Mass Relationships of Atoms · 2015-09-08 · However, two atoms of the same element can have...
Transcript of Mass Relationships of Atoms · 2015-09-08 · However, two atoms of the same element can have...
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Mass Relationships of Atoms
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Atoms,Molecules,and Ions
Symbols of Elements
Elements are symbolized by one or two letters.
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© 2012 Pearson Education, Inc.
Atoms,Molecules,and Ions
Symbols of Elements
All atoms of the same element have the same number of protons, which is called the atomic number, Z.
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Atoms,Molecules,and Ions
Symbols of Elements
The mass of an atom in atomic mass units (amu) is the total number of protons and neutrons in the atom.
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Atoms,Molecules,and Ions
Isotopes
• Isotopes are atoms of the same element with different masses.
• Isotopes have different numbers of neutrons.
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Atoms,Molecules,and Ions
Average Mass
• Because in the real world we use large amounts of atoms and molecules, we use average masses in calculations.
• Average mass is calculated from the isotopes of an element weighted by their relative abundances.
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Atomic number and mass numberAtomic number (Z) = the number of protons in the nucleus.
Mass number (A) = the sum of the number of protons + neutrons in the nucleus.
ZA X
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Symbols for a few atoms
11H 4
2He 126 C 16
8 O 6530 Zn
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Symbols for a few atoms
11H 4
2He 126 C 16
8 O 6530 Zn
Mass number (A)
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Symbols for a few atoms
11H 4
2He 126 C 16
8 O 6530 Zn
Atomic number (Z)
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ElementAn element is a form of matter in which all of the atoms have the same atomic number.
However, two atoms of the same element can have different mass numbers.
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IsotopeAtoms that have the same atomic number but different mass numbers are called isotopes.
- same number of protons in nucleus- differ in number of neutrons
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Some isotopes
11H 12
6 C 168 O 235
92 U
21H 13
6 C 178 O 238
92 U
31H 14
6 C188 O
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Atomic masses- synonymous with atomic weight- is a relative scale- mass-12 isotope of carbon (carbon-12) is thereference atom and assigned an atomic mass ofexactly 12- one atomic mass unit (amu) is defined as a mass exactly equal to 1/12th the mass of one carbon-12 atom
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relative masses of carbon-12 and carbon-13 in a random sample carbon has a ratio of 1.0836129
136 C
126 C
= 1.0836129
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= (1.0836129)(12 amu)
Since the atomic mass unit is defined such that the mass of 12C is exactly 12 atomic mass units, then
136 CMass of
= 13.003355 amu
136 C
126 C
= 1.0836129
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The Mole- the fundamental SI measure of “amount ofsubstance”- the amount of substance that contains as many elementary entities as there are atoms in exactly 12 g of carbon-12- this number of atoms is 6.022045 x 1023
Avogadro’s number
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Stoichiometry
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Avogadro’s Number
• 6.02 x 1023
• 1 mole of 12C has a mass of 12.000 g.
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The Mole vs. The Dozen
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The Dozen - the amount of substance that contains 12 entities.
The Mole - the amount of substance that contains Avogadro’s number (6.022x1023) of entities.
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Dozen Apples = 10 Lbs.
Mole of Helium atoms = 4.0026g
Dozen Apples = 12 Apples
Mole of Helium atoms = 6.022x1023 atoms
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Example
1.3 Lbs
How many dozens of apples are represented by 1.3 Lbs. of apples.
Converting to Dozens
10 Lbsx
1 dozen.13 dozen=
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Example
How many moles of He are in 6.46 g of He?
Converting to Moles
1.61 mol=6.46 g He4.003 g1 molx
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The Mole
- since 6.022045 x 1023 atoms of carbon have a mass of 12 grams,
6.022045 x 1023 atoms x1 atom
12 amu12 g=
6.022045 x 1023 amu 1 g=
then
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Stoichiometry
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Mole Relationships
• One mole of atoms, ions, or molecules contains Avogadro’s number of those particles.
• One mole of molecules or formula units contains Avogadro’s number times the number of atoms or ions of each element in the compound.
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Molar mass of an elementThe mass of 6.022 x 1023 atoms of an elementis equal to its atomic mass in grams.BUT: what does the periodic table tell us aboutthe atomic mass of carbon?Atomic mass is weighted average of mixture of isotopes
6C
12.01
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Atomic weight of carbon= (atomic mass ) (fraction ) 12
6 C 126 C
+ (atomic mass ) (fraction ) 136 C 13
6 C
= (12.0000 amu)(0.9889)+(13.0035 amu)(0.0111)
= 11.8670 amu + 0.1441 amu
= 12.0111 amu
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ExampleCopper, a metal known since ancient times, is used in Electrical cables and pennies, among other things. The atomic masses of its two stable isotopes, (69.09%) and (30.91%), are 62.93 amu and 64.9278 amu, respectively. Calculate the average atomic mass of copper. The percentages in parentheses denote the relative abundances.
6329Cu 65
29Cu
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Answer
= (62.93 amu)(0.6909)+(64.9278 amu)(0.3091)
= 43.47 amu + 20.07 amu
= 63.54 amu
6329Cu= (atomic mass ) (fraction ) 63
29Cu
6529Cu+ (atomic mass ) (fraction ) 65
29Cu
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Mass Relationships of Atoms - Extended
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ExampleCalculate the number of grams of lead (Pb)In 12.4 moles of lead.
12.4 mol 2569 g=207.2 g
1molx
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ExampleWhat is the mass in grams of one silver atom?
107.9 g1 mol
= 17.91 x 10–23 g 1 atom
6.022 x 1023 atoms x
1 mol
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ExampleCalculate the number of atoms in 0.551 g ofPotassium (K).
0.551 g39.10 g1 mol
x1 mol
6.022 x 1023 atomsx
= 8.48 x 1021 atoms K
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ExampleCalculate the number of molecules in a sampleof oxygen gas (O2) with a mass of 64.0g.
1.20 x 1024 molec. O2=
64.0 g x6.02 x 1023 molec.
1 mol32.0 g
1 molx
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Experimental DeterminationOf Atomic & Molecular Masses
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Atomic mass is measured by mass spectrometry
Atom is bombarded by stream of highEnergy electrons.
e–
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Atomic mass is measured by mass spectrometry
Electron collides with atom, “bounces” off
e–
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Atomic mass is measured by mass spectrometry
and transfers some of its energy to it.
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Atomic mass is measured by mass spectrometry
Atom dissipates its excess energy by expellingone of its electrons.
+
e–
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Ion is deflected by magnetic field
+
amount of deflection depends on mass to chargeratio
highest m/zdeflected least
lowest m/zdeflected most
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Ions are detected after passagethrough magnetic field
+ ++
++
+
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Ions are detected after passagethrough magnetic field
++++ ++mixture of ions ofdifferent mass givesseparate peak for each m/z
intensity of peak proportionalto percentage of each atom ofdifferent mass in mixture
separation of peaksdepends on relative mass
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19 20 21 22 23
90.92%
0.26%8.82%
The mass spectrum ofthe three isotopes of neon.
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Atoms,Molecules,and Ions
Atomic Mass
Atomic and molecular masses can be measured with great accuracy using a mass spectrometer.
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