1.3 Understanding Atomic Mass

A Review: Subatomic particles

Electron

Proton

Neutron

Name Symbol ChargeRelative mass

Actual mass (g)

e-

p+

no

-1

+1

0

1/1840

1

1

9.11 x 10-28

1.67 x 10-24

1.67 x 10-24

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Structure of the Atom

There are two regions:

1. The Nucleus: • With protons and neutrons• Positive charge• Almost all the mass

2. Electron cloud• Most of the volume of an atom• The region where the electrons can be found

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If the atom was the size of a stadium,

the nucleus would be the size of: a marble.

http://www.andrew.cmu.edu/user/jgrao/Manchester%20United/giant%20stadium.jpg

http://www.akronmarbles.com/marble-frag-2.jpg

Standard Atomic Notation

Includes the symbol of the element, the mass number and the atomic number

X Massnumber

Atomicnumber

# protons

# protons + # neutrons mass number

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• Find the – number of protons– number of neutrons– number of electrons– Atomic number– Mass number

F19 9

= 9

= 10

= 9

= 9

= 19

+

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http://chemistry.beloit.edu/Stars/EMSpectrum/fluorine.gif

Find the – number of protons– number of neutrons– number of electrons– Atomic number– Mass number Br80

35

= 35

= 45

= 35

= 35

= 80

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Find the – number of protons– number of neutrons– number of electrons– Atomic number– Mass number

Na2311

1+

Sodium ion

= 11

= 12

= 10

= 11

= 23

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http://www.webelements.com/webelements/elements/media/nearingzero/Na.gif

http://discover.edventures.com/images/termlib/i/isotope/support.gif

Isotopes• Atoms of the same element can have

different numbers of neutronsThis gives them different mass

numbers.They have the same atomic numbers.They look, act and react the same.

C-12 vs. C-14

Isotopes of Hydrogen

1 p+ 1 e-1 p+

1 n 1 e-1 p+2 n 1 e-

Protium Deuterium Tritium

(ordinary hydrogen) (heavy hydrogen) (radioactive hydrogen)

H11

H21

H31

Ralph A. Burns, Fundamentals of Chemistry 1999, page 100

H-2 H-3H-1

Isotopes of Hydrogen

•• ProtiumProtium (H-1)1 proton, 0 neutrons, 1 electronMost abundant isotope

•• DeuteriumDeuterium (H-2)1 proton, 1 neutron, 1 electronUsed in “heavy water”

•• TritiumTritium (H-3)1 proton, 2 neutrons, 1 electronradioactive

Naming Isotopes

• Put the mass number after the name of the element

• Examples:

• Carbon-12 and Carbon-14

• Uranium-235 and Uranium-238

Isotopes of Three Common Elements

92.21%

4.70%

3.09%

27.977

28.976

29.974

28

29

30

Si

Si

Si

Silicon

Chlorine

Carbon

Average Atomic Mass

Fractional AbundanceSymbolElement

126

136

3517

3717

2814

2914

3014

12.01

35.45

28.09

1.11%13.00313C

99.89%12 (exactly)12C

Mass (amu)

75.53%

24.47%36.96637Cl

34.96935Cl

Mass

Number

LeMay Jr, Beall, Robblee, Brower, Chemistry Connections to Our Changing World , 1996, page 110

Which isotopes are represented in this sketch?

http://discover.edventures.com/images/termlib/i/isotope/support.gif

Calculations of Average Atomic Mass

Why is the mass number on a periodic table never a nice whole

number?

Mass Number vs AverageAtomic Mass

• The Mass Number is specific to one isotope.

• The Average Atomic Mass is the calculated based on the abundance of each naturally occurring isotope of that element.

• The Average Atomic Mass is shown on Periodic Tables.

Average Atomic Mass• How heavy is an atom of oxygen?• There are different kinds of oxygen atoms.

oxygen-16, oxygen-17 and oxygen-18

• We are more concerned with average atomic mass, based on the abundance of each element in nature.

• We don’t use grams because the numbers would be too small.

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Atomic Mass Unit (amu or u)

• This unit is used instead of grams.

• It is 1/12 of the mass of a carbon-12 atom.

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Radioactive Isotopes

• Radioactive: – Capable of spontaneously emitting radiation in

the form of particles and/or gamma rays

• Radioisotope:– A radioactive isotope of an element that occurs

naturally or is produced artificially.

Radioisotopes• Many elements have one or more isotopes

that are unstable. • Atoms of unstable isotopes decay, emitting

radiation as their nucleus changes.• The changes can happen quickly or slowly,

depending on the isotope.• Radiation emitted can be harmless or very

dangerous to living cells.

Applications of Nuclear Science

Radiology:Anthropology:

Nuclear Reactors:http://blogs.warwick.ac.uk/images/flingard/2004/11/08/carbon_dating.jpg

http://www.johnharveyphoto.com/RoadTrip/NuclearReactor.jpghttp://mednews.stanford.edu/story_images/radiology-xray-051805.jpg

Radioisotopes

• Radioactive isotopes• Many uses

– Medical diagnostics– Optimal composition of

fertilizers– Abrasion studies in engines

and tires Radioisotope is injected into the bloodstream toobserve circulation.

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