Transcript Writing Chemical Formulas

Chemical Formulas
Writing Binary Compounds
Tim Bass
Updated 11-22-01
(c) 2001 Tim Bass
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Chemical Formulas

A shorthand method of representing the
makeup of chemicals using chemical symbols
and oxidation numbers.
Positive Ions always go 1st in the formula.
Negative Ions always go last.
Ionic Bonded chemical formulas are always
written as empirical formulas.
Empirical Formula: The smallest whole number
ratio of elements in a compound.
Updated 11-22-01
(c) 2001 Tim Bass
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Golden Rule For Formulas:
The sum of the oxidation #s =
zero!!
+2 + -2 = 0
Mg+2 Cl-1
2
+2 + -2 = 0
K+1
2
Updated 11-22-01
Sum of the oxidation numbers
always equal zero!!!!
O-2
(c) 2001 Tim Bass
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Charged Particles
Ion: a particle with an electrical charge.
Cation: a positively charged ion.
Anion: a negatively charged ion.
Cation always goes 1st in the formula.
Anion always goes last in the formula.
Updated 11-22-01
(c) 2001 Tim Bass
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Oxidation Number
Oxidation Number: The apparent
charge on an atom.
The charge on an ion.
Also called the valence.

Example: Ca+2
Cl-1
Calcium:
Chlorine:
20P
17p
20N
18N
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Noble Gas
The greater the stability the less active a
substance becomes.
The noble gasses are the least active
substances (therefore are very stable and
low energy).
When the outer shells of an element are full
the atom is at a much lower energy level.
Updated 11-22-01
(c) 2001 Tim Bass
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Shorthand Notation
10Ne:
18Ar:
36Kr:
1s2 2s2 2p6
• (8 valence electrons)
1s2 2s2 2p6 3s2 3p6
• (8 valence electrons)
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6
• (8 valence electrons)
All noble gases (except He) have 8
valence electrons.
Updated 11-22-01
(c) 2001 Tim Bass
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Noble Gas (Octet) Law
Noble Gas Law: When forming
compounds, elements tend to gain or lose
electrons until they have an electron
configuration similar to one of the noble
gases.
Metals tend to lose electrons (become
cations)
Nonmetals tend to gain electrons (become
anions)
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(c) 2001 Tim Bass
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Metals and Nonmetals
Nonmetals
Metals
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(c) 2001 Tim Bass
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Oxidation Examples
Na + Cl - Na+1Cl-1
+2+2
-1 -1
Mg
ClCl
Mg++ Cl
2ClMg
Mg
2
Al +
+ 3Cl
Al
Al+3+3Cl
Cl-13-1
Al
Cl 
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(c) 2001 Tim Bass
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Oxidation Numbers
The sum of the oxidation numbers in a
chemical formula must equal zero.
This is the golden rule of writing formulas for
inorganic compounds!
There is only one category that does not follow this
rule. They follow the rule also, but the rules
change a little to apply to them.
Nonmetals bonded to nonmetals
.
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(c) 2001 Tim Bass
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Updated 11-22-01
(c) 2001 Tim Bass
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Subscripts
Subscript: Small numbers to the
lower right of a symbol.
Represent the number of atoms of that
element in the compound.
Affects only the element immediately in front
of the subscript.
Coefficient: Large number in front of a
chemical formula.
Affects everything following the coefficient.
Updated 11-22-01
(c) 2001 Tim Bass
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H2O
Hydrogen:
Oxidation # X Subscript
+1
X 2
=
+2
Oxygen:
Oxidation # X Subscript
-2
X 1
=
-2
Sum of oxidation numbers = 0
(+2) + (-2) = 0
Updated 11-22-01
(c) 2001 Tim Bass
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A Shortcut
Make the oxidation of one
element the subscript of
the other element.
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(c) 2001 Tim Bass
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A Shortcut
There is one problem!
+2
-2
CaO
C a2 O2
These subscripts will reduce.
Remember: Ionic bonded compounds are
always written as an empirical formula.
Updated 11-22-01
(c) 2001 Tim Bass
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Polyatomic Ion
 An ion made up of more than one
atom that behaves like one atom.
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(c) 2001 Tim Bass
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Polyatomic Ion
Never change subscripts within a
polyatomic ion. (Changes the polyatomic
ion.)
If more than one polyatomic ion is needed,
put parenthesis around the ion and add the
subscript needed outside the parenthesis.
NH4OH
(NH4)2O
Al(MnO4)3
Updated 11-22-01
Ammonium Hydroxide
Ammonium Oxide
Aluminum Permanganate
(c) 2001 Tim Bass
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Polyatomic Ion
The sum of the oxidation numbers in an ion
is equal to the charge on the ion.
You must learn the names, formulas, and
charges of the most common ions.
MnO4-1
OH-1
CN-1
NH4+1
Updated 11-22-01
Permanganate
Hydroxide
Cyanide
Ammonium
(c) 2001 Tim Bass
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Warnings
1. Once the sum of the oxidation #s = 0,
do not change the formula.
2. Parenthesis are only for polyatomic
ions, and only if they are needed two
times or more in a chemical formula.
3. Always make sure that the final chemical
formula is an empirical formula!
Updated 11-22-01
(c) 2001 Tim Bass
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Chemical Formulas
End
Updated 11-22-01
(c) 2001 Tim Bass
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