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Atomic Mass
One atomic mass unit (amu) : the mass exactly equal to one-twelfth the mass of one carbon-12 atom that has six protons and six neutrons. 1 atom of carbon-12 = 12 amu
1 amu =
mass of one C-12 atom
1 amu = 1.66054 x 10 -24^ g and 1 g = 6.02214 x 10^23 amu
Average Atomic Mass : the weighted average of the masses of the naturally occurring isotopes of the element; the mass of the atom in atomic mass units
Average Atomic Mass = ∑ (fractional abundance of isotope n) × (mass of isotope n)
Isotopes : atoms with identical atomic numbers but different mass numbers (that is, same number of protons but different numbers of neutrons)
Average atomic mass of carbon = (0.09890)(12.00000 amu) + (0.0110)(13.00335 amu) = 12.0107 amu
Example
If chlorine is 75.78 % Cl-35 with a mass of 34.9689 amu and the rest Cl-37 with a mass of 36.9659 amu , find chlorine’s atomic mass.
Cl atomic mass = 0.7578 x 34.9689 + (1 - 0.7578) × 36.9659 = 35.45 amu
Atomic Number
Atomic Symbol Atomic Weight
Carbon Isotopes
Atomic Number (number of protons)
Mass number (number of protons plus neutrons)
Atomic mass = (Fraction of isotope 1 x Mass of isotope 1) +
(Fraction of isotope 2 x Mass of isotope 2) +
(Fraction of isotope 3 x Mass of isotope 3) + ……
Practice Problems
- Three isotopes of silicon occur in nature. 28 Si (92.23%), which has an atomic mass of 27.97693 amu; 29 Si (4.68%), which has an atomic mass of 28.97649 amu; and (^30) Si (3.09%), which has an atomic mass of 29.97377 amu. Calculate the atomic weight of silicon.
- Gallium has two naturally occurring isotopes: Ga-69 with mass 68.9256 amu and a natural abundance of 60.11%, and Ga-71 with mass 70.9247 amu and a natural abundance of 39.89%. Calculate the atomic mass of gallium.
- Bromine has two naturally occurring isotopes (Br-79 and Br-81) and an atomic mass of 79.904 amu. (a) If the natural abundance of Br-79 is 50.69%, what is the natural abundance of Br- 81? (b) If the mass of Br-81 is 80.9163 amu, what is the mass of Br-79?
- Titanium has five common isotopes: 46 Ti (8.25%), 47 Ti (7.44%). 48 Ti (73.72%), 49 Ti (5.41%), 50 Ti (5.18%). What is the average atomic mass of titanium?
Isotope Aundance (fraction) Atomic mass (amu) (^46) Ti 0.0825 45. (^47) Ti 0.0744 46. (^48) Ti 0.7372 47. (^49) Ti 0.0541 48. (^50) Ti 0.0518 49.
References: Tro, Chemistry: A Molecular Approach 2 nd^ ed., Pearson Brown/LeMay/Bursten, Chemistry: The Central Science, 12th^ ed., Pearson
Answers
47.7 amu 4. (a) 49.31% (b) 78.91 amu 3. 69.72 amu 2. 28.09 amu 1.
