ThursdayOctober 18, 2012

(Average Atomic Mass)

Bell RingerThursday, 10-18-12

Which one of the listed elements has

the smallest atomic radius – Mg, Ca, Ba

the greatest ionization energy – Sr, Ag, I

the least electron affinity – C, Ga, In

the largest ionic radius – Cs, Hg, At

the least electronegativity – Al, Ga, Tl

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Assignment Currently Open

Summative or

Formative?Date Issued Date Due Date Into

GradeSpeed Final Day

QUIZ 6 S3 10/5 10/5 10/19

QUIZ 7 S4 10/12 10/12 10/26

WS – The Periodic Table F1 10/11 10/17 10/19

WS – The Periodic Law F2 10/12 10/17 10/19

WS – Periodic Properties F3 10/15 10/17 10/19

Pop Quiz – The Periodic Table

and the Periodic Law

F4 10/16 10/16

Average Atomic MassThe mass number of an element is the number listed (usually) under

the atomic symbol on a Periodic Table tile.

Notice it is not a whole number, but contains

decimal places.

The mass number is the top number in isotopic

notation, and is rounded off.

Average Atomic MassWhy is the atomic mass not a whole

number?

The atomic mass number is an

average of all of the masses of the different isotopes of that particular

element.

Average Atomic Mass

What is an isotope?

Isotopes are atoms of the same element that have different

numbers of neutrons.

Average Atomic MassWhat is the relationship

between isotopes and atomic mass?

The average atomic mass is the weighted average of the atomic

masses of the naturally occurring isotopes of an element.

Average Atomic MassThe average atomic mass of an element depends on both

the mass and the relative abundance of each of the element’s isotopes.

For example, naturally occurring copper consists of 69.17% copper-63, which has an atomic mass of 62.929 599 amu, and 30.83% copper-65, which has an atomic

mass of 64.927 793 amu.The average atomic mass of copper can be calculated by

multiplying the atomic mass of each isotope by its relative abundance (expressed in decimal form) and

adding the results.(0.6917 × 62.929 599 amu) + (0.3083 × 64.927 793 amu)

= 63.55 amu

The calculated average atomic mass of naturally occurring copper is 63.55 amu.

Average Atomic MassWhy is it important to know the percent

abundance of each isotope in an element?Knowing this information allows us to

calculate the average atomic mass of an element.

What do you get when you add up all of the percent abundances of all of the isotopes?

100%What information do you need to calculate the average atomic mass of an element?

Average Atomic MassSample Problem

Uranium has three naturally occurring isotopes as follows:uranium-234: abundance 0.005%, atomic mass

234.040947

uranium-235: abundance 0.720%, atomic mass 235.043924

uranium-238: abundance 99.275%, atomic mass 238.050784

Calculate the average atomic mass of uranium.

The Least You Need to Know

The average atomic mass is the weighted average of the

atomic masses of the naturally occurring isotopes of an

element.Isotopes are atoms of the same

element that have different numbers of neutrons.

Worksheet

Average Atomic Mass