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Chemical Interactions
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Chemical Formulas and Equations – Chapter 5 and 6
Writing Chemical Formulas 5.2 (pg. 161)
A
chemical formula is a combination of symbols that represents the elements in a compound and
their proportions.
Writing formula information adapted from:
A subscript number tells how many of that specific atom is in the
http://www.kentchemistry.com/links/naming/
formula - H20 has 2 hydrogen atoms; 1 is not written on the oxygen, it
formulawriting.htm and Physical Science,
Prentice Hall Science Explorer, 2009
is understood that there is at least one atom if the symbol is there
Ionic Compounds

Follow these steps to write a chemical formula for ionically bonded substances
1. Identify the symbol of the cation (first part of the name – positive ion) and the anion (the second
part of the name – negative ion usually; check to see if it is a polyatomic ion)
2. Identify the valence or charge of each symbol and write it just above the symbol
3. Balance the total positive and negative charge on the cation (positive ion) and anion (negative
ion). You ask yourself do the total positive charge and total negative charge add up to zero. If
the answer is no then we ask how many of each ion must we have in order to balance charge
(find the least common multiple). We must have the same number of positive charges as we do
of negative charges. Another way of saying that is that they must add up to zero. Another way of
doing this is the criss-cross method (go back and check) – see example below
Ex. Calcium chloride
Ca2+
Ca
Cl1Cl2 = CaCl2
Charge check one 2+ for calcium cancels two 1- changes for chloride (total of 2-). Total charge equals
zero. NOTICE THE 1 IS ASSUMED ON THE Ca!
Ex. Aluminum sulfate Al3+ SO42Al2 (SO4)3 = Al2(SO4)3
Charge check two 3+ for aluminum (total of 6+) cancels three 2- charges (total of 6-). Total charge
equals zero (used a least common multiple of 6). USE PARENTHESIS TO INDICATE A POLYATMOIC ION
WITH A SUBSCRIPT!
4. Once you have determined the number of units of the cation and anion those become the
subscripts that are placed right after the respective symbol.
Cations (Positive Ions)
All Group 1 elements in the Periodic Table are +1 in
compounds.
All Group 2 elements in the Periodic Table are +2 in
compounds.
Transition elements (have a few charges) will have a
Roman Numeral to tell you what positive charge to use.
Anions (Negative Ions)
Group 17 are 1- (will end with -ide)
Group 16 are 2- (will end with -ide)
Group 15 are 3- (will end with -ide)
Polyatomic Ions -just use its charge. (Generally ends with -ate or
-ite, watch out for hydroxide OH- and cyanide CN-)
Silver is 1+, Zinc is 2+ and Aluminum is 3+
Polyatomic Ions -just use its charge.
Chapter 5 and 6 - Chemical Formulas and Equations Notes
8th Grade Science
1
Ex. What is the formula of Copper (I) Oxide?
1. Identify the symbols of the cation and anion
Copper is Cu and Oxide is O
2. Identify the charge for each and place above the symbol
For Copper I that would be 1+ and for Oxide that would be 23. Balance the positive and negative charges
Since each Copper is 1+ and each Oxide is 2- then it will take two Cu+ to balance one oxide with
a 2- so that 2(1+) + 1(2-) = 0. The numbers outside the parenthesis become the subscripts in
the formula.
4. Write the formula placing the subscripts right after the symbol they go with.
Cu2O
that there is no subscript 1 after the Oxide symbol. That is because a subscript one is
understood to be there if the symbol is written. If it was zero, it wouldn't appear at all in the formula.
Notice
Covalent Compounds
Follow
these steps to write a chemical formula for covalent bonded substances
1. Identify and write the chemical symbols for the elements listed in the name.
2. Write the corresponding subscript to the symbol if any numerical prefixes are
used. (Mono will be dropped if it’s the first element and the subscript 1 is
understood)
Ex. Tetraphosphorous decaoxide = P O
4 10 = P4O10
Ex. Sulfur dioxide = S O
1 2
= SO2
Mono = 1
Di = 2
Tri = 3
Tetra = 4
Penta = 5
Hexa = 6
Hepta = 7
Octa = 8
Nona = 9
Deca = 10
Naming Chemical Compounds (from formulas)
5.2 (pg. 161)
Binary
compounds have two types of atoms (not diatomic which has only two atoms). The positive ion
comes first.
Metals (groups 1 and 2) and Non-metals
Metal _________ + Non-Metal _________ide
Example: Sodium Chloride NaCl
Transition Metals (groups 3 - 12) and Non-metals
Metal ______ + Roman Numeral (__) + Non-Metal ________ide
Example: FeBr3 is Iron (III) Bromide
Compare with FeBr2 as Iron (II) Bromide
Subscripts criss-cross back the other way to give the oxidation number (charge)
Chapter 5 and 6 - Chemical Formulas and Equations Notes
8th Grade Science
2
This chart shows some possible ions
for transition metals. Usually the
oxidation number will be specified in
the equation. That is your Roman
numeral.
Oxidation number - the charge left over after the
atoms becomes “happy”
Transition Metals (groups 3 - 12) and Non-metals – OLD
Metal (Latin root) _______ + ous or ic + Non-Metal ________ide
Example: FeBr2 is Ferrous Bromide
Compare with FeBr3 as Ferric Bromide
Write the root of the name for the first element followed by
“ous” for the lower of the possible oxidation numbers or "ic" for
the higher of the possible oxidation numbers.
