parts of an atom and organization of the periodic table...parts of an atom and organization of the...
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ATOMIC MASS
October 8, 2014
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Atomic Mass (Mass #)
• Definition: the ~ mass of one atom of an element (equals #protons + # neutrons)
• Tells us how much one atom weighs in atomic mass units.
Mass
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Organization of The Periodic TableOrdered in columns by increasing atomic mass.Columns arranged to form groups with similar properties
Group/Family
Period
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Assessment
• Which element has the greatest atomic mass?
– Lithium
– Sodium
– Potassium
– Rubidium
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Reactivity
• The ease and speed with which an element combines, or reacts, with other elements or compounds
Pure sodium reacts explosively with air
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Groups/families of elements have similar reactivity
Group 1: metals that react violently with water
Group 18: Gases that barely react at all
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Atoms have neutral charge
• # Protons (+) = # Electrons (-) = 0 charge
• Example: Hydrogen– 1 proton + 1 electron
– +1 + (-1) = 0
– Thus # protons = # electrons
• How many electrons does C have?– 6 electrons
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RECAP!
• Atoms are composed of protons (p+), neutrons (n0), and electrons (e-)
• Protons and neutrons are in the nucleus
• Electrons orbit
• Atoms have an overall neutral charge equal to zero
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The Nucleus of the Atom
The nucleus is…
1. Positively charged
2. Extremely small(diameter is 10,000 x smaller than the atom)
3. Extremely dense
4. Contains virtually all of the atom’s mass
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Particle Comparison
Particle Symbol Charge Relative mass
Mass (g)
Electron e- -1 1/1840 9.11 x 10-28
Proton p+ +1 1 1.673 x 10-24
Neutron n0 0 1 1.675 x 10-24
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Atomic Mass Units
• Due to extremely small mass we use amu’s (atomic mass units)
• Atomic standard is the carbon atom1 carbon atom = 12 amu
1 amu = 1/12 carbon atom mass
*the values for 1 amu and the masses for 1 proton and neutron are slightly different
e- .000549 amu
p+ 1.007276 amu
n0 1.008665 amu
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Atomic Mass
• The weighted average mass of the isotopes of that element
*different than mass number (# of protons + # of neutrons)
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Atomic Mass: How Do We Get It?
• All isotopes of an element do not exist in equal abundance in the world
• The atomic mass is the weighted average mass of the isotopes of that element
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Example #1
Using the data table, calculate the average atomic mass
6Li = .075 x 6.015 = .4517Li = .925 x 7.016 = 6.490
6.941 amu
Isotope Mass (amu)
Percent abundance
6Li 6.015 7.5%
7Li 7.016 92.5%
Convert the percent to a decimal by dividing by 100!
So, 7.5%/100 = .075
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Example #2
• A sample of cesium is 75% 133Cs, 20% 132Cs, and 5% 134Cs. What is its atomic mass?
.75 x 133 = 99.75
.20 x 132 = 26.4
.05 x 134 = 6.7
132.85 amu
+
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Example #3
• Assume the following three isotopes of element X exist: X-122, X-124, X-128. If the atomic mass of X is 127.55 amu, which of these isotopes is most abundant?
ANSWER: The most abundant is the one closest to the atomic mass…so it would be X-128!