The Structure of an Atom Ch.4 Notes. Section 1: Early Theories of Matter Atom- the smallest particle...
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The Structure of an AtomCh.4 Notes
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Section 1: Early Theories of Matter
•Atom- the smallest particle of an element that retains the properties of the element.
•Size? The world’s population is 6,840,000,000 •The number of atoms in a penny is 29,000,000,000,000,000,000,000
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Early Theories of Matter• Ancient Greek philosopher Democritus speculated
matter was made up of atomos
• The theory was rejected for 2000 years due to being challenged by Aristotle
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Discovery of the Electron
• John Dalton- proposed his atomic theory in 1803 based on his research that was not completely accurate
• Atoms can be subdivided
• Atoms of the same element can have slightly different masses
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•Discovering the electron
•Sir William Crookes - used a cathode ray tube to do experiments on the atom based on the relationship between mass and charge. Electrons are negatively charged
Discovering the Electron
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•JJ Thomson (1856-1940)- in the 1890’s found a cathode ray could be deflected by magnetic and electric fields. He concluded the ray was a stream of negatively charged particles dislodged from an atom with a mass much less than a hydrogen atom.
Discovering the Atom
http://www.youtube.com/watch?v=GzMh4q-2HjM
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Discovering the Atom
•Robert Millikan (1868-1953) – in 1909 he accurately determined the mass of an electron through the Oil Drop Experiment, it had a charge of -1
•The actual mass of an electron is 9.1 x 10-28 g
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• Plum Pudding Theory-( aka the chocolate chip cookie model)
• Matter is neutral, so Thomson proposed: spherical atoms have uniform (+) charge with (-) electrons embedded throughout.
Nucleus
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Nucleus
• Ernest Rutherford, in 1911
• To test the theory, he aimed a thin beam of (+) charged Alpha particles at a thin sheet of gold.
• He expected the alpha particles to pass through with little deflection
• Instead, most passed through, but few bounced back and deflected at big angles.
• He concluded atom is mostly empty space with large (+) charged nucleus in center
http://www.youtube.com/watch?v=wzALbzTdnc8
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Atomic Summary
*Nucleus -protons + neutrons
-contains all the mass-positive charge
*Electrons-located in the cloud-fast moving-very small
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Properties of Subatomic Particles
Particle
Electron
Proton
Neutron
Symbol Location ChargeRelative
MassActual Mass
e-
p+
n0
Electron Cloud
Nucleus
-1
Nucleus
+1
none 1
1
1/1840
1.675 x 10-24
1.673 x 10-24
9.11 x 10-28
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How atoms differ:
•Atomic number is how an atom is identified, it is equal to the number of protons in an atom, & determines the location of the atom in the periodic table.
•The periodic table is arranged left to right & top to bottom by increasing atomic number.
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1
1.008
H
atomic #
Chemical Symbol
Average atomic mass
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Number of protons in nucleus of an element
This number identifies the element
Periodic table is arranged in sequence of increasing atomic numbers
Atoms are neutral: (+) charges must equal (-) charges
Atomic Number
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We know: atomic # = #protons = # electrons
•How many protons and electrons are there in:
•Be: p= 4 e= 4
•F: p= 9 e= 9
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Ions
• Charged particles: Number of electrons has changed- proton # stays the same
• Be2+ has lost 2 electrons:
• F1- has gained one electron:
p= 4 e= 2p=9 e=10
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•To identify an isotope a number is added to the end of the name, it is called a mass number.
•Mass Number = # of protons + # of neutrons
•Ex: Carbon-14, Neon-22, Potassium-41
Isotopes
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•Isotopes
• All atoms of same element have some number of protons
• Most have different numbers of neutrons
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Mass Number
• An isotope is identified by its mass number
• Mass number= number of protons + number of neutrons
• Number of neutrons = mass number – atomic number
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Atomic Mass
• Atomic mass units (amu) = ½ the mass of a Carbon-12 atom
• Atomic mass given in Periodic Table is not given in whole numbers
• Atomic mass is the weighted average of the atomic masses of all naturally occurring isotopes. This weighted average takes into account the mass and abundance of each isotope
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Atomic Mass• Average atomic mass= mass * abundance for
each isotope: then add
Copper-63 Copper-65
Number of protons 29 29
Number of neutrons 34 36
Atomic mass 62.930 amu 64.928 amu
Abundance 69.17% 30.83%
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• Mass Contribution = mass x abundance
• Cu-63= (62.930)(.6917)= 43.53
• Cu-65= ( 64.928)(.3083) =20.02
• 43.53+20.02= 63.55