Isotopes
• Atoms of
the same
element
have the
same
atomic
number
• Elements
have
Isotopes
• Isotopes are atoms that have the
same number of protons, but
different numbers of neutrons.
• They can be a radioactive form
of an element.
– Atoms of the same element all have the
same number of protons.
– Isotopes of the element have different
numbers of neutrons.
• How to
name
isotopes?
• How to
write the
isotope
symbol?
• To name isotopes use the name of
the element followed by the
isotope’s mass number.
Carbon 14 Protons =
Neutrons =
Electrons =
Carbon 12
Protons =
Neutrons =
Electrons =
Writing isotopes
in the language of chemistry
• Example
of
Isotopes
of
chlorine
atom
Isotopes of chlorine
Mass number
35Cl
Mass number
37Cl
Atomic number
17
Atomic Number 17
chlorine 35
chlorine 37
Protons =
Protons =
Neutrons =
Neutrons =
Electrons =
Electrons =
• To name isotopes, use the name of
the element followed by the
isotope’s mass number
Learning Check
An atom has 14 protons and 14 neutrons.
1.
Its atomic number is
a) 14
b) 0
c) 28
2. Its mass number is
a) 14
b) 0
c) 28
3. The element is
a) Si
b) Ca
c) Se
4.
Another isotope of this element is
a) 28X
16
b) 34X
14
c) 29X
14
Atomic Mass
• What is
the
atomic
mass?
• Listed on the periodic table
Na
22.99
• Gives the mass of “average” atom
of each element compared to 12C
• Average atom based on all the
isotopes and their % abundance.
• Atomic mass is not a whole
number
Masses of Atoms
• Why is
 A scale was designed for atoms to give
the
their small atomic masses in atomic
atomic
mass units (amu)
mass not
 An atom of 12C was assigned an exact
a whole
mass of 12.00 amu
number?
 The atomic mass is the average mass of
all the isotopes of the element and their
percent abundance.
• Calculating
Atomic
Mass
Need the Percent(%)
abundance of isotopes
Also, need the Mass of each
isotope of that element
Atomic mass=
mass isotope1(%) + mass isotope2(%) + …
100
100
Learning Check 7
Gallium is a metallic element found in small
lasers used in compact disc players. In a
sample of gallium, there is:
60.2% of gallium 69 atoms
39.8% of gallium 71 atoms
What is the atomic mass of gallium?
Solution
Ga 69 (%/100) 69Ga
69 amu x 60.2 =
41.5 amu
100
Ga 71 (%/100) 71Ga
71 amu x 39.8 =
28.3 amu
100
Atomic mass Ga = 69.8 amu