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Transcript
Atomic Structure
The Idea of the Atom
Early Models



Greek philosophers – 450 B.C. what is the
smallest particle?
Democritus - Particles are atomos
An atom is the smallest particle of an
element that retains the chemical identity
of that element.
Aristotle Wins





Greek society - slave based
Beneath them to work with hands – no
experiments
Settled disagreements by arguments
Aristotle more famous than Democritus
Aristotle wins
Didn’t believe or look into atoms until…
Late 1700s

Chemists believed elements:

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
couldn’t be broken down by ordinary means
Combine to form compounds
Advancements in balances  better
experiments
Laws Discovered



Law of Conservation of mass: mass is neither
created nor destroyed during chemical or physical
changes
Then discovered no matter where or how a
compound is made it is made of fixed proportions
of elements  Law of Definite Proportions
Ex: Sodium Chloride, table salt, always has
39.34% Na and 60.66% Cl by mass
Laws Discovered


Elements combine to form more than one
compound Ex: carbon monoxide, CO, and
carbon dioxide CO2
Law of Multiple Proportions
John Dalton

1.
2.
1808 –DALTON’S ATOMIC THEORY
All matter is composed of small particles
called atoms.
*All atoms of a given element are
identical, but they differ from those of any
other element.
Dalton’s Atomic Theory
3.
4.
5.
*Atoms cannot be subdivided, created, or
destroyed
Atoms of different elements combine in
whole-# ratios to form compounds
In a chemical reaction, atoms are
combined, separated, or rearranged.
1800s – The atom IS divisible

Atom has two regions:

Nucleus: center of atom
Protons: (+) charged
 Neutrons: no charge



Electron cloud: region around nucleus
containing electrons, (-) charged
Subatomic particles: p+, no, and e-
J.J. Thomson’s Experiment
Cathode - Ray Tube
Voltage source
-
+
Moving electrical current (Cathode Ray) through glass
tube with inert gas (Noble Gas)
J.J. Thomson’s Experiment
Voltage source
+
•
By adding magnets, found the moving (-) particles
Thomson’s Model



Same properties no
matter what element
used
All atoms had
ELECTRONS!!
Thomson’s Model
Plum pudding:
bunch of positive
stuff, with the
electrons able to be
removed.
Ernest Rutherford - 1911




Atoms have e- = (-) charge
Atoms are neutral  need (+) to balance
Must be other particles to account for mass
of atom…
Rutherford used alpha particles to test.


Alpha particle: (+) charge, (2 p+ and 2 no)
Experiment: beam of high speed alpha
particles into thin gold foil
Lead
block
Uranium
Florescent
Screen
Gold Foil
Mr. Green. Mr. Green’s Homepage.
10 Sept. 2003.
http://www.tvgreen.com/index.htm.
21 Aug. 2004
His prediction….
Mr. Green. Mr. Green’s Homepage.
10 Sept. 2003.
http://www.tvgreen.com/index.htm.
21 Aug. 2004
What he expected
Mr. Green. Mr. Green’s Homepage.
10 Sept. 2003.
http://www.tvgreen.com/index.htm.
21 Aug. 2004
What he happened
Mr. Green. Mr. Green’s Homepage.
10 Sept. 2003.
http://www.tvgreen.com/index.htm.
21 Aug. 2004
Results…



Most particles passed through
1 in 8000 bounced back
Some particles completely reversed path!!!
Plum-pudding is WRONG!



Atom is mostly empty
Small dense,
positive piece
at center, nucleus
Alpha particles
are deflected by
nucleus if they get close.
Mr. Green. Mr. Green’s Homepage.
10 Sept. 2003.
http://www.tvgreen.com/index.htm.
21 Aug. 2004
+
+
Mr. Green. Mr. Green’s Homepage.
10 Sept. 2003.
http://www.tvgreen.com/index.htm.
21 Aug. 2004
Electron Cloud Model


Current model of atom
Nucleus surrounded by negative cloud
“electron cloud”
Size of Atom




Atom = size of football stadium
Nucleus = smaller than a dime in center
Electron = each one smaller than
Roosevelt’s eye on the dime
Atom is mostly empty space!
Composition of Atom

