IsotopesIsotopes

Background & HistoryBackground & History Famous scientist John Dalton Famous scientist John Dalton

maintained that all atoms of a maintained that all atoms of a particular element were particular element were identicalidentical Their Their massesmasses were the same, were the same,

and also their #’s of and also their #’s of pp++, n, noo, and , and ee-- were identical were identical

In 1912, J.J. Thomson In 1912, J.J. Thomson discovered that this was not discovered that this was not accurateaccurate In an experiment measuring In an experiment measuring

the mass-to-charge ratios of the mass-to-charge ratios of positive ions in neon gas, he positive ions in neon gas, he made a remarkable discovery:made a remarkable discovery:

91% of the atoms had one 91% of the atoms had one massmass

The remaining atoms were The remaining atoms were 9% heavier9% heavier

All of the atoms had 10 All of the atoms had 10 protons, however some had protons, however some had 10 neutrons, 11 neutrons and 10 neutrons, 11 neutrons and 12 neutrons12 neutrons

Basic InfoBasic Info

Atoms of the same element MUST have the Atoms of the same element MUST have the same same number of protonsnumber of protons (otherwise they would (otherwise they would not be the same element)not be the same element)

The number of The number of neutrons may varyneutrons may vary Isotopes:Isotopes:

Atoms that have the same number of Atoms that have the same number of protonsprotons but but different numbers of different numbers of neutronsneutrons

Refers to the Refers to the type (s)type (s) of atom that can be found in of atom that can be found in naturenature

Even though the neutrons are different, the isotopes Even though the neutrons are different, the isotopes are are chemically alikechemically alike in most ways in most ways

Conventional Notation - SymbolsConventional Notation - Symbols

The element carbon has 3 naturally The element carbon has 3 naturally occurring isotopes: one with 6 neutrons, occurring isotopes: one with 6 neutrons, one with 7 neutrons and one with 8 one with 7 neutrons and one with 8 neutronsneutrons

When writing out isotopes, we include the When writing out isotopes, we include the mass numbermass number in the top left and the in the top left and the atomic atomic numbernumber in the bottom left: in the bottom left:

Conventional Notation - WordsConventional Notation - Words

Isotopes may also be expressed in wordsIsotopes may also be expressed in wordsSpell the element name, hyphen, followed Spell the element name, hyphen, followed

by the mass numberby the mass numberFor example, from the following slide:For example, from the following slide:

Carbon - 12 Carbon - 13 Carbon - 14

Determining the # of NeutronsDetermining the # of Neutrons

Use the same basic formula as always:Use the same basic formula as always:atomic mass = # protons + # of neutronsatomic mass = # protons + # of neutrons

For example – How many neutrons are in For example – How many neutrons are in Neon – 22?Neon – 22?Atomic Mass = 22Atomic Mass = 22Protons = 10Protons = 1022 – 10 = 12 neutrons22 – 10 = 12 neutrons

Applications of IsotopesApplications of Isotopes

Important Terminology:Important Terminology:Naturally occurring abundancesNaturally occurring abundances

The isotopes of an element that The isotopes of an element that occur naturally in occur naturally in the worldthe world

% natural abundances% natural abundancesThe The amount (in percentages) amount (in percentages) of each isotope of an of each isotope of an

element that occurs naturally in the worldelement that occurs naturally in the world

Application of Isotopes – Atomic MassesApplication of Isotopes – Atomic Masses

We are used to calculating #’s of pWe are used to calculating #’s of p++, n, noo and e and e-- using whole numbers; however on the Periodic using whole numbers; however on the Periodic Table we often see a decimal number Table we often see a decimal number Why? Why?

Atomic Mass (on the Periodic Table) Atomic Mass (on the Periodic Table) The average of the isotopic masses, weighted The average of the isotopic masses, weighted

according to the naturally occurring abundances of according to the naturally occurring abundances of the isotopes of the elementthe isotopes of the element

In a weighted average we must assign greater In a weighted average we must assign greater importance – give greater weight – to the quantity that importance – give greater weight – to the quantity that occurs more frequentlyoccurs more frequently

Units are “amu” or atomic mass unitUnits are “amu” or atomic mass unit

Comparative Example – Your GradesComparative Example – Your Grades

To calculate your overall To calculate your overall average, we use a weighted average, we use a weighted average instead of a simple average instead of a simple average since different tasks average since different tasks are worth moreare worth more

For example:For example:

(35/100 x 80) + (25/100 x 75) + (35/100 x 80) + (25/100 x 75) + (10/100 x 70) + (30/100 x 50) = (10/100 x 70) + (30/100 x 50) = 68.8% (Yikes! Don’t bomb the 68.8% (Yikes! Don’t bomb the final!)final!)

/100/100 Your Your markmark

ExamsExams 3030 80%80%

Applied Applied ScienceScience

1010 70%70%

Course Course WorkWork

3030 75%75%

FinalFinal 3030 50%50%

Calculating the Atomic MassCalculating the Atomic Mass

Calculating the atomic mass can be Calculating the atomic mass can be performed successfully using the following performed successfully using the following method:method:

atomic mass = (fractional abundance of isotope 1 x mass of isotope 1) atomic mass = (fractional abundance of isotope 1 x mass of isotope 1) + (fractional abundance of isotope 2 x mass of isotope 2) + …+ (fractional abundance of isotope 2 x mass of isotope 2) + …

Fractional abundance is the percent Fractional abundance is the percent abundance divided by 100 percentabundance divided by 100 percent

In depth example:In depth example:

In J.J. Thomson’s experiment, he found In J.J. Thomson’s experiment, he found that the percent abundances of neon are that the percent abundances of neon are as follows:as follows:Neon – 20 = 90.51%Neon – 20 = 90.51%Neon – 21 = 0.27%Neon – 21 = 0.27%Neon – 22 = 9.22% Neon – 22 = 9.22%

Calculate the average atomic mass of Calculate the average atomic mass of neon showing all of your workneon showing all of your work

In depth exampleIn depth example