Chemical Formulas Review: Nomenclature
and Formula Writing
Naming Simple Compounds
There are four naming systems you should familiarize yourself
with to succeed on the SAT II Chemistry exam. The trick is recognizing which naming
system to use. Here are the guidelines:
- If the compound starts with H,
it is an acid. Use the naming acids rules.
- If the compound starts with C and contains
quite a few H’s and perhaps some O’s, it is organic. Use the naming
organic compounds rules.
- If the compound starts with a metal, it is
most likely ionic. Use the naming binary ionic compounds rules.
- If the compound starts with a nonmetal other than
H or C, use the naming binary molecular compounds rules.
It is also essential that you memorize
some common polyatomic ions. Polyatomic ions behave as a unit. If
you need more than one of them, enclose them in parentheses when
you write formulas. You need to know their names, formulas, and
charges. If you learn the nine that follow, you can get many others
from applying two simple patterns.
|Name of polyatomic ion
||Formula and charge
- Pattern 1: The -ates “ate”
one more oxygen than the -ites and their charge doesn’t
change as a result! For instance, if you know nitrate is NO3-,then
nitrite is NO2-.If
you know phosphate is PO43-,then
you know phosphite is PO33-.You can
also use the prefixes hypo- and per- with
the chlorate series. Perchlorate, ClO4-,was
really “hyper and ate yet another oxygen” when compared to chlorate,
is a double whammy: it is -ite and therefore “ate” one
less oxygen than chlorate and it is hypo-,
which means “below,” so it “ate” even one less oxygen than plain
chlorite, so its formula is ClO-. You
can also substitute the other halogens for Cl and make additional
sets of the series.
- Pattern 2: The -ates with
charges less than negative 1 (that is, ions with charges of -2,
-3, etc.) can have an H added to them to form new polyatomic ions.
For each H added, the charge is increased by a +1. For instance,
an H added and become HCO3-.HCO3-is
called either the bicarbonate ion or the hydrogen
carbonate ion. Since phosphate is -3, it can add one or two hydrogens to
make two new polyatomic ions, HPO42-and
are named hydrogen phosphate and dihydrogen phosphate, respectively.
If you keep adding hydrogen ions until you reach neutral, you’ve
made an acid! That means you need to see the naming acids rules.
- Pattern 3: The following periodic table will
also come in handy. Notice there are simple patterns for determining
the most common oxidation states of the elements based on their
family’s position in the periodic table. Notice the 1A family is
+1, while the 2A family is +2; then skip across to the 3A family
and see that aluminum is +3. Working backward from the halogens,
or 7A family, the oxidation states are most commonly -1, while the
6A family is -2, and the 5A family is -3. The 4A family is “wishy-washy”:
they can be several oxidation states, with the most common being
How do you know it’s an acid? The compound’s formula begins
with an H, and water doesn’t count! Naming acids is extremely easy
if you know your polyatomic ions. There are three rules to follow:
- H + element: When the acid
has only an element following the H, use the prefix hydro-,
followed by the element’s root name and an -ic ending.
HCl is hydrochloric acid; H2S is hydrosulfuric
acid. When you see an acid name beginning with hydro-,
think: Caution, element approaching! HCN is an exception since it
is a polyatomic ion without oxygen, so it is named hydrocyanic acid.
- H + -ate polyatomic
ion: If the acid has an -ate polyatomic
ion after the H, that makes it an -ic acid. H2SO4 is
- H + -ite polyatomic
ion: When the acid has an -ite polyatomic
ion after the H, that makes it an -ous acid. H2SO3 is
Acids have enough H+ added
to the anion to make the compound neutral. Supply either the acid’s
name or its formula to complete the table below:
||Hyperchloric acid (or perchloric
Naming Organic Compounds
How do you know it’s organic? The formula will start with
a C followed by H’s. Most of the organic carbons you will encounter
will be either hydrocarbons or alcohols, and luckily for you, these
are the simplest of all to name. Learn the list of prefixes in the
table following this section: they correspond to the number of carbons
present in the compound. The following silly statement will help
you remember the order of the first four prefixes since they are
not ones you are familiar with: “Me eat peanut butter.”
This corresponds to meth-, eth-, prop-,
and but-, which correspond
to one, two, three, and four carbons, respectively.
Now that we have a stem, we need an ending. There are
three common hydrocarbon endings; the ending changes depending on
the structure of the molecule:
- -ane = alkane (all single
bonds and saturated); CnH2n+2; saturated:
it contains the maximum number of H’s
- -ene = alkene (contains double bond,
- -yne = alkyne (contains triple bond,
unsaturated); CnH2n-2; polyunsaturated:
it contains more than one double or triple bond
For any hydrocarbon, you can remove one H and replace
it with a hydroxyl group, or —OH group, to form an alcohol. Do not be
fooled—this looks like a hydroxide group but isn’t! The OH does not make
this hydrocarbon an alkaline or basic compound, nor do
you name it as a hydroxide! C2H6 is
ethane, while C2H5OH
is ethanol. Fill in the missing formulas and names for each compound
in the table:
|No. of carbon atoms = n
||Prefix or stem
CnH2n+1 + OH
||Must have 2 carbons
Naming Binary Ionic Compounds
How will you know a compound is ionic? You’ll know because
the formula will begin with a metal cation or the ammonium cation.
Formulas often end with a polyatomic anion. If only two elements
are present, they are usually from opposite sides of the periodic
table, like in KCl. If the metal is one of the transition metals,
be prepared to use a Roman numeral to indicate which oxidation state
the metal is exhibiting. Silver, cadmium, and zinc are exceptions
to the Roman numeral rule! First, let’s name the ions.
Naming positive ions (usually metals)
- Monatomic, metal, cation: simply the name
of the metal from which it is derived. Al3+ is
the aluminum ion (these are often referred to as group A metals).
- Transition metals form more than one ion;
Roman numerals (in parentheses) follow the ion’s name. Cu2+ is
copper (II) ion. Exception: mercury (I) is Hg22+, that
is, two Hg+ bonded together covalently.
- Roman numerals are not usually written with silver, cadmium,
and zinc. Arrange their symbols in alphabetical order—the first
one is 1+ and the other two are 2+.
Naming negative ions (usually nonmetals or polyatomic
- Monatomic, nonmetal, anion: add the suffix
-ide to the stem of the nonmetal’s name. Halogens
are called the halides. Cl- is
the chloride ion.
- Polyatomic anion: you must memorize the polyatomic ion’s
name. NO2-is the nitrite
Naming ionic compounds: The positive ion
name is given first (remember, if it’s a transition metal,
the Roman numeral indicating its charge is part of its name), followed
by the name of the negative ion. No prefixes are
Naming Binary Molecular Compounds
How will you know if it’s a molecular compound? Well,
it will be a combination of nonmetals, both of which lie near each
other on the periodic table. Use the following set of prefixes,
and don’t forget the -ide ending to the name.
(usually used only on the second element, such as carbon monoxide
or nitrogen monoxide)
If the second element’s name begins with a vowel, the a at
the end of the prefix is usually dropped. N2O5 is
dinitrogen pentoxide, not dinitrogen pentaoxide.
PCl5 is phosphorous pentachloride, not phosphorous