THE NATURE OF MATTER
Chapter # 02
Define nucleon number
Define and state isotope of same atoms with
different nucleon number
ATOMIC STRUCTUREATOMIC STRUCTURE
the number of protons in an atom
the number of protons and
neutrons in an atom
number of electrons = number of protons
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
mass number is the total number
of protons and neutrons in an
Number of neutrons in the
oxygen atom is: 16 – 8 = 8.
Interpreting Chemical Symbol
Definition of Isotopes
Isotopes are atoms of the same
element which contains the same
number of protons but different
number of neutrons.
Oxygen-16 Oxygen-17 Oxygen-18
n = 8 n = 17 – 8
n = 18 – 8
Isotopes have the same chemical
properties but slightly different
Q. Do all isotopes of the same element have the
Isotopes of Some Elements and Their
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.
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
Ar=(Average atomic mass)
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
Stable vs. Unstable
• A stable isotope does NOT
undergo radioactive (or nuclear)
• An unstable isotope undergoes
radioactive (or nuclear) decay.
Unstable isotopes are also known
as radioisotopes or radionucleides
Uses of Radioactive Isotopes
• Food irradiation
• Archaeological dating (carbon-dating)
• Smoke detectors
• Radioactive tracers
• Stable isotopes:
• They do not naturally decay but exist in
natural materials in differing proportions
• Unstable iso tope
• Isotopes that continously break down into
lower atomic weight isotopes