Atoms, Molecules and Ions

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Chapter 2

• Atoms, Molecules and Ions

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I. Particulate nature of matter
A) Submicroscopically matter is composed of
particles. Matter has lots of holes in it. There
is empty space between the particles.
MATTER IS NOT CONTINUOUS
B) This idea was not accepted easily.
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It took 2200 years from the time the idea was
first proposed by a Greek philosopher,
Democritus, who expressed a belief (not based on
scientific evidence) that all matter is composed
of tiny, indivisible particles called atomos
(uncuttable), to when the evidence acquired in
the 18th and 19th centuries from experiments
provided overwhelming evidence to scientists that
matter is particulate in nature.
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C) Evidence obtained in the 18th and 19th
centuries came from "scientists" doing
experiments involving careful measurements of the
mass of the reactants and the products in
chemical reactions. Some of this evidence was
summarized into LAWS.
A law is a concise statement or mathematical
equation about a fundamental relationship or
regularity of nature.
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1) Law of Conservation of Mass-mass is neither
created nor destroyed in a chemical reaction.
2) Law of Definite Proportions-for a given
compound, the elements are always combined in the
same proportions by mass.
For example water contains 1 g of hydrogen for
every 8 g of oxygen. Carbon dioxide contains
1.00 g of carbon for every 2.67 g of oxygen.
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D) John Dalton (1808-1809) proposed his atomic
theory based on his knowledge of the experimental
results of other scientists which led to the laws
mentioned above.
The main points of his theory follow
1. Matter consists of tiny indivisible particles
called atoms. An atom is an extremely small
particle of matter which retains its identity
during a chemical reaction.
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2. An element is a type of matter composed of
only one kind of atom. Each atom of a given kind
has the same properties. Atoms of a given element
have a characteristic mass.
3. A compound is a type of matter composed of 2
or more elements chemically combined in fixed
proportions.
4. Atoms are not created nor destroyed in a
chemical reaction. A chemical reaction consists
of a rearrangement of the atoms present in the
reacting substances to give new chemical
combinations.
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A good theory predicts new laws or facts.
E) Law of Multiple Proportions, deduced by Dalton
from his theory states that when two elements
form more than one compound, the different masses
of one element that combine with the same mass of
the other element are in the ratio of small whole
numbers.
For example
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Where is the ratio of small whole numbers?
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Applying the atomic theory, the formulas of the
compounds are N2O, NO, NO2.
The deduction of the law of multiple proportions
from atomic theory aided in convincing chemists
of the validity of the theory.
F) ATOMS AND MOLECULES
1) An atom is the smallest particle of an element
that can enter into chemical combination. Our
model is a sphere. Chemical properties of
elements differ because atoms differ.
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WHAT IS IT LIKE INSIDE A BALLOON FILLED WITH
HELIUM?
2) Molecules are the smallest part of an element
or compound that can have a stable independent
existence.
There are monatomic, diatomic, triatomic,
tetraatomic, etc. molecules.
Name some molecular substances
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II. What are the components of the atom?
A) Determination of the existence and the nature
of the electron(s).
1) Sir William Crookes used an electrical
discharge tube, (a Crookes' tube - a cathode ray
tube - CRT) to discover the nature of the
"electron".
DEMONSTRATIONS WITH CRT's
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2) From these experiments it was deduced that
cathode rays are streams of negatively charged
particles. Different metals as the cathode and
gave the same result, all substances contain
these particles. These particles were named
electrons. J.J. Thomson, 1897, tried to measure
the characteristics of the electron.
Due to the design of the experiment, he wasn't
able to obtain either the mass or the charge of
the electron alone, but he was able to obtain the
ratio of the charge to the mass of the electron.
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Charge/Mass -1.759 X 108 Coulombs/gram
What does this mean?
C) 1906 - 1916 - American physicist, Robert
Millikan measured the charge on the electron with
his famous oil drop experiment
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He discovered that the charge on one electron is
-1.602 X 10-19 coulombs. HOW?
D) By combining J. J. Thomson's value and
Millikan's value for the charge, we can obtain
the mass of an electron by arithmetic
Charge/mass -1.759 x 108 C/g
-1.602 x 10-19 C/m -1.759 x 108 C/g
m -1.602 x 10-19C -1.759 x 108 C/g
m _______________
With respect to mass, electrons barely exist!!
