1. Writing Formulas Chemistry 7(B)
In chemistry, you will be dealing with many different compounds, each with their own unique properties. In order to distinguish compounds from one another, you will need to be able to write their formulas correctly.
Writing Formulas
Chemistry 7(B)

2. Writing Formulas Lesson Objectives Write formulas for
Common polyatomic ions
Ionic Compounds
Containing main group or transition metals
Bases
Acids
Covalent compounds
This presentation will teach you how to write formulas from IUPAC chemical names. By the end of this presentation, you will know how to write formulas for common polyatomic ions, ionic compounds containing main group or transition metals, bases, acids, and covalent compounds.

3. Chemical Formulas
Formulas express the types and numbers of atoms in a compound
Symbols are used to represent the types of elements
Subscripts are used to represent the number of atoms of each element
Ex) NH3 H N
Formulas express the types and numbers of atoms in a compound. Symbols are used to represent the types of elements. Subscripts are used to represent the number of atoms of each element. In this example, the symbols N and H represent nitrogen and hydrogen. The subscript of 3 behind hydrogen indicates there are three hydrogen atoms in the molecule. Nitrogen does not have a subscript behind it in the formula. This is because 1’s are not written in chemical formulas. When a symbol is written without a subscript, it means there is one atom of that element.

4. Ionic Formulas
Ionic formula – shows the number and types of ions in a formula unit
Formula unit – the lowest whole number ratio of ions in a compound
Has no overall charge
Ex) NaCl
Formula unit
There are two types of formulas: ionic and covalent. Ionic formulas show the number and types of ions in a formula unit. An example of this is sodium chloride or table salt. Unlike covalent compounds, ionic compounds do not form individual molecules. Instead, large numbers of ions, held together by ionic bonds, form ionic crystals. In order to make it easier to keep track of ions, we use formula units. Formula units are the lowest whole number ratio of ions in a compound. The formula unit of salt consists of one sodium ion and one chloride ion. The positive and negative charges in the formula unit balance each other out so that there is no overall charge.

5. Formulas for Covalent Compounds
Molecular formula – a formula that shows the number of atoms of each element in a molecule
Does not contain ions H O
Ex) H2O2
Covalent compounds are also known as molecular compounds. Unlike ionic compounds, covalent compounds do not contain ions. Instead, atoms are held together by covalent bonds and form individual molecules. So, molecular formulas show the number of atoms of each element in a molecule. Therefore, molecular formulas are not always given in their lowest whole number ratios. An example of this is the dihydrogen dioxide molecule (more commonly known as hydrogen peroxide). This molecule contains two hydrogen atoms and two oxygen atoms so the formula is written H2O2 (not HO).

6. Writing Molecular Formulas
Least electronegative element is written first
Prefixes in the name determine the number of atoms of each element in the compound
Number of Atoms
Prefix 1
mono- 2
di- 3
tri- 4
tetra- 5
penta- 6
hexa- 7
hepta- 8
octa- 9
nona- 10
deca-
Let’s learn how to write formulas for covalent compounds. The least electronegative element is written first. Use prefixes in the name to determine the number of atoms of each element in the compound. The prefixes you will use are listed in the chart on the right.

7. Writing Covalent Formulas
Number of atoms will become the subscript
No prefix on the first element’s name means there is one atom of the element
1’s are not written in formulas
Number of Atoms
Prefix 1
mono- 2
di- 3
tri- 4
tetra- 5
penta- 6
hexa- 7
hepta- 8
octa- 9
nona- 10
deca-
The number of atoms will become the subscript. If there is no prefix on the first element’s name, there is one atom of the element. This is because 1’s are not written in chemical formulas. For example, let’s write the formula for nitrogen dioxide. Nitrogen does not have a prefix on it so, there is only one atom of nitrogen in the molecule. The prefix di- in the word dioxide indicates that there are two atoms of oxygen in the molecule. So, the O in the formula will get a subscript of 2. Notice that the formula is written in the same order as the name, from least to most electronegative element.
Ex) Nitrogen dioxide = N O2

