THE NATURE OF MATTER
Chapter # 02
Topic :ISOTOPES
Objective
Define nucleon number
Define and state isotope of same atoms with
different nucleon number
ATOMIC STRUCTURE
Nucleus
Electron
Orbit
Energy Levels
ATOMIC STRUCTURE
Atomic number
the number of protons in an atom
Atomic mass
the number of protons and
neutrons in an atom
2
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He
number of electrons = number of protons
Interpreting Chemical Symbol
Mass=P+N
mass number
(nucleon number)
mass number is the total number
of protons and neutrons in an
atom.
Number of neutrons in the
oxygen atom is: 16 – 8 = 8.
proton number
(atomic number)
P=E
No. of
protons= no.
of electrons
16
O
chemical symbol
8
atomic number (proton number) is the number of
protons in an atom.
Each oxygen atom has 8 protons.
As the number of electrons is equal to the number of
protons in an atom, each oxygen atom contains 8
electrons.
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Definition of Isotopes
Isotopes are atoms of the same
element which contains the same
number of protons but different
number of neutrons.
Isotopes
mass
number
proton
number
16
17
18
8
8
8
O O O
Oxygen-16
n = 8
Oxygen-17
n = 17 – 8
= 9
Oxygen-18
n = 18 – 8
= 10
Isotopes
Isotopes have the same chemical
properties but slightly different
physical properties
Q. Do all isotopes of the same
same mass?
element have the
Isotopes of Some Elements and Their
Atomic Mass
The atomic mass of an element
• is listed below the symbol of each element on
the periodic table.
• Gives the mass of an “average” atom of each
element compared to 12C.
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The calculation for atomic mass requires the
• percent(%) abundance of each isotope.
• atomic mass of each isotope of that element.
• sum of the weighted averages.
mass of isotope(1)x (%) + mass of isotope(2) x (%) =Ar
100
100
Ar=(Average atomic mass)
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24Mg
= 23.99 amu x 78.70/100
= 18.88 amu
25Mg
= 24.99 amu x 10.13/100
= 2.531 amu
26Mg
= 25.98 amu x 11.17/100
= 2.902 amu
Atomic mass (average mass) Mg
= 24.31 amu
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