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E) Eugen Goldstein discovered canal rays - which
were really positive particles.
1) These were called protons.
2) Each one had a charge of 1.602 X 10-19
Coulombs.
3) Each one had a mass of 1.6727 X 10-24 g.
F) As a result, we believe that atoms are
composed of positive and negative particles.
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G) Discovery of atomic bullets in 1890's, was
necessary for further investigation of the atom.
1) Radioactivity is discovered - the spontaneous
emission of particles and energy from radioactive
elements.
2) Three types of rays were discovered - alpha,
beta and gamma rays first three letters of the
Greek alphabet.
3) Becquerel, the Curies, and others were people
who worked in this area of study.
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4) Further investigation of these rays showed
that alpha rays were really particles, helium
atoms stripped of their electrons. Beta rays were
really electrons, (particles??) and gamma rays
were high energy radiation.
III. Where are the protons and electrons located
in an atom?
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A) Rutherford in 1911 set out to prove Thomson's
theory which said that atoms are like "plum
pudding" (Britain). Americans might use the
analogy of chocolate chip ice cream or raisin
muffins.
There is a sphere of protons with the electrons
inside the sphere. The atom has a constant
density throughout as the electrons would not add
much mass to the position which they held.
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Why should they think in this manner?
THE EXPERIMENT
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2) The unexpected happened, some were deflected
slightly, a few bounced straight back to the
origin.
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3) Rutherford and his graduate student, Geiger,
were astonished! How could a piece of tissue
paper bounce back a 15 inch cannon ball???
B) What did Rutherford conclude from this
experiment? What was the evidence?
1)Since most of the particles went straight
through, the volume of the atom must be largely
empty space.
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2)Since some alpha particles were deflected and
repelled, each atom must contain a positively
charged massive body.
3) This massive body is called the nucleus which
contains 99.9 of the mass of the atom and has a
diameter of 1 X 10-12 cm.
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4) The electrons must be outside this nucleus and
surround it to the give the atom a diameter of
about 1 nanometer (10-7 cm) or 100,000 times
bigger than the nucleus.
IV. Neutrons
A) All of the mass of atoms of most elements
could not be accounted for by adding the mass of
the protons and the mass of the electrons.
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B) In 1920, Rutherford postulated the existence
of neutral particles of mass about equal to that
of protons.
C) In 1932, James Chadwick detected neutrons.
Neutrons have a charge of 0, and a mass of 1.675
X 10-24 g. (About the same as the mass of a
proton.)
V. The Nuclear Atom
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A) Protons are positively charged particles.
Since the charge of one proton in coulombs is
very small, it is given a charge unit of 1.
Since the mass in grams is very small as well, a
relative mass unit has been invented so that
protons have a relative mass of 1 atomic mass
unit or to more significant figures 1.00728 amu's.
B) Electrons then are given a charge unit of -1
and a relative mass of 0 amu (5.5 X 10-4 amu).
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C) Neutrons have a charge of 0 and a relative
mass of 1 amu (1.00866amu).
D) For an atom to be neutral - have no net charge
- the number of protons must be equal to the
number of electrons.
VI. Characterizing Elements
A) The number of protons in the nucleus
identifies an atom as a particular kind. The
atomic number is the number of protons in an atom
- its symbol is Z.
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EXAMPLES Any atom having 2 protons is a
________________ atom.
Any atom having 26 protons is a ________ atom.
By now you should know the symbols of the common
elements and the spelling of their names. See
table 2.1 page 44 of the text.
B) The symbol contains 1 capital letter or 1
capital letter and 1 lower case letter, NOT A
SMALL CAPITAL LETTER. It is best to print the
capital letter and write the small letter if
there is one.
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PROBLEM ELEMENTS Mg and Mn F - fluorine
C) Most periodic charts have the atomic number
above the symbol.
D) The other number which is important in the
identification of an atom as a particular kind is
the mass number, symbol - A. This represents the
total number of neutrons and protons present in
the nucleus of an atom of a particular element.
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E) In most cases, all atoms of a given element do
NOT have the same mass.
ISOTOPES are atoms of the same element that have
different numbers of neutrons - different A - but
the same number of protons - same Z.