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Proton has (+) charge equal in magnitude to (-)
charge of electron
Atoms neutral  # p+ = # ep+ and no about same mass
e- = 1836 times smaller than p+
Nucleus has majority of mass
Nuclear Forces: short-range forces that hold
nucleus together

p+-p+, p+-no, no-no
Counting Atoms

Atomic Number (Z): number of p+ of
each atom of the element

Identifies Elements!!
Atomic Number



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What is the atomic number for…
Hg
What is the element with …
20 p+
Neutral atoms  # p+ = # eIn neutral atoms, atomic number = # e-
Isotopes


Def: atoms of same element with different
# of no (different masses)
3 H isotopes:

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Protium: 1 p+ 99.9885%
Deuterium: 1 p+ 1 no 0.0115%
Tritium: 1 p+ 2 no
Mass Number

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Use mass number to determine isotopes
Mass #: # of p+ and n0
Atom with 17 p+ and 20 no has mass # of…
Chlorine-37 – Hyphen Notation
Writing Isotopes

Nuclear symbol:
Mass number
Atomic number
37
Cl
17
Elemental symbol
Finding # of Neutrons


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# of neutrons = mass # - atomic #
How many p+, no and e- are in…
Selenium – 79
 27Al
13

Oxygen - 18
Isotopes

Write the complete chemical/nuclear
symbol for the isotope with 21 protons, 24
neutrons, and 21 electrons.
Relative Mass of Atom

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Mass of Oxygen-16 = 2.656 x 10-23 g
Use relative atomic masses
Pick standard  other masses are
expressed in relation to standard
Standard: Carbon –12 atom
Carbon - 12






Has mass of 12 atomic mass units (amu)
1 amu: is exactly 1/12 the mass of a
carbon-12 atom
Carbon-12: how many p+ and no?
1 p+  1 amu (1.007276 amu)
1 no  1 amu (1.008665 amu)
1 e-  0 amu (0.005486 amu)
Carbon - 12


What element is 4 x mass of Carbon –12?
What element is 1/3 mass of Carbon –12?
Average Atomic Masses




Most elements are mixture of isotopes
Mass of element is average mass of
isotopes
We need the % of each isotope
Average Atomic Mass: weighted average
of the atomic masses of the naturally
occurring isotopes of an element.
Weighted Average

Box with 100 marbles of 2 sizes:




25% have mass of 2.00g
75% have mass of 3.00g
What is the average mass of a marble?
2.75g (sig figs!!)
Average Atomic Mass

Or (atomic weight) found on periodic table
(ROUND TO 2 DECIMAL PLACES!!)

Average Atomic Mass =
(% Iso. #1)(Mass Iso.#1) + (%Iso.#2)(Mass Iso.#2) + …
100
Average Atomic Mass



Find the atomic mass of Li if…
7.5% is Lithium – 6 = 6.015 amu
92.5% is Lithium – 7 = 7.016 amu
Example – AAM = 16.00 amu
Isotope
Fractional Abundance
Oxygen – 16 (15.99 amu) 99.757%
Oxygen – 17 (17.00 amu) .038%
Oxygen – x
.235%
The Mole
Quantities

1 dozen =


1 gross =


144
1 ream of paper =


12
500
In chemistry: 1 mole = 6.022 x 1023
The Mole





SI unit for amount of substance (mol)
Def: the number of particles in exactly 12 g
of carbon – 12.
12 g of carbon –12 has 6.022 x 1023 atoms
Avogadro’s #, after Amadeo Avogadro
The number is HUGE!!
Molar Mass



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Def: mass of 1 mole of a pure substance
1 mole Carbon –12 = 12 g
1 atom Carbon – 12 = 12 amu
Mass of 1 mole of He atoms?
4.00g/mol
Same as atomic mass from periodic table,
different units
Molar Mass
So: 4.00g He, 6.94 g Li and 200.59 g Hg all
have 6.022 x 1023 atoms = 1 mole!!!
 Molar mass (g/mol)  conversion factor!!
Gram/Mole Conversions

Mass, in g, of 3.6 mol of C?


How many moles are in 23.5 g S?


43g
0.733 mol
p. 85 problems
Conversions with Avogadro’s #
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Diatomic Molecules: elements that exist as
2-atom molecules in natural state
7 diatomic molecules
H2, O2, N2, F2, Cl2, Br2, I2
Remember!!! HONClBrIF
Conversions with Avogadro’s #


How many moles of oxygen are in
2.00x1022 molecules of oxygen?
How many atoms of sodium are in 3.80
mol of sodium?
Conversions with Avogadro’s #



How many molecules of hydrogen are in
0.020 g of hydrogen?
Mass of 5.0x109 atoms of neon?
p.86 problems