8. Writing Ionic Formulas
Determine the charges of the ions in the formula
Generally, charge can be determined by an ion’s group number 1+ 2+ 3+ 3- 2- 1-
Group 1A
Group 2A
Group 3A
Group 4A
Group 5A
Group 6A
Group 7A
Group 8A
Now, let’s learn how to write ionic formulas. The first step is to determine the charges of the ions in the formula. Generally, charge can be determined by an ion’s group number. Remember that noble gases don’t form ions because they already have stable electron configurations. A nonmetal’s charge corresponds to the number of groups it is away from the noble gases on the periodic table. Group 7A is one group away from the noble gases so, elements in it tend to form ions with a –1 charge. Group 6A is two groups away from the noble gases so elements in it tend to form ions with a –2 charge. Group 5A is three groups away from the noble gases so elements in it tend to form ions with a –3 charge. Metallic groups have charges that are even easier to remember because they directly correspond to the group number. Remember that metals form cations. So, group 1A metals form ions with a +1 charge, group 2A metals form ions with a +2 charge, and group 3A metals form ions with a +3 charge.

9. Writing Ionic Formulas
Determine the charges of the ions in the formula
Exceptions — multiple oxidation state metals
Most transition and post transition metals can form more than one cation
Oxidation state will be written in the name as a Roman numeral
Ex) Iron(II) = Fe2+
There are some exceptions to these rules. Most transition and post transition metals can form more than one cation. For these ions, the oxidation state (or charge) will be written in the name as a Roman numeral. For example, iron can have an oxidation state of either +2 or +3. The roman numeral in the name is needed to indicate which oxidation state the metal will have in the formula. As seen here, iron with the Roman numeral II in parentheses, would indicate that the iron ion has an oxidation state of +2.

10. Ex) Write the formula for lead(IV) oxide
Writing Ionic Formulas
Write ions with charges
Cations written first
Cross and drop charges
Use the absolute value of each ion’s charge as the subscript for the other ion
Reduce subscripts to the lowest whole number ratio
Ex) Write the formula for lead(IV) oxide
Step 2: Pb4+ O-2
Step 3: Pb2O4
Step 4: PbO2
Let’s look at an example as we go through the steps for writing ionic formulas. Once we identify the charges of the ions involved, we’ll write them down next to each other. For our example, let’s write the formula for lead(IV) oxide. Remember that cations are always written first. So, lead is written first in our example. The next step is to cross and drop the absolute value of each ion’s charge and make it the subscript for the other ion. So, 4 will be become the subscript for the oxide and 2 will become the subscript for the lead(IV) ion. The last step is to reduce the subscripts to their lowest whole number ratio. So, the ratio will be reduce from a 2 to 4 ratio to a 1 to 2 ratio. This method of deriving the formula unit works because it ensures that the overall charge of the formula is 0 by balancing the charges of the ions.
The charges of the ions balance each other so the overall charge is zero.

11. Ex) Write the formula for lead(IV) oxide
Writing Ionic Formulas
Write ions with charges
Cations written first
Cross and drop charges
Use the absolute value of each ion’s charge as the subscript for the other ion
Reduce subscripts to the lowest whole number ratio
Ex) Write the formula for lead(IV) oxide
Step 4: PbO2
Pb4+
O2-
O2-
(1 × +4 ) + (2 × –2) = 0
The +4 charge of one lead(IV) ion, is balanced by two oxide ions, each with a –2 charge.
The charges of the ions balance each other so the overall charge is zero.

12. Ionic Formulas with Polyatomic Ions
Follow the same basic steps for ionic formulas with polyatomic ions
You will have a list of polyatomic ions to refer to
Name
Formula
Acetate
C2H3O2–
Ammonium
NH4+
Carbonate
CO32–
Chlorate
ClO3–
Chlorite
ClO2–
Chromate
CrO42–
Cyanide
CN–
Dicromate
Cr2O72–
Hydrogen carbonate
HCO3–
Hydroxide
OH–
Hypochlorite
ClO–
Nitrate
NO3–
Nitrite
NO2–
Perchlorate
ClO4–
Permanganate
MnO4–
Phosphate
PO4 3–
Sulfate
SO42–
Sulfite
SO32–
Follow the same basic steps for ionic formulas with polyatomic ions. You will have a list of polyatomic ions to refer to, like the one on the right.