These were discovered when very pure samples of
elements were placed in a MASS SPECTROMETER.
(More on this device later)
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F) A nuclide is a particular nucleus
characterized by a definite atomic number and
mass number.
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• If it is an ion, an atom that has gained or lost
  electrons, the charge on the ion appears in the
  upper right hand corner.
• If it has lost electrons, there are more protons,
  more positive charges, the symbol will include a
  n.

Xn
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3) If the atom has gained electrons, more
negative charges than positive charges, then the
symbol will include a _______ .
4) For H there are 3 isotopes protium,
deuterium, and tritium.
1H 2H 3H
All must have ______ proton(s).
All must have ______ electron(s).
How do they differ?
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3) Usually we don't have names for the different
isotopes usually we call 60Co, cobalt-60.
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If an element has an atomic number of 48 and a
mass number of 114, what is
the number of protons? the number of
electrons? the number of neutrons?
the nuclide symbol?
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VII. ATOMIC MASS
A) Early scientists could not measure the mass of
one atom and they felt it was crucial to have a
number which related to the mass of an atom. What
could they do?
When they measured the mass of a pile of atoms,
since they didnt know how many were in the pile,
they could not divide by that number to find the
mass of one.
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B) They developed the concept of relative mass.
C) They started by comparing the mass of elements
with the least dense gas - hydrogen. Hydrogen
atoms were given a relative mass of 1.0 atomic
mass unit.
D) They still had a problem. How did they know if
they were comparing equal numbers of particles?
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They knew that 1 g of hydrogen combined with 8 g
of oxygen to give 9 g of water. They assumed that
the formula for water was HO and gave oxygen a
relative mass of _____ g.
When they discovered the formula for water was
H2O, then the relative mass of oxygen has to be
changed to 16 amu... WHY???
E) Since lots of compounds have O in them, O
became the standard, and in 1961, carbon-12
became the standard. That is, a carbon-12 atom
has a mass of exactly 12.00000 amu.
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F) 1 amu the mass of a C-12 atom in amus
divided by 12.
G) A hydrogen atom is 8.400 as massive as the
standard C-12 atom, therefore the relative mass
of a hydrogen atom is 0.08400 X 12.00 or 1.008
amu.
VIII. WHY ISN'T THE ATOMIC MASS OF CARBON
12.00000 ON The Periodic Chart?
A) A natural sample of the element carbon is a
mixture of different isotopes as found in the
mass spectrometer.
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B) The mass spectrometer is a device used to
obtain the most accurate atomic masses relative
to C-12 in the most direct way. It can tell you
the masses of the isotopes as well as the age of
each isotope in a sample.
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This indicates that we have 3 naturally occurring
isotopes of neon 90.92 of Ne-20, 0.257 of
Ne-21, and 8.82 of Ne-22.
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The mass on the chart is a weighted average mass
obtained by multiplying the mass of each isotope
by its fractional abundance (the changed to a
decimal) and adding the results together.
For boron, B-10 atoms have a fractional abundance
of 0.1960 and B-11 atoms have a fractional
abundance of 0.8040.
10.0129 X 0.1960 1.963
11.0093 X 0.8040 8.851
10.814
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Since the chart says 10.811 with the notation of
OR -.005, this answer is good enough for us. We
will use 10.81 as the atomic mass for boron. In
general we will use the atomic masses as given on
the chart to the hundredths place, except for
hydrogen which we will use to the thousandths
place.
REMEMBER There are no atoms of boron which have
a mass of 10.81. In a collection of naturally
occurring boron, the average mass of a boron atom
is 10.81 amu.
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XI. THE PERIODIC TABLE
A) Mendeleev (1834-1907) and Meyer (1830-1895)
worked independently on categorization of the
elements according to their properties. Mendeleev
used primarily chemical properties, while Meyer
used physical properties to arrange the elements
in groups. They also arranged the elements in
order of increasing relative mass. The vertical
lines separating the elements described the
groups, while the horizontal lines separating the
elements described the periods.
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Why is Mendeleev given credit for the modern
periodic table?
Placing the elements in order of their atomic
masses was problematic with respect to their
relationships in columns with like elements.
