Download 3.3 Atomic Mass and Isotopes File

Ch. 3-3a
Distinguishing and Counting Atoms
POINT > Define Atomic Number
POINT > Define Mass Number
POINT > Describe and identify isotopes
POINT > Determine the number of neutrons in an
isotope
POINT > Define the atomic mass unit (U)
POINT > Calculate average atomic mass
POINT > Define Atomic Number
The atomic number is the number of
protons in the nucleus of an atom
6
C
Carbon
Ex. Carbon has 6 protons
Each element contains a different number of
protons. Therefore…
The atomic number identifies each element
POINT > Define Atomic Number
The Periodic Table is organized by
increasing atomic number
6
C
Carbon
Hydrogen has 1 proton, helium has 2 protons,
lithium has 3 protons, etc
6
POINT > Define Atomic Number
Protons have a charge of 1+
Atoms are neutral in charge
C
Carbon
the
So the atomic number also tells you how
many electrons there are (same as # of
protons)
WB CHECK:
What is the atomic number of aluminum (Al)?
What is the atomic number of cesium (Cs)?
How many electrons does an atom of oxygen
have?
POINT > Define Mass Number and isotopes
Recall that the mass of an atom is almost
entirely the protons and neutrons
The mass number is the total number of
protons and neutrons in an atom
The mass number usually isn’t shown on the
Periodic Table
POINT > Define Mass Number and isotopes
Mass number is represented one of two ways:
(Nuclear symbol)
or
gold-197
(Hyphen notation)
WB CHECK:
What is the mass number of this krypton atom?
How many protons does krypton have?
POINT > Define Mass Number and isotopes
Most elements have isotopes. Isotopes are
atoms of an element with different numbers
of neutrons
POINT > Define Mass Number and isotopes
Isotopes have different mass numbers due
to different number of neutrons present
Isotopes have the same atomic number
since the number of protons is not different
WB CHECK:
Carbon-12, carbon-13 and carbon-14 are
the isotopes of carbon. How many protons
does carbon-14 have?
How many electrons does carbon-12 have?
POINT > Define Mass Number and isotopes
The number of neutrons can be calculated:
Mass number - atomic number = # of neutrons
Ex.
Mass#
Atomic #
48
- 22
26 neutrons
WB CHECK:
How many neutrons in this krypton atom?
How many electrons does krypton have?
POINT > Define the atomic mass unit (U)
The atomic mass unit (amu or U) is a relative
quantity, with carbon-12 chosen as reference
Carbon-12 = 12U
Notice that 12 is the number of protons + neutrons
Protons and neutrons are very close to 1.0 U
POINT > Define the atomic mass unit (U)
Carbon-12 = 12U
So, 1/12 of a carbon-12 atom = 1 U
Atomic masses of other elements are determined
by comparison to carbon -12
POINT > Calculate atomic mass (atomic weight)
The average atomic mass (or atomic weight) is a
value that takes into account the abundance of
different isotopes. This number appears in the Table
1. We estimate the mass of protons and neutrons to be
=1.0 U
2. We weight the isotopes of an element by their
naturally occurring abundance
POINT > Calculate average atomic mass
Multiply the mass of each isotope by its natural
abundance (as a decimal). Then add the products.
Example: Chlorine has two isotopes, chlorine-35 and
chlorine-37
75.77% of all chlorine atoms are chlorine-35 and
24.23% are chlorine-37
POINT > Calculate average atomic mass
Example – Chlorine
17
Cl
Chlorine
Chlorine-35
Chlorine-37
(35u x 0.7577)
+ (37u x 0.2423)
35.453
=
= 35.48u
Actual value using AMU = 35.45u
WB CHECK:
Lithium has two isotopes, Li-6 and Li-7.
92.5% is Li-7 and 7.5% is Li-6.
Calculate the atomic weight of lithium.
6.93u
 Read
pages 73-78
 Practice problems #1-3 page 76
 Determining Atomic Mass W.S.