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