Chapter 4
Section 4
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There are about 118 known types of atoms.
Each element has it’s own type of atom.
All atoms of an element have to have one
common factor, just like all cells of a particular
person will have the same DNA.
The identifying factor of an atom is the
ATOMIC NUMBER.
The atomic number is the number of p+ in the
atom.
Neutron
e-
n
P+
Proton
Electron
Positive
charge
No
charge
Negative
Charge
1amu
1amu
0amu
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contains protons and neutrons
The number of protons in all atoms of the same
element will be the same
is positively charged
Contains all of the mass of the atom
Each proton and each neutron will add 1amu
to the mass of the atom
The atomic number (found on the periodic
table) is the number of protons
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The atomic mass of an atom is equal to the number
of protons + the number of neutrons
Atomic mass = p+ + n
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To find the number of neutrons, subtract the
atomic number from the atomic mass
Ex: Phosphorus
Phosphorus
atomic number = 15
15 p+
16n
mass # = 31
number of neutron = 31-15
16 neutrons
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Atoms that have the same number of p+ but
different numbers of n
Neon-20
Neon-21
Neon-22
# p+
#n
Mass #
# eWhen an element name or symbol appears as
name-x, the x stands for the mass number of
that isotope.
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The number appearing on the periodic table is
actually an average atomic mass
This number is the weighted average of the
masses of the isotopes of an element
This number accounts for the relative
abundance of different isotopes of the element
1.
2.
Multiply the mass of each isotope by its
natural abundance, expressed as a decimal.
Add the products.
Example: Element X has two natural isotopes. The isotope with a mass of
10.012amu has a relative abundance of 19.91%. The isotope with a mass of
11.009amu has a relative abundance of 80.09%. Calculate the atomic mass of
this element.
Isotope
Mass x
Abundance as Product
a Decimal
=
X-10
10.012
.1991
1.993amu
X-11
11.009
.8009
8.817amu
Add the Products 10.810
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Multiply your percentage per category by the weight of
each category expressed as a decimal.
Add your answers.
Homework
Lab
Quiz
Tests
10%
25%
25%
40%
60%
75%
86%
50%
Category
Weight of
Grade in
Category as a Category
Decimal
Product
HW
.10
60
6
Lab
.25
75
18.75
Quiz
.25
86
21.5
Test
.40
50
20
Add the products! 66.25%
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Multiply the mass of the isotope by the percent
abundance (expressed as a decimal)
Repeat the first step for each isotope
Add the products of the answers from your
first two steps.
Isotope
Mass of the
Isotope
%
abundance
(as a
decimal)
Boron - 10
10.013
.199
Boron - 11
11.009
.801
Average Atomic Mass
Product