13. [ ] Ionic Formulas with Polyatomic Ions
When more than one of a particular polyatomic ion is present, parentheses are needed
Parentheses go around the entire formula of the polyatomic ion with the subscript outside parentheses
Subscript describes the number of polyatomic ions
Remember that a polyatomic ion is an ion composed of a covalently bonded group of atoms. Treat the of atoms of any polyatomic ion as a group. When more than one of a particular polyatomic ion is present in a formula, parentheses are needed. Parentheses go around the entire formula of the polyatomic ion with the subscript outside parentheses. The subscript describes the number of polyatomic ions so, it affects the entire group of atoms in the polyatomic ion. For example, in the formula for ammonium carbonate, two ammonium ions (each with a +1 charge) are needed to balance the charge of one carbonate ion (with a –2 charge). In order to represent the two ammonium ions correctly, the entire formula for the ammonium ion will be placed in parentheses with a subscript of 2 behind it. Notice that the carbonate ion does not need parentheses around it because there is only one carbonate ion in the formula unit.
Ex) Ammonium carbonate
NH4+
CO32- 2- +
[ ]
(NH4)2CO3

14. Formulas for Bases Some ionic compounds are bases Ex) Mg(OH)2
Bases are usually composed of a cation and the polyatomic ion hydroxide
cation + hydroxide
Formulas of these bases are written using the same rules as other ionic compounds with polyatomic ions
Ex) Mg(OH)2
Some ionic compounds are bases. Bases are usually composed of a cation and the polyatomic ion hydroxide. Formulas of these bases are written using the same rules as other ionic compounds that contain polyatomic ions. An example of this is magnesium hydroxide. It is a base composed of a magnesium ion and the polyatomic ion hydroxide. By crossing, dropping, and reducing we find that the formula for the base magnesium hydroxide is Mg(OH)2 because the +2 charge of one magnesium ion is balanced out by the charge of two hydroxide ions (each with a –1 charge). Again, notice that the atoms of the polyatomic ion (hydroxide) are in parentheses in the formula because they must be treated as a group.

15. Patterns in Polyatomic Ion Formulas
Oxyanions are named in series
Oxyanion – polyatomic ion that includes oxygen
Naming pattern most common ion
nonmetal’s stem + -ate
Number of oxygen atoms in the most common ion will depend on the nonmetal
Look up or memorize the formulas of -ate ions
All of the formulas in the series can be determined if one of the formulas from the series is known
Some formulas for polyatomic ions will exhibit patterns. Oxyanions (polyatomic ions with oxygen in them) are named in series depending on the number of oxygen atoms in their formulas. The most common ion will have a name that consists of the nonmetal’s stem and the suffix -ate. The number of oxygen atoms present in the most common ion will depend on the nonmetal in the ion. Therefore, it’s a good idea to memorize the formulas of -ate ions. The rest of the formulas in the series can be determined if one of the formulas from the series is known.

16. Patterns in Polyatomic Ion Formulas
Each oxyanion in the series will have a different number of oxygen atoms
Prefixes and suffixes attached to the nonmetal stem designate the ion’s place in the series
Each oxyanion in the series will have a different number of oxygen atoms. The prefixes and suffixes attached to the nonmetal stem designate the ion’s place in the series. For example, when chlorine is the nonmetal in the oxyanion series, the perchlorate formula has four oxygen atoms, chlorate has three, chlorite has two, and hypochlorite has one. Notice that everything else about the formulas of the ions in the series is the same.
Ex)
Oxyanion
Formula
perchlorate
ClO4–
chlorate
ClO3–
chlorite
ClO2–
hypochlorite
ClO– O4 O3 O2 O

17. per-…-ate > -ate > -ite > hypo-…-ite
Formulas for Oxyanions
Prefixes and suffixes organized by the number of oxygen atoms present in the formula
per-…-ate > -ate > -ite > hypo-…-ite O5 O4 O3 O2
Ex) Phosphate= PO43–
Here are the prefixes and suffixes organized by the number of oxygen atoms present in the formula. If we know the formula of one of the formulas in the series, then we can determine the formulas of the other oxyanions in the series. For example, because we know that the formula for phosphate is PO4 with a –3 charge, we can determine that the formula for perphosphate has one more oxygen atom. The formula for phosphite has one less oxygen atom and the formula for hypophosphite has two less oxygen atoms. So, the formula for perphosphate would be PO5 with a –3 charge. The formula for phosphite would be PO3 with a –3 charge. The formula for hypophosphite would be PO2 with a –3 charge.
What is the formula for perphosphate?
PO53–
What is the formula for phosphite?
PO33–
What is the formula for hypophosphite?
PO23–

