R-12
Atomic Structure Notes
I. Atoms

The atom is the ________________unit of matter.

Smallest particle of an element that retains the ____________________ of that element.

Three basic subatomic particles:
XXXXX
Charge
Mass Number
Location
Protons
Neutrons
Electrons
 The atom is divided into two separate areas, the _______________ and the ________________ _______________.
1. Nucleus
 Found in _______________ of an atom
 _______________and _______________
 Contains _______________ and _______________, so is _______________ charged
 Contains most of the _______________ of an atom
 The number of_______________ is ALWAYS the_______________ .
 The number of_______________ can _______________.
2. Electron Cloud
 Located _______________ the nucleus

_______________ dense
 Contains _______________, so is _______________ charged
 Mass of electrons is _______________
 _______________ _______________ _______________ !
 Each box on the periodic table provides three important pieces of information:
1. Symbol
2. Atomic Number:
 The number of _______________ in the nucleus
 The number of protons _______________ the element
 Examples:
 Because atoms are electrically _______________, each atom contains the _______________ number of
protons as electrons
 Examples:
3. Atomic Mass (Mass Number):
 Atomic Mass = # protons __________ # neutrons
 Examples:

_______________________
II.
X
_______
Ions
 An ion is a _______________ particle
 # of protons __________ __________ __________ the # of electrons
 The only way to form an ion is to _______________ the # of _______________
 Positive ion (_______________): #electrons _____ #protons
 Negative ions (_______________): #electrons _____ #protons
 Examples #1:
 Example #2:
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C
 Remember: Electrons control behavior in ordinary chemical reactions!
III. Isotopes

Atoms of the same element that differ from each other by the number of ____________ they contain

Most elements have at least _____ naturally occurring isotopes

Isotopes of the same element:
1) Have the _______________ atomic number
2) Have _______________ mass numbers
3) Have _______________ behavior
 To write an isotope (nuclear) symbol:
Must contain:
1) Element symbol
2) Mass of THAT isotope
3) Atomic number
Examples:
 To name an isotope using a hyphen notation symbol:
Must contain:
1) Element name
2) Dash
3) Mass of THAT isotope
Examples:
IV. Relative Abundance and Average Atomic Mass:
 The naturally occurring isotopes of each element are present in specific amounts known as
_______________ _______________
 The ______________ the % Abundance, the more _______________ the isotope.
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 _______________ ____________ _________ (mass # on the periodic table) is a WEIGHTED average of the
mass and abundance of all of the naturally occurring isotopes of an element.
 To calculate average atomic mass:
1) Multiply mass and % abundance for each isotope.
2) Add all answers from step # 1 to determine average atomic mass
***HINT:
Your final answer should equal Mass # on the periodic table.
Example 1:
Boron has two naturally occurring isotopes. Calculate the average atomic mass.
Example 2:
Two isotopes of copper occur in nature. 69.17% of copper atoms have a mass of 62.94 amu and 30.83% have a
mass of 64.93amu. What is the average atomic mass?
Example 3:
Uranium has 3 isotopes with the following masses and % abundance. Calculate its average atomic mass.
234 U
92
0.0058%
235 U
92
0.71%
238 U
92
99.23%
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