WRITING FORMULAS FOR MOLECULAR ELEMENTS
1. METALS:
Write the symbol for the metal
Al, Pb, Hg
2. NONMETALS:
a. monatomic He, Ne, Ar, Kr, Xe, Rn (Noble Gases)
b. Diatomic: H2, F2, Cl2, I2, O2, N2, Br2, At2
c. Network solids: C, Si
d. Others S8, P4
3. COMMON FORMULAS:
Ozone - O3
Methane – CH4
Sucrose - C12H22O11
Hydrogen peroxide H2O2
Water - H2O
Ethanol – C2H5OH
Methanol - CH3OH
Ammonia – NH3
BINARY COMPOUNDS
2 elements


Molecular compounds – made up of non-metal atoms.
Ionic compounds – made up of many metal ions and many nonmetallic ions arranged in an
orderly pattern.
1. The chemical formula of a molecular compound is called a molecular formula. The formula
indicates the number and kinds of atoms present in a molecule of that compound. Prefixes
are used to describe the # of atoms of each kind in a binary compound. Suffix is “ide”…
Prefix + element name + prefix + element root + “ide”
Do not use mono prefix in first part of name: CO - carbon monoxide; CO2 - carbon dioxide
Note: the final “a” or “o” of a prefix is often omitted if the name of the atom or ion being
counted begins with either an a or an o.
E.g. - N2O5 = dinitrogen pentoxide; rather than dinitrogen pentaoxide
If a binary molecular compound contains hydrogen we do not use prefixes.
H2S - Hydrogen Sulfide; HF - Hydrogen fluoride
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2. The chemical formula for an ionic compound is called a formula unit. The formula indicates
the kinds of ions and lowest whole number ratio of the ions. The charges of the ratio must
balance so the compound as a whole is neutral.
NaCl
1 sodium ion + 1 chloride ion (1+ 1- = 0)
CaCl2
1 calcium for every 2 chloride ions (2+ 1- 1- = 0) Ca2+ Cl- ClThe formula for barium oxide is: Ba2+ O2- = BaO (Reduce to smallest ratio if crisscross
method is used: Ba2O2 becomes BaO)
Naming Binary Ionic Compounds
What is the name for CuCl? CuCl2?
When the metal can form 2 or more stable ion as most transition metals do, we must have a
method for naming each compound. A Roman numeral in parenthesis following the name of
the metal is the “Stock system” for a “choice of charge” metal. The numeral indicates the
charge of the metallic ion; therefore, CuCl = copper (I) chloride; CuCl2 = copper (II)
chloride
Only use roman numerals when you have a “choice of charge” metal!
AlCl3 Aluminum chloride (only one stable Al ion exists)
Is a Roman numeral needed in these names? PbO; FeO; Fe2O3
What charge is indicated? Lead (IV) oxide; silver bromide; stannic sulfide
This new method replaces the older “Classical method”
The suffix “ous” describes the lesser ionic charge; the suffix “ic” refers to the higher
ionic charge. Cu1+ = copper(I) or cuprous; Cu2+ = copper(II) or cupric
NON BINARY IONIC COMPOUNDS
More than 2 elements present in the compound: metal, H, or NH4 with a complex ion, or a
nonmetal. Complex ions are a group of atoms held together as a group, acting as a simple ion, and
carry a charge. These are listed on the back of your periodic table – take note that only one carries
a “+” charge (ammonium – NH4+)
Writing formulas: look for suffix “ate” or “ite” in the name of the compound. A few end in
“ide”, such as hydroxide and cyanide.
Write the formulas for metal (or ammonium) with its charge
Write the formula for the complex ion with its charge
Crisscross to balance the charges
Reduce to smallest ratio if needed
Magnesium chlorate - Mg2+ ClO3- = Mg(ClO3)2
Sodium sulfate - Na+ SO42- = Na2SO4
Naming non-binary complex ions (2 parts)
1. Write the name of the metal (or hydrogen or ammonium) Remember to include the Roman
numeral of the metal if needed
2. Determine which complex ion is present and write its name. Pay attention to any changes in
oxygen numbers, or whether hydrogen has been added…
NaHSO4 = Na+ H+ SO4- is named: sodium hydrogen sulfate or sodium bisulfate
NAMING ACIDS
1. binary acid: only 2 elements H + nonmetal
name contains the word “acid”
formula contains (aq)
HCl(aq) hydrochloric acid
HCl


Hydrogen chloride
Hydrogen ________ide

HCl(aq)
hydrochloric acid
hydro________ic acid
2. oxy acids: acids based in oxygen and hydrogen
H2SO4
Hydrogen sulfate
hydrogen_________ate

H2SO3
Hydrogen sulfite
Hydrogen________ite



H2SO4(aq)
sulfuric acid
________ic acid
H2SO3(aq)
sulfurous acid
________ous acid
HNO3(aq), HNO2(aq), HF(aq), H2S(aq)
Nitric acid
Nitrous acid
Hydrofluoric acid
Hydrosulfuric acid
HYDRATED COMPOUNDS
Produce water when heated
AlCl33H2O  AlCl3 + 3H2O
Examples: CuSO45H2O
MgSO47H2O
Ba(OH)28H2O
Name the first part of the formula according to the ions present.
Use a prefix to indicate the # water molecules/formula unit and add to the word “hydrate”
The dot indicates that the water can leave the compound very easily with the application of heat;
it is not a “multiplication dot”! It is only used in the formula, not within the name.
Copper (II) sulfate pentahydrate
Magnesium sulfate heptahydrate
Barium hydroxide dihydrate