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= # electrons in a neutral atom
ATOMIC NUMBER Elements are different because they contain different numbers of protons. Atomic number Chemical symbol Chemical name Average atomic mass Atomic number = # protons = # electrons in a neutral atom

82 82 O 8 8 Zn 30 30

1 neutron = 1 amu Atomic Mass Units amu: atomic mass units
× grams amu: 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

Mass Number – Mass of a specific atom
Mass number = # protons + # neutrons

The total number of protons
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 Mass The 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% % %

Average atomic mass (amu)
Example #1 Calculate the average atomic mass of Neon Atomic 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 Mass Neon-20 Which isotope is most abundant?
If Neon has 3 isotopes: Neon-20, Neon-21, Neon-22, Weighted average atomic mass is 20.2 amu.

Example #1 Calculate the average atomic mass of Neon Atomic 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

Average atomic mass (amu)
Example #1 Calculate the average atomic mass of Lithium Atomic 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

Average atomic mass (amu)
Example #3 Calculate the average atomic mass of Cesium Atomic 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) = 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: Atomic Mass 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. These illustrations show how the atomic model has changed as scientists learned more about the atom’s structure.

The Development of Atomic Models
5.1 The Development of Atomic Models The timeline shows the development of atomic models from 1913 to 1932. These illustrations show how the atomic model has changed as scientists learned more about the atom’s structure.

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