Unit 6 Skill Builder: Writing Ionic Formulas
Name _______________________
Date ______________ Period ____
BACKGROUND: A chemical formula is a way to show the types of elements present in a compound, and
the ratio of those elements. Two examples: NaCl and MgBr2. NaCl is the formula for sodium chloride. The
elements sodium and chlorine are present in a 1:1 ratio. MgBr2 is the formula for magnesium bromide. The
elements magnesium and bromine are present in a 1:2 ratio. For ionic compounds, the ratio of metal to
nonmetal ions is determined by what it takes for each element to become stable. There are a couple of
methods that can be used to determine the correct formula for an ionic compound.
PURPOSE: To learn and practice the skill of formula writing.
Method One: Electron Dot diagrams (aka Lewis structures)
The electron dot diagram shows only the electrons in the last level (the valence electrons that are involved
in bonding).
Look at the example. The example shown is for sulfur. Since sulfur is in column VIA (“6A”), a
sulfur atom has 6 valence e-. So 6 dots are drawn around the symbol for sulfur.
PROCEDURE A. Draw the electron dot diagram for the following elements:
a) sodium
b) chlorine
Example:
c) calcium
d) oxygen
e) aluminum
f) barium
g) bromine
S
The transfer of electrons can be shown with electron dot diagrams. When an ionic compound is formed the
metal loses electrons to form a positive ion and the nonmetal gains electrons to form a negative ion. Each
loses or gains electrons until the ion formed has a full or stable octet of electrons. An arrow is used to show
the transfer of electrons.
Look at the example. The example is for sodium sulfide. Each sodium (Na) atom needs to lose 1e-. But
a sulfur atom needs to gain 2e-. So it will take 2 sodium atoms to make 1 sulfur atom stable. So the ratio
is 2:1. The formula shows this by placing a subscript of 2 following sodium’s symbol.
PROCEDURE B. Draw the electron dot diagrams to predict the formulas for the following compounds:
a) sodium chloride
Example:
b) sodium oxide
c) calcium chloride
d) calcium oxide
e) aluminum chloride
f) aluminum oxide
g) barium bromide
Formula: Na2S
h) barium oxide
Na
Na
S
Method Two: Ionic Charge
Charges on the ions can also be used to predict the formula of the ionic compound. The ions combine in
simple whole number ratios to form a neutral compound. The sum of the charges on the ions must add up to
be zero.
PROCEDURE C. Use the charges on the ions to predict the formulas for the following compounds. Write
both the name and the formula. Use the charges on the ions to determine the ratio
Example:
of metal ions to nonmetal ions required to get a sum of zero. Write in subscripts
to reflect the ratio.
magnesium bromide
Mg+2 Br-1
a) calcium iodide
d) aluminum oxide
g) aluminum chloride
+2 + (-1)( -1) = 0
b) magnesium chloride
e) lithium sulfide
h) sodium chloride
1:2 ratio
c) barium sulfide
f) potassium bromide
i) magnesium iodide
1 Mg and 2 Br
MgBr2
The charge method also allows you to predict the formulas for compounds
involving transition metals and polyatomic ions.
Transition metals do not follow our patterns of the periodic table. Located on the periodic table in the B
columns, these metals lose a variable number of electrons. The nomenclature to name the compound must
indicate which ion is present. A Roman numeral in parenthesis following the element name shows the
charge on the ion.
Example: Iron may lose either two or three electrons. If it loses two electrons, an ion with a charge of
positive two is formed and named iron (II) – Fe(II). If it loses three electrons, an ion with a charge of
+3
positive three is formed and named iron (III) – Fe(III). So, iron (III) chloride must contain the Fe ion.
PROCEDURE D. Use this nomenclature and the charges on the ions to predict the formula for the
following compounds. Write both the name and the formula.
Example:
a) copper (II) chloride
f) lead (II) nitrate
b) iron (III) oxide
g) lead (IV) oxide
iron (II) bromide
c) tin (IV) chloride
h) lead (II) iodide
Fe+2 Br-1
d) nickel (III) chloride
i) iron (III) chloride
+2 + (-1)( -1) = 0
e) gold (III) sulfide
j) copper (I) sulfate
1:2 ratio
1 Fe and 2 Br
FeBr2
Polyatomic ions are a group of atoms held together by a covalent bond with a net charge. The table below
shows the most common polyatomic ions.
Nitrate
Sulfate
Carbonate
Phosphate
Hydroxide
Ammonium
NO3-
SO42-
CO32-
PO43-
OH-
NH4+
PROCEDURE D. Use the charges for the polyatomic ions/ions to predict the formulas for the following
compounds. Write both the name and the formula.
Example:
calcium nitrate
Ca+2 NO3-1
+2 + (-1)(-1) = 0
1:2 ratio
1 H and 2 SO4
H(SO4)2
a) calcium nitrate
b) potassium sulfate
c) barium sulfate
d) sodium carbonate
e) magnesium hydroxide
f) ammonium chloride
g) aluminum hydroxide
h) lithium phosphate
i) calcium phosphate
j) sodium sulfate