Mendeleev was not happy with tellurium and
iodine, whose relative atomic masses suggested
that iodine be placed before tellurium. Mendeleev
violated his periodic law and placed iodine and
tellurium in their "correct" positions.
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Argon was found to be slightly heavier than
potassium, another violation of the periodic law.
Clearly, atomic weight was not the basis of the
periodic law. It wasn't until the concept of
atomic number was brought forth by X-ray studies
of Moseley in the early 1900's that the true
basis of the atomic law was found.
B) The Modern Periodic Law states that the
chemical and physical properties of the elements
vary in a periodic way with their atomic number.
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C) The periodic table is divided into
1) 7 horizontal rows called periods or series.
2) 32 vertical columns called groups or families.
D) Elements are classified into 3 general
categories.
1) METALS - good conductors of heat and
electricity, have a luster, are malleable and
ductile.
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2) NONMETALS - nonconductors of electricity,
brittle, many are gases at room temperature.
3) METALLOIDS - have characteristics of both
metals and nonmetals. They are the elements of
the computer and semiconductor age B, Si, Ge,
As, Sb, Te, At.
4) TRENDS - FROM LEFT TO RIGHT - metallic
character decreases. FROM TOP TO BOTTOM -
metallic character increases.
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E) GROUP NAMES and numbering systems
1) THREE current systems of numbering columns IA
- VIIIA, 1A - 8A, AND 1 - 18.
2) ALKALI METALS - column 1A, 1, IA
3) ALKALINE EARTH METALS - column 2A, 2
4) HALOGENS - column 17, 7A
5) NOBLE GASES - column 18, 8A
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6) MAIN GROUP OR REPRESENTATIVE ELEMENTS
columns labeled A.
7) TRANSITION ELEMENTS - columns 3-12 or Group B
elements.
8) INNER TRANSITION ELEMENTS - at the bottom of
the periodic chart.
a) first row - lanthanides b) second row -
actinides
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XII. NOMENCLATURE of INORGANIC COMPOUNDS -
REQUIRES practice, practice, practice -
A) Four categories of INORGANIC COMPOUNDS
1) Ionic, 2) Molecular, 3) Acids and Bases, and
4) Hydrates
B) IONIC COMPOUNDS Ionic Salts-Consist of Anions
and Cations.
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Anions (-) and cations () are charged species
formed by the transfer of electrons from one atom
(or group of atoms) to another.
Polyatomic ions-Ions consisting of multiple atoms.
Since salts do not consist of molecules, formula
units are used to represent ionic compounds
symbolically.
Metals typically form cations, while non-metals
typically form anions.
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Ions are held together by ionic bonds.
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For the A group metals, the group number defines
the most common charge associated with the metal
ion.
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For the group A non-metals, the most common
charge found is the group number minus 8. For
example, O is in group VIA, and therefore forms a
-2 anion (6 - 8 -2). Cl has a -1 charge, N has
a -3charge, S has a -2 charge, etc.
For the transition metals, several different ions
can be formed from the same type of atom.
Examples- Fe2 and Fe3 and Cu and Cu2.
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Examples of Ionic Compounds
NaCl consists of a 11 ion ratio of Na and Cl-,
not individual NaCl molecules.
Ca3(PO4)2 consists of a 32 ion ratio of Ca2 and
PO43-.
You will need to memorize some common cations of
the transition elements and so common polyatomic
ions. The tables on pages 65 and 66 of the 8th
addition of Ebbing.
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RULES FOR WRITING IONIC FORMULAS
To write a correct formula, the formula must be
written to balance the charge.
Examples
K and S2- forms K2S
Ca2 and PO43- forms ____________________
Na and SO42- forms _____________________
As a rule of thumb, switch the charges and use
the absolute values of the charges as the
subscripts in the formula.
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RULES FOR NAMING IONIC SUBSTANCES
1) Name the cation first - has the same name as
the metal (ammonium ion exception).
2) Name the anion second. If only one element,
the ending changes to -ide.
AlCl3 ________________________
K3N _________________________
CaCl2 ________________________
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3) If the metal is not from Group 1, 2, or Al,
Zn, Cd, Ag, then a roman numeral indicating the
charge on the ion (i.e. its oxidation number),
follows the name of the metal. Then the anion is
named.
CuO __________________________
4) How do we know what charge to assign Cu?
