Isotopes

Background & History Famous scientist John Dalton maintained that all atoms of a particular element were identical Famous scientist John Dalton maintained that all atoms of a particular element were identical Their masses were the same, and also their #’s of p +, n o, and e - were identical Their masses were the same, and also their #’s of p +, n o, and e - were identical In 1912, J.J. Thomson discovered that this was not accurate In 1912, J.J. Thomson discovered that this was not accurate In an experiment measuring the mass-to-charge ratios of positive ions in neon gas, he made a remarkable discovery: In an experiment measuring the mass-to-charge ratios of positive ions in neon gas, he made a remarkable discovery: 91% of the atoms had one mass 91% of the atoms had one mass The remaining atoms were 9% heavier The remaining atoms were 9% heavier All of the atoms had 10 protons, however some had 10 neutrons, 11 neutrons and 12 neutrons All of the atoms had 10 protons, however some had 10 neutrons, 11 neutrons and 12 neutrons

Basic Info Atoms of the same element MUST have the same number of protons (otherwise they would not be the same element) Atoms of the same element MUST have the same number of protons (otherwise they would not be the same element) The number of neutrons may vary The number of neutrons may vary Isotopes: Isotopes: Atoms that have the same number of protons but different numbers of neutrons Atoms that have the same number of protons but different numbers of neutrons Refers to the type (s) of atom that can be found in nature Refers to the type (s) of atom that can be found in nature Even though the neutrons are different, the isotopes are chemically alike in most ways Even though the neutrons are different, the isotopes are chemically alike in most ways

Conventional Notation - Symbols The element carbon has 3 naturally occurring isotopes: one with 6 neutrons, one with 7 neutrons and one with 8 neutrons The element carbon has 3 naturally occurring isotopes: one with 6 neutrons, one with 7 neutrons and one with 8 neutrons When writing out isotopes, we include the mass number in the top left and the atomic number in the bottom left: When writing out isotopes, we include the mass number in the top left and the atomic number in the bottom left:

Conventional Notation - Words Isotopes may also be expressed in words Isotopes may also be expressed in words Spell the element name, hyphen, followed by the mass number Spell the element name, hyphen, followed by the mass number For example, from the following slide: For example, from the following slide: Carbon - 12Carbon - 13Carbon - 14

Determining the # of Neutrons Use the same basic formula as always: Use the same basic formula as always: atomic mass = # protons + # of neutrons For example – How many neutrons are in Neon – 22? For example – How many neutrons are in Neon – 22? Atomic Mass = 22 Atomic Mass = 22 Protons = 10 Protons = – 10 = 12 neutrons 22 – 10 = 12 neutrons

Applications of Isotopes Important Terminology: Important Terminology: Naturally occurring abundances Naturally occurring abundances The isotopes of an element that occur naturally in the world The isotopes of an element that occur naturally in the world % natural abundances % natural abundances The amount (in percentages) of each isotope of an element that occurs naturally in the world The amount (in percentages) of each isotope of an element that occurs naturally in the world

Application of Isotopes – Atomic Masses We are used to calculating #’s of p +, n o and e - using whole numbers; however on the Periodic Table we often see a decimal number  Why? We are used to calculating #’s of p +, n o and e - using whole numbers; however on the Periodic Table we often see a decimal number  Why? Atomic Mass (on the Periodic Table) Atomic Mass (on the Periodic Table) The average of the isotopic masses, weighted according to the naturally occurring abundances of the isotopes of the element The average of the isotopic masses, weighted according to the naturally occurring abundances of the isotopes of the element In a weighted average we must assign greater importance – give greater weight – to the quantity that occurs more frequently In a weighted average we must assign greater importance – give greater weight – to the quantity that occurs more frequently Units are “amu” or atomic mass unit Units are “amu” or atomic mass unit

Comparative Example – Your Grades To calculate your overall average, we use a weighted average instead of a simple average since different tasks are worth more To calculate your overall average, we use a weighted average instead of a simple average since different tasks are worth more For example: For example: (35/100 x 80) + (25/100 x 75) + (10/100 x 70) + (30/100 x 50) = 68.8% (Yikes! Don’t bomb the final!) /100 Your mark Exams3080% Applied Science 1070% Course Work 3075% Final3050%

Calculating the Atomic Mass Calculating the atomic mass can be performed successfully using the following method: Calculating the atomic mass can be performed successfully using the following method: atomic mass = (fractional abundance of isotope 1 x mass of isotope 1) + (fractional abundance of isotope 2 x mass of isotope 2) + … Fractional abundance is the percent abundance divided by 100 percent Fractional abundance is the percent abundance divided by 100 percent

In depth example: In J.J. Thomson’s experiment, he found that the percent abundances of neon are as follows: In J.J. Thomson’s experiment, he found that the percent abundances of neon are 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 neon showing all of your work Calculate the average atomic mass of neon showing all of your work

In depth example