Example: Fe2(SO4)3 = ferric because the two oxidation numbers
for iron are +2 and +3
Non-metals and Non-metals
Prefix _______ Non-metal _________ + Prefix _______ Non-Metal ________ide
Prefixes tell how many atoms are in the formula.
Mono = 1
Di = 2
Tri = 3
Tetra = 4
Penta = 5
Hexa = 6
Hepta = 7
Octa = 8
Nona = 9
Deca = 10
Examples:
CO2 Carbon dioxide
CO Carbon monoxide
PCl3 Phosphorus trichloride
CCl4 Carbon tetrachloride
N2O5 Dinitrogen pentoxide
CS2 Carbon disulfide
Chapter 5 and 6 - Chemical Formulas and Equations Notes
8th Grade Science
3
Acids (with H in front)
Binary acids (without oxygen in formula)
Hydro _________ ic Acid
Examples: HCl Hydrochloric acid HBr Hydrobromic acid
Oxy acids (with oxygen in formula)
-ate goes to –ic and –ite goes to –ous
Examples:
HNO3 Nitric acid
H2SO4 Sulfuric acid
H3PO4 Phosphoric acid
HNO2 Nitrous acid
H2SO3 Sulfurous acid
H3PO3 Phosphorous acid
Chapter 5 and 6 - Chemical Formulas and Equations Notes
8th Grade Science
4
Chemical Formulas and Equations 6.2 (pg. 194-199)
The
formula of a compound identifies the elements in the compound
and the ratios in which their atoms are present.
A
subscript number tells how many of that specific atom is in the
formula - H20 has 2 hydrogen atoms; 1 is not written on the oxygen,
it is understood that there is at least one atom if the symbol is there.
Sometimes
a coefficient number (number at the beginning of a
formula) is used to also describe how many atoms or molecules are present.
Chemical
equations use chemical formulas and other symbols instead of
words to summarize a reaction.
The
reactants are the things that change around how their atoms bond to
each other; they are on the left of the equation.
The
products are the result of changing bonds between atoms; they are
on the right of the equation.
Conservation of Mass 6.2 (pg. 194-199)
The
principle of conservation of mass states that in a chemical reaction, the total mass of the
reactants must equal the total mass of the products.
Matter
is not created or destroyed. All atoms present at the start of the reaction are present at the end.
Information adapted from:
Paul Surko at New Dimensions
High School in Poinciana, FL
and Physical Science, Prentice
Hall Science Explorer, 2009
Chapter 5 and 6 - Chemical Formulas and Equations Notes
8th Grade Science
5
It
may seem that some reactions violate this principle - but that’s because it may not always be easy to
measure the matter.
An
open system allows matter to enter or escape to the
surroundings, for example a burning match.
A
closed system prevents matter from entering or leaving a reaction, for example molding food in a
sealed plastic container.
Balancing Chemical Equations 6.2 (pg. 194-199)
To
describe a reaction accurately, a chemical equation must show the same number of each type of
atom on both sides of the equation.
Example 1
Magnesium
(MgO).
metal (Mg) reacts with oxygen gas (O2), forming magnesium oxide
To
write a balanced equation for this reaction, first write the equation using the
formulas of the reactants and products, then count the number of atoms of each
element.
Next,
use coefficients to balance the atoms (you cannot change the formula).
Check
that there is an equal number of each type of atom on each side of the
equation.
Example 2
Ammonia
equation.
(NH3) decomposes (breaks down) into Nitrogen gas (N2) and Hydrogen gas (H2). Balance this
1. Write equation:
2. Count atoms:
NH3
1
3
N2
2
+
H2
2
3. Use coefficients to balance:
2NH3
N2
4. Check by recounting atoms:
26
2
+ 3H2
6
Both sides match!
Chapter 5 and 6 - Chemical Formulas and Equations Notes
8th Grade Science
6
Practice
Naming Practice:
Acid Naming Practice:
CaF2
Calcium Fluoride
HF Hydrofluoric Acid
K2S
Potassium Sulfide
CoI2
Cobalt (II) Iodide or Cobaltous Iodide
SnF2
Tin (II) Fluoride or Stannous Fluoride
CuI2
Copper (II) Iodide or Cupric Iodide
CuI
Copper (I) Iodide or Cuprous Iodide
H3PO4 Phosphoric Acid
HNO3
H2SO4
Nitric Acid
Sulfuric Acid
Formula Practice:
Sodium phosphate _Na3PO4__
Iron (II) sulfide
_FeS_____
Calcium hydroxide _Ca(OH)2_
Potassium iodide
_KI______
Sulfur dioxide
_SO2_____
Na2CO3 Sodium carbonate
SO2
Sulfur dioxide
Balancing Equation Practice:
Cl2 + 2 KI
2 KCl + I2
2 Na + MgF2
2 NaF + Mg
C3H8 + 5 O2
3 CO2
2 NaN3
Zn + 2 HCl
2 Na
+ 4 H2O
+ 3 N2
ZnCl2
+ H2
Chapter 5 and 6 - Chemical Formulas and Equations Notes
8th Grade Science
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