10.81
12.01
14.01
16.00
19.00
11
12
13
14
15
16
17
Na Mg
22.99
24.31
19
20
K
Ca
39.10
40.08
Al
26.98
20.18
18
Si
P
S
Cl
Ar
28.09
30.97
32.07
35.45
39.95
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Element Name
Beryllium
Atomic Number
4
(# of protons and electrons)
Element Symbol
Atomic Mass
(# of neutrons and protons)
Be
9.01
Atomic Mass - Atomic Number = # of neutrons
+
+
++
9-4=5
5 Neutrons
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Mass Number
Mass number is the number of
protons and neutrons in the nucleus
of an isotope: Mass # = p+ + n0
p+
n0
e- Mass #
8
10
8
18
Arsenic - 75
33
42
33
75
Phosphorus - 31
15
16
15
31
Nuclide
Oxygen - 18
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What are the numbers of protons, electrons and neutrons?
Nitrogen
7
N
P=7
E=7
N =7
14.01
9
What are the numbers of protons, electrons and neutrons?
Fluorine
9
F
P=9
E=9
N = 10
19.00
10
What are the numbers of protons, electrons and neutrons?
Chlorine
17
Cl
P = 17
E = 17
N = 18
35.45
11
15 18
3
4
17
14
1
2
P=
N=
5
8
Energy Level
Max # of
Electrons
1
2
2
8
3
8
11
12
10
7
9
16
13
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The Bohr Model
Valence Electrons – The electrons on the outer shell
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Complete Symbols
• Contain the symbol of the element,
the mass number and the atomic
number.
Mass
Superscript →
number
Subscript →
Atomic
number
X
Symbols
 If an element has 78
electrons and 117 neutrons
what is the
a) Atomic number
b) Mass number
c) number of protons
d) complete symbol
Symbols
 If an element has 91
protons and 140 neutrons
what is the
a) Atomic number
b) Mass number
c) number of electrons
d) complete symbol
Symbols

If an element has an atomic
number of 34 and a mass
number of 78, what is the:
a) number of protons
b) number of neutrons
c) number of electrons
d) complete symbol
Symbols

Find each of these:
a) number of protons
b) number of
neutrons
c) number of
electrons
d) Atomic number
e) Mass Number
80
35
Br
Ions
• An atom or bonded group of atoms with a positive
or negative charge.
-1
Cl
PO
+2
Mg
-3
4
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Measuring Atomic Mass
• Instead of grams, the unit we use
is the Atomic Mass Unit (amu)
• It is defined as one-twelfth the
mass of a carbon-12 atom.
– Carbon-12 chosen because of its isotope purity.
• Each isotope has its own atomic
mass, thus we determine the
average from percent abundance.
Elements usually have differing amount of neutrons.
These different forms are called isotopes.
Let’s take hydrogen as an example:
As you can see from the atomic mass
(1.01) the most naturally abundant form
of hydrogen does not have a neutron.
However, in rare instances isotopes form.
Below are the isotopes of hydrogen:
1
Hydrogen
H
1.01
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Protium
Deuterium
Tritium
…. are isotopes of hydrogen.
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Isotopes are atoms of the same element having
different masses, due to varying numbers of
neutrons.
Isotope
Hydrogen–1
(protium)
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium)
Protons Electrons
Neutrons
1
1
0
1
1
1
1
1
2
Nucleus
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- Page 117
Question
Knowns
and
Unknown
Solution
Answer
22
add or subtract
a proton
ATOM
add another
atom
Add or subtract
A neutron
Add or subtract
an electron
Terms
New Element
Ion
Isotope
Molecule
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