Download Unit 1 - Mount St. Mary Catholic High School

9/7/2010
Chapter 2

Democritus (460-370 BC)
◦ thought the material world was made up of tiny
indivisible particles called atomos

Plato (427 – 347 BC) & Aristotle (384 – 322
BC)
◦ believed that ultimately nothing is indivisible
1.
2.
3.
4.
Each element is composed of extremely
small particles called atoms.
All atoms of a given element are identical to
other atoms of the same element and
different from atoms of other elements.
Atoms are not created or destroyed in
chemical reactions.
Compounds are formed when atoms of
more than one element combine. A given
compound always has the same ratio of
types and numbers of atoms.
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together
these
particles
crate the
nucleus


proton (p+) relatively “heavy”
neutron (n0) mass similar to proton

electron (e-) very, very light

J. J. Thomson- used Cathode ray tubes
-
Voltage source
+
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-
Voltage source
+
Thomson’s Experiment
-
Voltage source
+
Thomson’s Experiment

Voltage source
+
Passing an electric current makes a beam appear to move from
the negative to the positive end
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Thomson’s Experiment

Voltage source
+
Passing an electric current makes a beam appear to move from
the negative to the positive end
Thomson’s Experiment

Voltage source
+
Passing an electric current makes a beam appear to move from
the negative to the positive end
Thomson’s Experiment

Voltage source
+
Passing an electric current makes a beam appear to move from
the negative to the positive end
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Voltage source

By adding an electric field
Thomson’s Experiment
Voltage source
+

By adding an electric field
Thomson’s Experiment
Voltage source
+

By adding an electric field
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Thomson’s Experiment
Voltage source
+

By adding an electric field
Thomson’s Experiment
Voltage source
+

By adding an electric field
Thomson’s Experiment
Voltage source
+

By adding an electric field
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Thomson’s Experiment
Voltage source
+





By adding an electric field he found that the moving pieces were
negative
Found the electron
Couldn’t find
positive (for a while)
Said the atom was
like plum pudding
A bunch of positive
stuff, with the
electrons able to be
removed
Atomizer
+
Oil
Microscope
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Atomizer
Oil droplets
+
Oil
Microscope
X-rays
X-rays give some drops a charge by knocking off
electrons
+
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-
-
+
+
They put an electric charge on the plates
-
-
+
+
Some drops would hover
-
-
-
-
- -
-
+
+
+
+
+
+ +
+
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-
+
-
+
Measure the drop and find volume from 4/3πr3
Find mass from M = D x V
-
+
-
+
From the mass of the drop and the charge on
the plates, he calculated the charge on an electron



Discovered by accident
Bequerel
Three types
◦ alpha- helium nucleus (+2 charge, large mass)
◦ beta- high speed electron
◦ gamma- high energy light
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



Used uranium to produce alpha particles
Aimed alpha particles at gold foil by drilling
hole in lead block
Since the mass is evenly distributed in gold
atoms alpha particles should go straight
through.
Used gold foil because it could be made
atoms thin
Lead
block
Uranium
Florescent
Screen
Gold Foil
What he expected
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Because
Because, he thought the mass was evenly distributed in
the atom
What he got
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How he explained it



Atom is mostly empty
Small dense, positive
piece at center
+
Alpha particles are
deflected by it if they
get close enough
+




The atom is mostly
empty space
Two regions
Nucleus- protons and
neutrons
Electron cloudregion where you
have a chance of
finding an electron
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


Z - atomic number = number of protons
determines type of atom
A - mass number = number of protons +
neutrons
Number of protons = number of electrons if
neutral
 Atoms
are composed of identical
protons, neutrons, and electrons
 Elements are different because they
contain different numbers of protons
 The “atomic number” of an element is
the number of protons in the nucleus
#
protons in an atom = # electrons
(in a neutral atom)
Atomic number (Z) of an element is
the number of protons in the nucleus
of each atom of that element.
Element
# of protons
Atomic # (Z)
Carbon
6
6
Phosphorus
15
15
Gold
79
79
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Mass number is the number of
protons and neutrons in the nucleus
of an isotope: Mass # = p+ + n0
p+
n0
e- Mass #
8
10
8
18
Arsenic - 75
33
42
33
75
Phosphorus - 31
15
16
15
31
Nuclide
Oxygen - 18
 Contain
the symbol of the element,
the mass number and the atomic
number.
Mass
Superscript →
number
Subscript →
Atomic
number
X
Symbols

