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Isotopes
Atomic Number
• Atomic number = number of protons
– Which makes it always a whole number
• Atomic number is always the same for a given
element
• Can be found on the periodic table
Isotope
• Atoms of the same element with different
number of neutrons
• They have different mass numbers
• They have the same number of protons
• Nuclide
– General name for an isotope of an
atom
Mass Number
• Atoms of the same element may
have different numbers of
neutrons
• Mass number
– Sum of protons + neutrons
• Mass number can not be found on
the periodic table
– Not found by rounding atomic mass
– Given through words or symbols
Atomic Mass
• Weighted average of the masses of naturally
occurring isotopes of an element
• Can be found on the periodic table
• It is not usually a whole number
Calculating Average Atomic Mass
• Depends on both mass and relative
abundance
• How do you find average atomic mass?
– Naturally occuring copper consists of 69.15% copper-63 which has an
atomic mass of 62.929601 amu and 30.85% copper-65 which has an
atomic mass of 64.927794
– Multiply the atomic mass of each isotope by its relative
abundance(expressed as a decimal) and add the results
– 0.6915 x 62.929601 amu + 0.3085 x 64.927794 amu = 63.55 amu
Calculating average atomic mass
• Three isotopes of Argon occur in nature. Calculate the
average atomic mass of Argon to decimal places:
Argon-36 (35.97 amu; 0.337%)
Argon-38 (37.96 amu; 0.063%)
Argon-40 (39.96 amu; 99.600%)
Answer: 39.95 amu
Isotopes of Hydrogen
• Protium
• Deuterium
• Tridium
Isotopes of Hydrogen
faculty.weber.edu/bdattilo/shknbk/notes/time.htm
Isotopes
• Hydrogen isotopes are different because most
isotopes do not have their own specific name
• They are identified by their mass number
Illustrating isotopes/nuclides
Nuclear Notation:
12
Hyphen Notation:
C-12
6
C
Isotope Stability
• Stable nucleus
– a correct ratio of neutrons and protons should be
present in the nucleus.
• Unstable nucleus
– radioactive
• Elements 83-112  radioactive
• Elements 93-112  man-made
Problems – determine the # of p+, n,
and e- in each of these.
1. Sodium-23
1. 11 p+, 11 e-, 12 n
2. calcium-40
2. 20 p+, 20 e-, 20 n
3.
4.
64
29
108
47
Cu
Ag
3. 29 p+, 29 e-, 35 n
4. 47 p+, 47 e-, 61 n
Problems – write the nuclear notation and
hyphen notation for
• 5. mass number 28, atomic number 14
• 6. 26 protons and 30 neutrons
• 7. 56 electrons and 82 neutrons
Answers
28
• 5. 14Si
56
• 6. 26Fe
138
• 7. 56Ba
silicon-28
iron-56
barium-138