Name
Class
Date
Writing Chemical Formulas Notes
Remember to respond to questions with complete sentences whenever possible. Show your work
and include units for all calculations.
Ionic Compounds
The process of writing chemical formulas can be made easier by using the ion charges of the elements
in the formula. Keep in mind that the total charge of a compound must be zero.
1.
2.
Review
a.
The ion charge describes the number of
compounds.
b.
Elements in the same
c.
Ionic compounds are made up of a
an atom gains or loses when forming ionic
on the periodic table will have the same charge.
plus a
.
Balance Method: To determine the correct formula for an ionic compound, such as lithium
oxide.
Step 1. Write the chemical symbols in the correct order, with the metal ion first.
Li
O
Step 2. Determine the charges for the two elements in the compound. Lithium is in Group 1, so
its charge is 1+. Oxygen is in Group 16, so its charge is 2-.
Step 3. Write the charges of each element as a superscript. For an ion with a charge of 1+ or 1-,
you can write only the sign and not bother writing the number 1.
Li+
O2-
Step 4. Compare the charges. If they are not equal, add another atom of whichever element has
the smallest charge, be sure to include its charge.
2 positive charges
Li+
O2-
2 negative charges
Li+
Step 5. Repeat step 4 until the positive charges and the negative charges balance each other.
Step 6. Count up the number of atoms of each element. Write the element symbols again in the
correct order. Add a subscript to each element indicating the number atoms of each
required to balance the formula. If there is only one atom of an element, do not write a
subscript. The one is implied.
Li2 O
3.
Practice: Use the balance method to determine the chemical formula for the following ionic
compounds.
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a. Barium oxide
4.
b. Magnesium iodide
Crisscross Method: Follow these steps to determine the chemical formula of an ionic compound
such as barium chloride using the crisscross method.
Step 1. Write the chemical symbols in the correct order, with the metal ion first.
Ba
Cl
Step 2. Determine the charges for the two elements in the compound. Barium is in Group 2, so its
charge is 2+. Chloride is in Group 17, so its charge is 1-.
Step 3. Write the charges of each element as a superscript. For an ion with a charge of 1+ or 1-,
you can write only the sign and not bother writing the number 1.
Ba2+
Cl-
Step 4. Rewrite the elements again under each of the ions. Then, crisscross the numbers only
(not the signs!!!) so that the charge on one element is now the subscript for the other. Don’t
bother writing the number 1 here either.
Ba 2+
Cl -
Ba
Cl 2
Step 7. Determine whether the formula is in its simplest form. Reduce the subscripts to their
simplest form by dividing by a common denominator. The formula for barium chloride is
BaCl2, which cannot be further simplified.
5.
Practice: Use the crisscross method to determine the chemical formula for the following ionic
compounds.
a. Aluminum chloride
b. Lithium sulfide
Covalent Compounds: Using Prefixes
To determine the chemical formula for covalent compounds, you simply look at the prefixes written
before each element in the compound.
1.
Review
a.
Covalent compounds are made up of two
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b.
In the chart below, write the prefix that would be given for the following number of atoms
of an element in a compound.
Number of Atoms
c.
2.
Prefix
Number of Atoms
1
6
2
7
3
8
4
9
5
10
Prefix
If the first element in a compound consists of only one atom, such as CO2, then it does not
have a
.
Example: Follow these steps to determine the chemical formula for covalent compounds such
as carbon tetrachloride.
Step 1. Write the chemical symbols in the order they appear in the compound name.
C Cl
Step 2. Look for a prefix on the first element listed. If it has a prefix, then determine the number
that corresponds with that prefix and write it as a subscript behind the element. Since
carbon does not have a prefix, we can assume that there is only one atom of carbon in the
compound. Do not write a subscript if there is only one atom of that element.
CCl
Step 3. Look for a prefix on the second element listed. The second element will always have a
prefix on it, even if it only has one atom of that element (mono-). Determine the number
that corresponds with that prefix and write it as a subscript behind the element.
Since the second element is tetrachloride, and tetra- means four, we write a subscript of four
behind chlorine.
CCl4
3.
Practice: Use the prefixes to determine the chemical formula for the following covalent
compounds.
a. Dinitrogen tetraoxide
b. Nitrogen monochloride
c. Carbon trifluoride
d. Tetraphosphorous dibromide
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