Find each of these:
a) number of protons
b) number of
neutrons
c) number of
electrons
d) Atomic number
e) Mass Number
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35
Br
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Symbols

If an element has an atomic
number of 34 and a mass
number of 78, what is the:
a) number of protons
b) number of neutrons
c) number of electrons
d) complete symbol
Symbols

If an element has 91 protons and
140 neutrons what is the
a) Atomic number
b) Mass number
c) number of electrons
d) complete symbol
Symbols
 If an element has 78 electrons
and 117 neutrons what is the
a) Atomic number
b) Mass number
c) number of protons
d) complete symbol
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 Dalton
was wrong about all elements of
the same type being identical
 Atoms
of the same element can have
different numbers of neutrons.
 Thus,
different mass numbers.
 These
are called isotopes.
 Frederick
Soddy (1877-1956)
proposed the idea of isotopes in
1912
 Soddy
won the Nobel Prize in
Chemistry in 1921 for his work with
isotopes and radioactive materials.
 Name
– mass number
◦carbon-12
◦carbon-14
◦uranium-235
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Isotope
Protons Electrons
Neutrons
Hydrogen–1
(protium)
1
1
0
Hydrogen-2
(deuterium)
1
1
1
1
1
2
Hydrogen-3
(tritium)
Nucleus
Elements
occur in nature
as mixtures of
isotopes.
How heavy is an atom of oxygen?
It depends, because there are different
kinds of oxygen atoms.
 We are more concerned with the average
atomic mass.
 This is based on the abundance
(percentage) of each variety of that
element in nature.

 We don’t use grams for this mass because
the numbers would be too small.
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
Instead of grams, the unit we use is the
Atomic Mass Unit (amu)

An amu is one-twelfth the mass of a carbon12 atom.
◦ Carbon-12 chosen because of its isotopic purity.

Each isotope has its own atomic mass, thus
we determine the average from percent
abundance.

Multiply the atomic mass of each isotope
by it’s abundance (expressed as a
decimal), then add the results.

If not told otherwise, the mass of the
isotope is expressed in atomic mass units
(amu)
Atomic mass is the average of all the
naturally occurring isotopes of that element.
Isotope
Symbol
Carbon-12
12C
Carbon-13
13C
Carbon-14
14C
Composition of
the nucleus
6 protons
6 neutrons
6 protons
7 neutrons
6 protons
8 neutrons
% in nature
98.89%
1.11%
<0.01%
Carbon = 12.011
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
Work Practice Exercise on page 48.
Periodic Table



Conductors
Lose electrons
Malleable and ductile
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


Brittle
Gain electrons
Covalent bonds
Alkali Metals
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Alkaline Earth Metals
Halogens
Transition metals
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Noble Gases
Inner Transition Metals



formed when an atom has gained or lost
electrons
+ ions have lost electrons (negatives)
- ions have gained electrons
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


Occurs during bonding to stabilize electrons
Each element wants 8 valence (outer shell)
electrons
Atoms gain or lose electrons to satisfy the
octet rule
+1 +2

-3 -2 -1
Molecular formula
◦ Gives actual number and types of atoms
◦ E.g. H2O2

Empirical formula
◦ Gives the simplest ratio of elements
◦ E.g. HO

Structural formula
◦ Shows the arrangement of atoms
◦ H-O=O-H
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Molecular (covalent) – two nonmetals
Ionic – metal + nonmetal
Acid – begins with a H
Charges must “balance”
Write the metal first
Memorize your common ions!
Watch out for transition metals
NH4Br
Cr2O3
Co(NO3)2
potassium sulfide
calcium bicarbonate
nickel (II) perchlorate
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1
2
3
4
5
6
7
8
9
– mono (use only on 2nd element)
- di
- tri
- tetra
- penta
– hexa
- hepta
– octa
– nona
Element further left on periodic table goes first
1st element: prefix (except mono) + name
2nd element: prefix + root of name + ide
SO2
PCl5
N2O3

silicon tetrabromide
disulfur dichloride
carbon monoxide

Name of compound prefix-hydrate




E.g. Na2SO4•10H2O
Sodium sulfate decahydrate
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