DISTINGUISHING AMONG ATOMS
IaN pg. 7
MAIN IDEAS
Key Concepts
1. What makes one element
different from another?
1. Atomic Number 2. How do you find the
2. Mass Number
number of neutrons in an
3. Isotopes
atom?
4. Atomic Mass
3. How do isotopes of an
element differ?
5. Periodic Table
4. How do you calculate the
atomic mass of an
Success is dependent upon
element?
effort.
5. Why is a Periodic Table
Sophocles
useful?
Distinguishing Among Atoms
Atoms are Neutral
The Periodic Table gives us information
about atomic structure…
Atomic Number
• The number of protons is what distinguishes
one element from another.
• The periodic table is arranged in order of
atomic number.
Sometimes you find elements
represented like this…
OR
Element
Symbol
He - 4
Mass
number
Mass Number = total # of protons & neutrons in
the nucleus.
# of Neutrons = Mass # - Atomic #
• Example:
How many neutrons are there in one atom of
24
12
Mg?
• Solution:

Mass number = 24
Atomic number = 12
Number of neutrons = mass number – atomic number
= 24 – 12 = 12
Fill in the rest of the chart…
Symbol of
the Element
Atomic
Number
Mass
Number
Number of
Electrons
Ba
Number of
Neutrons
82
8
10
33
Ca
Number of
Protons
40
42
•Now for some
perspective…
• How Small is an Atom?
Isotopes
• Isotopes are elements that have the same
number of protons, but different numbers of
neutrons.
• Isotopes and their percent abundance are
used to determine the atomic mass of an
element.
Atomic Mass
Mass Number = Mass of the nucleus
Atomic Mass = Weighted average mass of the
atoms in a naturally occurring sample.
• Mass
• Abundance of Isotopes
AMU – Atomic Mass Unit standard 1/12 the
mass of Carbon – 12.
Calculating Atomic Mass
Required Variables:
1. # of Stable Isotopes of the element
2. Mass of each Isotope
3. Percent abundance of each Isotope
(amu X %) + (amu X %) = atomic mass
For each Isotope of the element.
• For example:
– Carbon-12 makes up about 98.89% of all carbon
on Earth.
– Carbon-13 makes up about 1.11%
– Carbon-14 is only about 1x10-12%
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
• Carbon’s mass on the periodic table is 12.01amu.
mass numbers
Percents written as
decimals
• This is found by: (12 x .9889) + (13 x .0111) + (0)*
= 12.01amu
*Carbon-14’s contribution is too small to matter.