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4.2
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
82 82
O 8 8
30 30Zn
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
Mass Number
Mass Number – Mass of a specific atom
Mass number = # protons + # neutrons
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
Isotopes
• Each element may have different numbers of neutrons => ISOTOPES
• The mass number is different for different isotopes.
Isotopes – Naming (Two ways)
• Element name – mass number
• Ex. carbon-12 or neon-22
• Using the element symbol:
Isotopes - Naming
• How would you write Uranium-235 using the symbol? (atomic number = 92)
Atomic MassThe weighted average mass of the isotopes of that element.
*different than mass number
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%
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
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
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
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
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
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
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
The Development of Atomic Models
The timeline shoes the development of atomic models from 1803 to 1911.
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