A chemical formula is a
combination of elemental symbols and subscript numbers that is used to show the
composition of a compound. Depending of the type of compound that the formula
represents, the information that it provides will vary slightly. Before we go about
learning how to write chemical formulas, it is important that you clearly understand the
difference between molecular compounds and ionic compounds.
Ionic compounds are
composed of charged ions that are held together by electrostatic forces. A typical
type of ionic compound, called a binary compound because it is made up of
two elements, will be composed of metallic positive ions (cations)
and nonmetal negative ions (anions). Another type of ionic
compound, called a ternary compound as it contain three elements,
is composed of monatomic ions and polyatomic ions. When dealing with ionic formulas
it is very important to remember that the formula does not show how the compound actually
exists in nature. It only shows the ratio by which the individual ions
combine. For example, the ionic formula for calcium chloride is CaCl2.
Since calcium chloride is an ionic compound, this formula does not mean that there are
actually two chlorine atoms floating around attached to one calcium atom. Ionic
compounds are actually continuous, lacking the discrete units that make up a sample of a
molecular substance. Rather, the formula shows that a sample of calcium
chloride contains twice as many chlorine atoms as calcium atoms. Remember that ionic
compounds are not molecules, so the formula CaCl2 is said to represent
one formula unit of calcium chloride.
Molecular compounds are
held together by covalent bonds, or shared pairs of electrons. Molecular formulas do
show these molecules as they actually exist as discrete units in nature. When we say
that the molecular formula of water is H2O, we can see that the molecules of
water are made up of three atoms, two hydrogen atoms are covalently bonded to each oxygen
atom. A special type of chemical formula, called an empirical formula,
shows the composition of a molecule not as it actually exists, but in a simple whole
number ratio. The difference between empirical and molecular formulas will be
explained in lesson 5-5.
This lesson will concentrate on writing
simple chemical formulas when given a formula name. In learning how to write
chemical formulas, you will make use of the oxidation numbers that you learned about in lesson 5-2. For your convenience, print out the tables
from lesson 5-2 before you continue with this lesson, as they will be referred to from
time to time.
Writing Ionic Formulas
I. Binary Compounds - Binary
compounds are compounds that are composed of only two elements. When you write the
formulas for binary compounds, they will consist of two elemental symbols, and they may
also have one or two subscript numbers, if the elements don't combine in a one to one
ratio. You are probably familiar with the formula NaCl for table salt. This
formula shows no subscripts because one ion of Na will be present for each ion of Cl, in
any sample of table salt.
You will be given the name of a binary compound and you will be
expected to be able to write the proper formula for the compound. There will be two
sources of information for writing the correct formula. The compounds name will give
you the elements that make up the compound. The oxidation numbers of the ions
involved will show you the ratio by which they combine. Let's go through an example;
Example 1. Write the correct formula for Barium
Fluoride.
Step one - Write the symbols for the elements in the
compound. If you need to review the elemental symbols, see lesson 5-1.
Note that the ending "ide" is used for fluoride to show that it is a negative
ion of fluorine.
Barium = Ba
Fluoride = F
Step two - Look up the oxidation numbers of the
elements involved (in table 5-2b or some similar table), and write them as superscripts to
the right of the elemental symbols. Note that when no number accompanies a charge symbol, as in the case of
fluoride below, they charge value is understood to be "1".
Barium = Ba2+
Fluoride = F-
Step three - Use the correct combination of ions to
produce a compound with a net charge of zero. In this case, (2+) + 2(-1) = 0. So, two fluoride ions will cancel
out one barium ion. Since it would take two fluoride ions (each with a charge of
negative one) to cancel out one barium ion (with a charge of plus two) we use a subscript
of two after the symbol for fluorine to show the ratio.
BaF2
If this seems confusing to you, it will get
simpler over time.
Example 2. Write the proper formula for the
ionic compound lithium bromide.
Step one - Write the symbols for the elements in the
compound. Note that the
ending "ide" is used for bromide to show that it is a negative ion of bromine.
Lithium = Li
Bromide = Br
Step two - Look up the oxidation numbers of the
elements involved (in table 5-2b or some similar table), and write them as superscripts to
the right of the elemental symbols. Note that when no number accompanies a charge symbol, as in the case of
fluoride below, they charge value is understood to be "1".
Lithium = Li+
Bromide = Br-
Step three - Use the correct combination of ions to
produce a compound with a net charge of zero. In this case, (+1) + (-1) = 0. so, one lithium ion will cancel out the
charge of one bromide ion. This means that the two elements will combine in a one
to one ratio, and know subscripts will be needed.
LiBr
II. Ternary Compounds - Ternary compounds are
composed of three different elements. The most common types of ternary compounds
consist of a metallic cation (positive ion) and a polyatomic anion (negative ion).
The only common polyatomic ion with a positive charge is the ammonium ion. At any
rate, To write these formulas you will want to have reference tables with the information
provided on tables 5-2b and 5-2d.
Example 1. Write the proper chemical formula for potassium
hydroxide.
Step one - Write the symbols for the monatomic and polyatomic
ions in the compound. You will find the symbol
potassium on table 5-2b. Hydroxide is a polyatomic ion, which will be found on table
5-2d. Eventually you will recognize the name of a polyatomic ion, but for now if you
can't find an ion on one table, look on the other.
