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4.2
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ATOMIC NUMBER
Elements are different because they contain different numbers of protons.
Atomic number = # protons = # electrons in a neutral atom
Atomic number
Chemical symbol
Chemical nameAverage atomic mass
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82 82
O 8 8
30 30Zn
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Atomic Mass Unitsamu: atomic mass units
Unit used to measure the mass of VERY SMALL particles (atoms)
Standard is the carbon-12 atom
1 carbon-12 atom = 12 amu
1 amu = 1/12 carbon-12 atom mass
1 proton = 1 amu
1 neutron = 1 amu
1 atomic mass unit =1.66053892 × 10-24 grams
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Mass Number
Mass Number – Mass of a specific atom
Mass number = # protons + # neutrons
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REVIEW:
Atomic Number:
The total number of protons
Atomic number = # protons = # electrons
Mass Number:
The total number of protons and neutrons
Mass number = # protons + # neutrons
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Isotopes
• Each element may have different numbers of neutrons => ISOTOPES
• The mass number is different for different isotopes.
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Isotopes – Naming (Two ways)
• Element name – mass number
• Ex. carbon-12 or neon-22
• Using the element symbol:
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Isotopes - Naming
• How would you write Uranium-235 using the symbol? (atomic number = 92)
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Atomic MassThe weighted average mass of the isotopes of that element.
*different than mass number
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Calculating Average Atomic Mass All isotopes of an element do not exist in equal abundance in the world
Percent abundance in world:
90.48% 0.27% 9.25%
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Example #1
Calculate the average atomic mass of NeonAtomic Mass = (% abundance A)(mass A) +
(% abundance B)(mass B) + ….
Isotope Mass (amu) (M)
Percent abundance (P)
M x P÷100
Neon-20 20.0 90.48%
Neon-21 21.0 0.27%
Neon-22 22.0 9.25%
Average atomic mass (amu)
18.1
0.057
2.03
20.2
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Atomic MassWhich isotope is most abundant?
If Neon has 3 isotopes:
Neon-20, Neon-21, Neon-22,
Weighted average atomic mass is 20.2 amu.
Neon-20
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Example #1
Calculate the average atomic mass of NeonAtomic Mass = (% abundance A)(mass A) +
(% abundance B)(mass B) + ….
Another way to write it out:
Atomic Mass = (90.48/100)(20.0) +
(0.27/100)(21.0) +
(9.25/100)(22.0)
= 20.2 amu
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Example #1
Calculate the average atomic mass of LithiumAtomic Mass = (% abundance A)(mass A) + (% abundance B)(mass B) + ….
Isotope Mass (amu) (M)
Percent abundance (P)
M x P÷100
Lithium-6 6.01 7.5%
Lithium-7 7.01 92.5%
Average atomic mass (amu)
0.45
6.49
6.94
OR
Atomic Mass = (7.5/100)(6.01) + (92.5/100)(7.01) = 6.94 amu
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Example #3
Calculate the average atomic mass of CesiumAtomic Mass = (% abundance A)(mass A) + (% abundance B)(mass B) + ….
Isotope Mass (amu) (M)
Percent abundance (P)
M x P÷100
Cesium-132 132.0 75.0%
Cesium-133 133.0 20.0%
Cesium-134 134.0 5.0%
Average atomic mass (amu)
99.0
26.4
6.7
132.1
OR Atomic Mass = (75.0/100)(132.0)
+ (20.0/100)(133.0) + (5.0/100)(134.0) = 132.1 amu
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Questions
1) What particles make up an atom?
Protons, Neutrons and Electrons
2) What are the charges on these particles?
Protons (+), Neutrons (none), Electrons (-)
3) What particles make up the nucleus?
Protons and Neutrons
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More Questions:
Atomic Number tells us:
The identity of the element; the number of protons and electrons
Atomic Mass tells us:
The average mass in amu of all naturally occuring isotopes
Mass
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The Development of Atomic Models
The timeline shoes the development of atomic models from 1803 to 1911.
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End Show
Slide 20 of 18
© Copyright Pearson Prentice Hall
Defining the Atom > The Development of Atomic Models
The timeline shows the development of atomic models from 1913 to 1932.
5.1