In ordinary compounds, oxygen has a charge
(oxidation number) of-2.
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Cu2O ___________________________

Hg2Cl2 __________________________
Fe2O3 ____________________________
C) MOLECULAR COMPOUNDS
1) A molecular substance is usually composed of
nonmetals only.
2) A molecular substance is composed of
molecules, the formula of which gives the exact
number of different atoms in a molecule, for
example CO2 and H2O.
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Rules for naming inorganic binary (two different
atom) molecular compounds, compounds not
containing C atoms except for a few like CO2.
The name tells you the number of each kind of
atom, since prefixes are used to tell the number.
4) the name starts with a prefix if necessary,
the name of the first element, a prefix, then the
name of the second element, with the ide ending.
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Examples
N2O4 _________________________
S2Cl2 ___________________________
Name these
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D) NAMING ACIDS
1) What are acids?
We will use the Arrhenius definition first.
Acids give H ions in water solution. They have H
in them. The H will be next to an anion. The
anion determines the name.
2) Naming acids with H's and no O's
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If the anion attached to the H ends in ide,use
the prefix hydro and change the ide to ic.
HCl is _______________________________
HCN is ______________________________
H2S is ___________________________
3) Naming acids with H's and O's if only 2 acids
in a series.
Use -ous or -ic ending. The ous ending is for the
acid with the fewer O's, the ic ending is for the
acid with more O's.
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There are two H2SOx acids.
H2SO3 - fewer O's _____________________ H2SO4 -
more O's _______________________
4) Naming acids if more than two in a series
requires prefixes.
HClO - hypochlorous acid - fewest O's HClO2 -
chlorous acid -fewer O's HClO3 -
chloric acid - more O's HClO4
-perchloric acid - most O's
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D) NAMING IONIC COMPOUNDS RELATED TO ACIDS
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1) -ous ending gives the -ite salt - Na2SO3 -
____________________________
2) -ic ending gives the -ate salt - Na2SO4 -
_____________________________
3) the hypo_____ous acid gives the hypo____ite
salt - KClO - ________________
4) the per_____ic acid gives the per____ate salt
- KClO4 - _________________________
If you look on a bottle of CHLOROX, you will see
that it contains NaOCl (NaClO) ___________________
______ .
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F) HYDRATES
A hydrate is a compound with water molecules
weakly bound in its crystals. A common one is
epsom salts.
Its formula is MgSO47 H2O. Its scientific name
is magnesium sulfate heptahydrate.
You name the salt plus a prefix indicating the
number of water molecules and the word hydrate.
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XI REPRESENTATION OF CHEMICAL REACTIONS
A) Chemical equations describe chemical reactions
using symbolic representations.
Zn S ? ZnS
2Mg O2 ? 2MgO
B) Coefficients represent the number of each
element or compound.
C) The arrow represents the direction of the
reaction.
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D) The meanings of the symbols used
1) on the left side should be read as "reacts
with" not plus.
2) ? is read as to yield, to form, to produce.
3) on the right side is read as and.
E) When the coefficients are correctly written
the equation is balanced and the law of
conservation of mass (matter - atoms) is obeyed.
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Often the equation will and must for certain
purposes indicate the state of each of the
reactants and products.
(g) for gas, (l) for liquid, (s) for solid, (aq)
for water solution.
XII BALANCING EQUATIONS BY INSPECTION - TRIAL AND
ERROR
A) First balance those atoms which occur in only
one substance on each side of the equation
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C3H8(g) O2(g) ? H2O(g) CO2(g)
I would first balance the C's, then the H's, then
the O's.
C3H8(g) O2(g) ? H2O(g) 3CO2(g)
C3H8(g) O2(g) ?4H2O(g) 3CO2(g)
C3H8(g) 5O2(g) ? 4H2O(g) 3CO2(g)
BE CAREFUL THINK! THINK! THINK!
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Students have more trouble with the following
equation
C4H10(g) O2(g) ? H2O(g) CO2(g)
I would first balance the C's, then the H's, then
the O's.
C4H10(g) __O2(g) ? __H2O(g) __CO2(g)
Initially there will be a fraction in front of
the O2. We prefer whole numbers, except in
certain cases, so we have to multiply each
coefficient by ___.
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Your homework will be due the next lecture period.
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The End