Unit 4.2 Inorganic Binary Compounds Type I
How does shorthand work? Shorthand was a
very popular way of recording speech,
especially in dictating letters and in court
testimony. Instead of trying to write out all
the words, the person taking the dictation
would use a set of symbols that represented
syllables or words. The pages above show a
shorthand version of “A Christmas Carol”
written by Charles Dickens. Unless you know
shorthand, the passage is meaningless. But
knowing shorthand allows you to read this
classic story.
Different professions also use a type of shorthand in communication to save time. Chemists use chemical
symbols in combination to indicate specific compounds. There are two advantages to this approach:
1.
The compound under discussion is clearly described so there can be no confusion about its
identity.
2.
Chemical symbols represent a universal language that all chemists can understand, no matter
what their native language is.
Writing Formulas for Binary Ionic Compounds
If you know the name of a binary ionic compound, you can write its chemical formula. Start by writing
the metal ion with its charge, followed by the nonmetal ion with its charge. Because the overall
compound must be electrically neutral, decide how many of each ion is needed in order for the positive
and negative charge to cancel each other out. Consider the compound aluminum nitride. The ions are:
Al+3
N-3
AlN
Remember that the total charge must be equal to zero.
Since the ions have charges that are
equal in magnitude, one of each will be
the lowest ratio of ions in the
formula.
(+3) + (-3) = 0
The ions for the compound lithium oxide, two lithium ions are required to balance out the charge of one
oxide ion.
Li+1
O-2
Li2O
Again the total charge must be equal to zero.
2(+1) + (-2) = 0
An alternative way to writing a correct formula for an ionic compound is to use the crisscross
method. In this method, the numerical value of each of the ion charges is crossed over to become the
subscript of the other ion. Signs of the charges are dropped. Shown below is the crisscross method for
aluminum oxide.
The red arrows indicate that the 3
Al+3 O-2
Al2O3
The total charge must be equal to zero.
from the 3+ charge will cross over to
become the subscript of the O. The
2 from the 2− charge will cross over
to become the subscript of the Al.
2(+3) + 3(-2) = 0
Be aware that ionic compounds are empirical formulas and so must be written as the lowest ratio of the
ions. In the case of aluminum nitride, the crisscross method would yield a formula of Al 3N3, which is not
correct. It must be reduced to AlN. Following the crisscross method to write the formula for lead (IV)
oxide would involve the following steps:
Pb+4 O-2
Pb2O4
The crisscross first yields Pb2O4 for
the formula, but that must be
reduced to the lower ratio and
PbO2 is the correct formula.
PbO2
The total charge must be equal to zero.
(+4) + 2(-2) = 0
Inorganic Binary Compounds Type I
An inorganic binary compound type I is a compound composed of a monatomic metal cation and a
monatomic nonmetal anion. Monatomic meaning that it is a one atom ion. These would include ions such as
those made from Alkali metals, alkali Earth metals, or Halogens. It may include other ions.
Naming Binary Ionic Compounds
When examining the formula of a compound in order to name it, you must first decide what kind of
compound it is. For an inorganic binary compound type I, it must be made of two types of atoms only!
A metal will always be the first element in the formula, while a nonmetal will always be the second.
When naming, the metal cation is named first. Remember that the positive metal ions keep the name
given to the metal. For example the sodium ion, Na+1, is called sodium.
The nonmetal anion is named second. Monatomic, or one atom negative ions always drop the ending and
add –ide to the name. For example the ion made from chlorine, Cl-1, is called chloride.
The compound made from one sodium ion and one chloride ion, NaCl, is called sodium chloride.
Subscripts are used to balance the charge to equal zero net charge for the molecule. Subscripts in the
formula do not affect the name. The following chart shows three examples.
Naming Inorganic Binary Compounds Types I
Formula
KF
Na3N
Ca3P2
Name
Potassium fluoride
Sodium nitride
Calcium phosphide
Net charge
(+1) + (-1) = 0
3(+1) + (-3) = 0
3(+2) + 2(-3) = 0
Notice that in each of the formulas above, the overall charge of the compound is zero.
Potassium ion is K+1, while fluoride ion is F-1. Since the magnitude of the charges is equal, the formula
contains one of each ion.
K+1
KF
F-1
(+1) + (-1) = 0
This would also be the case for a compound such as magnesium sulfide, MgS, in which the ions are
Mg2+ and S2-.
Mg+2 S-2
MgS
(+2) + (-2) = 0
In order to make a neutral compound, three of the +1 sodium ions are required in order to balance out
the single -3 nitride ion.
3(+1) + (-3) = 0
Na+1 N-3
Na3N
The crisscross method is used with subscripts to
balance the compound to zero net charge.
So the Na is given a subscript of 3. For calcium phosphide, the calcium ion is Ca+2, while the phosphide ion
is P-3. The least common multiple of 2 and 3 is 6 (3 x +2 = 6, and 2 x -3 = 6). To make the compound
neutral, three calcium ions have a total charge of +6, while two phosphide ions have a total charge of -6.
The Ca is given a subscript of 3, while the P is given a subscript of 2.
Ca+2 P-3
Ca3P2
3(+2) + 2(-3) = 0
Summary

A binary ionic compound is a compound composed of a monatomic metal cation and a monatomic
nonmetal anion.

Monatomic ion is a one atom ion.



Formulas for binary compounds begin with the metal followed by the non-metal.
The metal cation keeps the name of the metal.


Positive and negative charges must cancel each other out.
For binary compounds, it is not necessary to indicate the number of ions in the compound using
The non-metallic anion is named by dropping the end of the element and adding –ide.
subscripts.

Ionic compound formulas are written using the lowest ratio of ions.
Review
1. What is a binary compound?
2. Which of the following is not a binary compound?
a. NaCl
b. KH2PO4
c. KBr
3. Name the following compounds:
a. NaBr
b. MgCl2
c. LiI
d. CaO
4. Write formulas for the binary ionic compounds formed between the following pairs of elements:
a. cesium and fluorine
b. calcium and sulfur
c. aluminum and chlorine
d. zinc and nitrogen
Answers
1. A molecule made of only two types of atoms.
2. KH2PO4 has more than two types of atoms.
3.
a. NaBr
b. MgCl2
Sodium bromide
c. LiI
Lithium iodide
d. CaO
Calcium oxide
Magnesium chloride
4. Write formulas for the binary ionic compounds formed between the following pairs of elements:
a. cesium and fluorine
CsF
Cesium fluoride
b. calcium and sulfur
CaS
Calcium sulfide
c. aluminum and chlorine
AlCl3
Aluminum chloride
d. zinc and nitrogen
Zn3N2
Zinc nitride