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Chapter 2 cont’Chapter 2 cont’Atoms and Elements
Recall:•Atomic Number
Number of protons Z
•Mass Number Protons + Neutrons Whole number A
•Abundance = relative amount (in %) found in a sample
NeonNeon
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9.25%221210Ne-22 or
0.27%211110Ne-21 or
90.48%201010Ne-20 or
Percent Natural Abundance
A, Mass Number
Number of Neutrons
Number of ProtonsSymbol
Ne2010
Ne2110
Ne2210
Atomic MassAtomic Masswe previously learned that not all
atoms of an element have the same mass◦ isotopes
we generally use the average mass of all an element’s atoms found in a sample in calculations
we call the average mass the atomic mass
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nn isotope of massisotope of abundance fractional MassAtomic
Mass SpectrometryMass Spectrometry masses and abundances of
isotopes are measured with a mass spectrometer
atoms or molecules are ionized, then accelerated down a tube
their path is bent by a magnetic field, separating them by mass◦ similar to Thomson’s Cathode
Ray Experiment a mass spectrum is a graph that
gives the relative mass and relative abundance of each particle
relative mass of the particle is plotted in the x-axis
relative abundance of the particle is plotted in the y-axis
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ExampleExample
Magnesium has three naturally occurring isotopes with masses of 23.99 amu, 24.99 amu, and 35.98 amu and natural abundances of 78.99%, 10.00% and 11.01% respectively. Calculate the atomic mass of magnesium
Lithium has two naturally occurring isotopes: lithium-6 and lithium-7. If the average atomic mass of lithium is 6.941 amu, which isotope is the most abundant? How do you know?
Collection TermsCollection Terms
A collection term statesa specific number of items.• 1 dozen donuts
= 12 donuts
• 1 ream of paper
= 500 sheets
• 1 case = 24 cans
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In chemistry, how do chemists know the number of atoms in an element or compound?
A Mole of AtomsA Mole of Atoms A mole is• a unit of measurement used in chemistry to
express amounts of a chemical substance, the same number of particles as there are carbon atoms in 12.0 g of carbon.
• a collection term “dozen”
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Avogadro’s Number = 6.0221421 x 1023
1 mole = NA = 6.022 x 1023 of anything
Relationship Between Moles and Relationship Between Moles and MassMass
The mass of one mole of atoms is called the molar mass
The molar mass of an element, in grams, is numerically equal to the element’s atomic mass, in amu
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E.g 1 H atom = 1.01 amu 1 mol H = 1.01 6.022 x 1023 atoms of H = ???? g
ExamplesExamples
Give the molar mass for each
A. 1 mole of Li atoms = ________ g
B.1 mole of Co atoms = ________g
C.1 mole of S atoms = ________g
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ExamplesExamples Assuming all pennies are pure copper and each has
a mass of 2.5 g◦ Without doing calculation, determine the number
atoms of copper present in 1 mole
◦ How many pennies does it take to make a mole?
ExamplesExamples In a 3.0 moles of O2 molecules
◦ How many oxygen molecules are there ?
◦ How many oxygen atoms are there?
Calculate the moles of carbon in 0.0265 g of pencil lead
Calculate the mass (in grams) of 0.473 moles of titanium
Converting between mass, Converting between mass, moles and atomsmoles and atoms
g C mol C
mol C g C
g C mol C atoms
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