Atomic Model Review
(Match Theory to Scientist)
1. Atoms are solid
spheres.
2. Electrons move around
the nucleus in specific
levels.
3. Protons are
concentrated in the
center.
4. Electrons move in everchanging paths within
certain energy levels.
5. Electrons are stuck in
the atom’s surface.
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A. Thomson
B. Dalton
C. Bohr
D. Rutherford
E. Electron cloud model
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• Atomic Number
– Number of protons or electrons in an element
– Identifies the element
• Atomic Mass
–
–
–
–
–
Nucleus contains most of the mass of an atom.
Protons and neutrons are each ~ 1.67 x 10-24 g.
Electrons are each ~ 9.11 x 10-28 g.
Use atomic mass unit (amu) instead of gram.
The mass of one proton is ~ 1 amu.
• Mass Number
– The sum of the number of protons and number of
neutrons in the nucleus
– Is approximately equal to the average atomic mass
shown on periodic table.
– Number of neutrons = mass number – atomic number
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• Isotopes
– Atoms of the same element with different numbers of
neutrons
– Have the same number of protons
– Example: Carbon-12 and Carbon-14
• Radioactive Isotopes
– Unstable in nature
– Can be used to date fossils and rocks
– The time it takes for half of the radioactive atoms in a
piece of the fossil to change to another element is its
half-life.
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Radioactive Isotopes
Radioactive Element
Changes to this radioactive
element
Half-life
uranium-238
lead-206
4510 million years
potassium-40
argon-40, calcium-40
1350 million years
rubidium-87
strontium-87
50,000 years
carbon-14
nitrogen-14
5,730 years
If 25% of the atoms in a rock are uranium-238, how old is the rock?
• 50% changed to lead-206 in
4.510 x 109 yrs
4.510 x 109 yrs
• Total is 75% changed in
9.020 x 109 yrs
• Age of rock is ~ 9,020 million years
•
50% of remaining changed to lead-206 in
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The Atom: Idea to Theory
• Democritus (~ 400 BC) called nature’s basic
particle an atom
• Atom comes from Greek word meaning
“indivisible”
• 1808: Dalton proposed a theory with
several statements which were later
verified, but his “model” of an atom was
that of a sphere.
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Dalton’s Atomic Theory
1. All matter is composed of extremely small
particles called atoms.
2. Atoms of a given element are identical in size,
mass and other properties.
3. Atoms cannot be divided, created or destroyed.
4. Atoms of different elements combine in simple
whole-number ratios to form chemical
compounds.
5. In chemical reactions, atoms are combined,
separated or rearranged.
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Modern Atomic Theory
• Democritus = Idea about an atom
• Dalton = Scientific theory which could be tested
• Dalton’s concepts which have “held true:”
– All matter is composed of atoms
– Atoms of any one element differ in properties from
atoms of another element
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• Law of Conservation of Mass :
Mass is neither created nor destroyed during
ordinary physical or chemical changes.
•Law of Definite Proportions:
A chemical compound contains the same
elements in exactly the same proportions by mass
regardless of the size of the sample.
•Law of Multiple Proportions:
If two or more different compounds are made of the
same two elements, then the ratio of the masses of the 2nd
element combined with a certain mass of the 1st element is
always a ratio of small whole numbers.
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The Mole
• A mole is the amount of a substance that
contains as many particles as there are
atoms in exactly 12 g of carbon-12.
• A mole is the SI unit for the amount of a
substance.
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Avogadro’s Number
• This is the number of particles in one mole.
• Avogadro was an Italian scientist who explained the relationship
between mass and number of atoms.
• 12.0 g of carbon-12 contains 6.022 1367 x 1023
carbon-12 atoms.
Avogadro’s number is rounded to 6.022 x 1023
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Molar mass
•
•
The mass of one mole of a pure substance
Units = __g_
mol
Molar mass of an element = atomic mass of the
element in amu.
Molar mass of Al is 26.98 g/mol
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Relationship between mass, moles
and molecules in a compound
Mass
(g)
X molar mass
(__g__
mole)
Amount
(moles)
Grams moles = moles
gram
X 6.022 x 1023
(units
mole)
# molecules
or
Formula units
moles units = units or molecules
mole
moles grams = grams
mole
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Molar mass
• Molar mass of a substance = mass in grams of one
mole of the substance.
• A compound’s molar mass is NUMERICALLY equal
to its formula mass.
2 mol H x 1.01 g H = 2.02 g H
1 mol H
1 mol O x 16.00 g O = 16.00 g O
1 mol O
molar mass
H2O =
18.02 g/mol
• Formula mass H2O = 18.02 amu
• Molar mass H2O = 18.02 g/mol
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Molar Mass Example
What is the molar mass of K2SO4?
2 mol K x 39.10 g K = 78.20 g K
1 mol K
1 mol S x 32.10 g S = 32.07 g S
1 mol S
4 mol O x 16.00 g O = 64.00 g O
1 mol O
molar mass K2SO4 =
174.27 g/mol
How many moles of each element are present in this
compound?
2 mol K, 1 mol S, 4 mol O
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What is the molar mass of C6H12O6?
6 mol C x 12.01 g C = 72.06 g C
1 mol C
12 mol H x 1.01 g H = 12.12 g H
1 mol H
6 mol O x 16.00 g O = 96.00 g O
1 mol O
molar mass C6H12O6 =
180.18 g/mol
How many moles of each element are present
in this compound?
6 mol C, 12 mol H, 6 mol O
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Converting to grams from moles
How many moles of glucose are in 4.15x10-3 g
C6H12O6?
4.15x10-3 g x 1 mol C6H12O6 = 2.30 x 10-5 mol C6H12O6
180.18 g
How many molecules of glucose are in 4.15x10-3 g
C6H12O6?
2.30 x 10-5 mol C6H12O6 x 6.022 x 10 23 molecules =
1 mol
(2.30 x 6.022)(10(-5+23)) = 13.90 x 10 –18 molecules
= 1.39 x 10 –19 molecules
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What is the mass in grams of 6.25 moles copper (II)
nitrate?
Cu 2+
NO3 - : formula is
Cu(NO3)2
Find molar mass of Cu(NO3)2 first.
1 mol Cu x 63.55 g Cu = 63.55 g Cu
1 mol Cu
2 mol N x 14.01 g N = 28.02 g N
1 mol N
6 mol O x 16.00 g O = 96.00 g O
1 mol O
molar mass Cu(NO3)2 = 187.57 g/mol
Now find mass in grams of 6.25 moles:
6.25 moles x 187.57 g = 1172 g Ans. 1170 g Cu(NO3)2
1 mol
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