Potassium = K
Hydroxide = OH
Step two - Look up the oxidation numbers of the ions
involved (in table 5-2b and 5-2d, or some similar tables), and write them as superscripts
to the right of the elemental symbols.
Potassium = K+
Hydroxide = OH-
Step three - Use the correct combination of ions to
produce a compound with a net charge of zero. Parenthesis must be used if you need more than one of a polyatomic ion. In this case, (+1) + (-1) = 0.
So, only one of each ion is used. No subscripts are necessary. If you needed
more than one hydroxide ion, it would be put in parenthesis with the subscript on the
outside.
KOH
Note the importance of upper and lower case
Example 2. Show the correct formula for
Calcium Nitrate.
Step one - Write the symbols for the monatomic and
polyatomic ions in the compound.
Calcium = Ca
Nitrate = NO3
Step two - Look up the oxidation numbers of the ions
involved (in table 5-2b and 5-2d, or some similar tables), and write them as superscripts
to the right of the elemental symbols.
Calcium = Ca2+
Nitrate = NO3-
Step three - Use the correct combination of ions to
produce a compound with a net charge of zero. Parenthesis must be used if you need more than one of a polyatomic
ion. In this case (+2) + 2(-1) = 0. We
need to show two nitrate ions in our formula. The subscript is put on the outside of
the parenthesis to show that the entire polyatomic ion is doubled.
Ca(NO3)2
The correct use of
parenthesis will seem hard at first, but you must master this skill with practice!
III. The Stock System - Some
elements, like iron and lead, have more than one oxidation number. If you were given
a compound name like lead chloride, you would not know if you should used an oxidation
number of +2 or +4 for the lead. The stock system is used to specify which form of
an element, that shows multiple oxidation numbers, is used in a particular compound.
A roman numeral is shown after the name of the positive ion (cation) to indicate
the oxidation number of the positive ion.
Example 1. Show the correct formula for
lead(IV) nitrate.
Step one - Write the symbols for the ions in
the compound.
Lead = Pb
Nitrate = NO3
Step two - Look up the oxidation number of the
negative ion involved (in table 5-2b and 5-2d, or some similar tables). The positive
ion will have a positive oxidation number equal to the roman numeral. Write the
numbers as superscripts to the right of the elemental symbols.
Lead = Pb4+
Nitrate = NO3-
Step three - Use the correct combination of ions to
produce a compound with a net charge of zero. Parenthesis must be used if you need more than one of a polyatomic
ion.
Pb(NO3)4
Example 2. Show the correct formula for
Copper(II) Fluoride
Step one - Write the symbols for the ions in
the compound.
Copper = Cu
Fluoride = F
Step two - Look up the oxidation number of the
negative ion involved (in table 5-2b and 5-2d, or some similar tables). The positive
ion will have a positive oxidation number equal to the roman numeral. Write the
numbers as superscripts to the right of the elemental symbols.
Copper = Cu2+
Fluoride = F-
Step three - Use the correct combination of ions to
produce a compound with a net charge of zero. Parenthesis must be used if you need more than one of a polyatomic ion.
CuF2
Writing Molecular Formulas
I. Binary Molecular Compounds - The
standard method for naming binary molecular compounds has changed over the years.
Currently, the stock system is commonly used for naming molecular
compounds. Names like "carbon dioxide", "carbon monoxide", and
"dinitrogen pentoxide" are really remnants of an older system that used prefixes
to identify the number of elements involved. When you are writing the formula for a
molecular compound using the stock system, you will not really notice any difference from
the methods described above, until you study bonding. You should be aware that you
are not dealing with ions when you are working with molecular formulas, rather you are
looking up what might be called the apparent charge on each atom.
Example 1. Write the correct formula for
nitrogen(IV) oxide.
Step one - Write the symbols for the elements
involved.
Nitrogen = N Oxide
= O
Step two - Use the roman numeral as the apparent
charge of the first element. Find the apparent chart of the second element by
looking on reference tables such as 5-2a.
Nitrogen = N4+
Oxide = O2-
Step three - Determine the ratio by which the
elements will bond to show a net charge of zero. Use subscripts to indicate the
number of atoms of each element present. In this case,
(+4) + 2(-2) = 0.
NO2
Example 2. Write the correct formula for
nitrogen(III) oxide.
Step one - Write the symbols for the elements
involved.
Nitrogen = N Oxide
= O
Step two - Use the roman numeral as the apparent
charge of the first element. Find the apparent charge of the second element by
looking on reference tables such as 5-2a.
Nitrogen = N3+
Oxide = O2-
Step three - Determine the ratio by which the
elements will bond to show a net charge of zero. Use subscripts to indicate the
number of atoms of each element present. In this case,
2(+3) + 3(-2) = 0.
N2O3
II. Other Molecular Formulas
- There are other types of molecular formulas, besides binary, which you will eventually
be required to write. These lessons will be presented at other times.
Please forward all questions, comments and criticisms to Gregory L. Curran.
© Copyright 2004 Fordham Preparatory School, All Rights Reserved.
Last Modified February 07, 2008 |
