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Isotope Note Handout
Atom Review
All matter is made up of elements (e.g. carbon, hydrogen, etc.).
The smallest part of an element is called an atom.
Atom of different elements contain different numbers of protons.
The mass of an atom is almost entirely due to the number of protons and neutrons.
The Atom
The atom consists of two parts:
1. The nucleus which contains:
2. Orbiting electrons.
(Draw the atom in the box and label)
Isotopic Notation
Mass number = __________________________________________________
Atomic number = ________________________________________________
(Draw the isotopic notation in the box of element X)
A= ________________________________
Z= ________________________________
A-Z= _______________________________
ISOTOPES
THE NUMBER OF _________________ FOR A GIVEN TYPE OF ATOM
_____________________________________________
THE NUMBER OF ____________________________ CAN CHANGE.
TWO ATOMS WITH THE SAME NUMBER OF PROTONS BUT DIFFERENT
NUMBERS OF NEUTRONS ARE CALLED _________________________.
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ISOTOPES
ISOTOPES ARE ATOMS WITH THE SAME NUMBER OF PROTONS BUT
____________________________ NUMBER OF _____________________________.
Examples of Uranium Isotopes
235
238
U
A
Z
Number of
protons
Number of
neutrons
U
A
Z
Number of
protons
Number of
neutrons
Atomic Weight
To calculate the atomic weight (average atomic mass) for a particular element we must
consider how much of one isotope of an element exists versus another isotope of the
same element. These are the "natural" abundances (% abundance) on earth.
Rule for Calculating Atomic Weight
To calculate the average atomic weight, each exact atomic weight is multiplied by its
percent abundance (expressed as a decimal). Then, add the results together and round off
to an appropriate number of significant figures.
Example #1: Nitrogen
mass number
exact weight
percent abundance
14
14.003074
99.63
15
15.000108
0.37
This is the solution for nitrogen:
(14.003074) (0.9963) + (15.000108) (0.0037) = 14.007
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Practice Problem # 1
Calculate the average atomic weight for magnesium:
mass number
exact weight
percent abundance
24
23.985042
78.99
25
24.985837
10.00
26
25.982593
11.01
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