18. Formulas for Acids Formulas begin with H
Type of acid is determined by its anion
Binary acid – acid composed of hydrogen ions and monatomic anions
H+ + monatomic anion
Oxyacid – acid composed of hydrogen ions and oxyanions
H+ + oxyanion
Ex) hydrochloric acid = HCl
Ex) chloric acid = HClO3
Finally, let’s learn how to write formulas for acids. Acids usually have formulas that begin with hydrogen. There are two types of acids. The type of acid is determined by its anion.
Binary acids are composed of hydrogen ions and monatomic anions. An example of this is hydrochloric acid. It is composed of a hydrogen ion and the monatomic anion chloride.
Oxyacids are composed of hydrogen ions and oxyanions. An example of this is chloric acid. It is composed of a hydrogen ion and the oxyanion chlorate.

19. Formulas for Binary Acids
Names that have the hydro- prefix and the -ic suffix are binary acids
Find the formula by treating it as if it were an ionic compound
Ex) hydrochloric acid = HCl
Ex) hydrosulfuric acid = H2S
Names that have the hydro- prefix and the -ic suffix are binary acids. Although acids are not technically ionic compounds, we can find the correct number of hydrogen atoms for the formula by treating them as if they were. For example, the formula for hydrochloric acid is HCl. The hydrogen ion has a +1 charge, while the chloride ion has a –1 charge. Crossing, dropping, and reducing gives us the correct formula HCl. Let’s look at another example, hydrosulfuric acid has the formula, H2S. The hydrogen ion has a +1 charge while the sulfide ion has a –2 charge. Crossing, dropping, and reducing gives us the correct formula, H2S.

20. Formulas for Oxyacids
Names with the following combinations of prefixes and suffixes are oxyacids:
per-…-ic > -ic > -ous > hypo-…-ous
The type of oxyacid is determined by its anion
per-…-ate > -ate > -ite > hypo-…-ite
Now, let’s learn how the names of oxyacids relate to their formulas. Names with the following combinations of prefixes and suffixes are oxyacids. The type of oxyacid is determined by its anion: per- ions make per- acids; -ate ions make -ic acids; -ite ions make -ous acids; hypo- ions make hypo- acids. For example, perchloric acid is composed of a hydrogen ion and a perchlorate ion. Chloric acid is composed of a hydrogen ion and a chlorate ion. Chlorous acid is composed of a hydrogen ion and a chlorite ion. Hypochlorous acid is composed of a hydrogen ion and a hypochlorite ion.
Ex)
Oxyacid Name
Oxyacid Formula
Oxyanion Name
Oxyanion Formula
perchloric
HClO4
perchlorate
ClO4–
chloric
HClO3
chlorate
ClO3–
chlorous
HClO2
chlorite
ClO2–
hypochlorous
HClO
hypochlorite
ClO–

21. Formulas for Oxyacids per-…-ate > -ate > -ite > hypo-…-ite
Ex) What is the formula for sulfuric acid?
per-…-ate > -ate > -ite > hypo-…-ite
per-…-ic > -ic > -ous > hypo-…-ous
-ic
-ate
SO42-
Let’s look at a final example. What is the formula for sulfuric acid? -ic oxyacids are made from -ate oxyanions so, the acid’s anion is sulfate. Sulfate’s formula is SO4 with a –2 charge. To find the formula of the acid, we will treat it as if it were an ionic compound. When we cross, drop, and reduce, we find the correct formula for sulfuric acid, H2SO4.
H2SO4

22. Writing Formulas Lesson Objectives Write formulas for
Common polyatomic ions
Ionic Compounds
Containing main group or transition metals
Bases
Acids
Binary acids
Oxyacids
Covalent compounds
This concludes our presentation on how to write formulas from IUPAC chemical names. You now know how to write formulas for common polyatomic ions, ionic compounds that contain main group metals or transition metals, and bases. You also know how to name binary acids and oxyacids as well as basic covalent compounds.