section 4.2—atomic structure. what are atoms? atom - smallest piece of matter that has the...
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Section 4.2—Atomic Structure
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What are atoms?
Atom - smallest piece of matter that has the chemical properties of the element.
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What’s in an atom?
An atom is made of three sub-atomic particles
Particle Location
Nucleus
Nucleus
Outside the nucleus
Mass
1 amu = 1.6710-27 kg
1 amu = 1.6710-27 kg
0.00055 amu
9.1010-31 kg
Charge
+1
0
-1
Proton
Neutron
Electron
1 amu (“atomic mass unit”) = 1.66 10-27 kg
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What gives an atom its identity?
What makes an atom “carbon” as opposed to “oxygen”?
Every atom has a different number of protons.
The number of protons determines the identity of the atom
The atomic number shows the number of protons.
Atomic number = protons
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The Nucleus & Mass
Since the nucleus has protons & neutrons, and the mass of each one is 1 amu…
The mass of the nucleus (in amu’s) is the number of protons + neutrons
Since electrons have relatively no mass (0.054% of one proton or neutron), we don’t need to worry about them when determining mass of an atom
Mass # = protons + neutrons
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Charges
Protons have a positive chargeElectrons have a negative chargeNeutrons have no chargeOverall charge = protons + (-1)×electrons
Charge = protons - electrons
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How do we show information about an element?
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XA C
Z #
Element symbols
Charge# protons - # electrons(assumed to be “0” if blank)
Element Symbol1 or 2 letters, found on
the periodic table
Mass number# protons + # neutrons
Atomic number # of protons Number
How many atoms do you have?
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Example: Element symbols
O16 -2
8
Charge-2
Element SymbolO = Oxygen
Mass number16
Atomic number 8 Number
Assumed to be “1” if blank
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Let’s Practice
Example:Fill in the missing values
Symbol Name Atomic #
Mass #
Charge Proton Neutron Electron
Magnesium-25 +2
82 126 82
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Let’s Practice
Example:Fill in the missing values
Symbol Name Atomic #
Mass #
Charge Proton Neutron Electron
Magnesium-25 +2
82 126 82
Remember: Atomic number is the identityAtomic number = protonsCharge = proton - electronsMass # = protons + neutrons
12 25 12 13 10
Lead-208 208 0 82
22512
Mg
Pb20882
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Isotopes
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What are isotopes?
Isotopes - n. Atoms of the same element with a different number of neutrons
Some isotopes are radioactive—but not all…many are quite stable!
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Isotopes Example
If they have different number of neutrons, and neutrons have a mass of 1 amu…
Then isotopes of the same element will have different masses!
But because their protons are the same, they are the same element!
Hydrogen-2Hydrogen-1
Mass # = 1 amu Mass # = 2 amu
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Identifying Isotopes
12C 13C
Carbon-12 Carbon-13
Isotopes can be differentiated by their different mass numbers in the element symbol
Or by the mass number following their name.
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Mass Number versus Atomic Mass
Mass Number Average Atomic Mass
# of protons + # of neutrons Average of actual masses
For one specific isotope only
Is not found on the periodic table
Weighted average of all isotopes
Is found on the periodic table
Always a whole number Not a whole number
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Calculating Average Atomic Mass
Average atomic mass
= Abundance of isotope
Mass of isotope
( )
Average atomic mass is a weighted average (it takes into account how often each isotope occurs).
“Sum of”
What fraction of the time is that isotope present?
Actual mass (not mass number)
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Example of Finding Avg Atomic Mass
966.362422.0969.347578.0 amuMassAvg
Example:Find the atomic mass of
chlorine if Chlorine-35 has a mass of
34.969 amu and Chlorine-37 has a
mass of 36.966 amu and is present 24.22% of the time.
Remember that percents add up to 100.
So they said the second isotope is present 24.22% of the time.
This means that the first isotope is present 100-24.22 = 75.78% of the time
Isotope Mass Percent Decimal
1 34.969 amu 75.78 0.7578
2 36.966 amu 24.22 0.2422
This chart summarizes the information in the problem:
= 35.45 amu (this is what’s on the periodic table for Cl!)