2.6: Ionic Formula Writing
- Page ID
- 466582
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- Relate the Lewis structure to the chemical formula for binary ionic compounds.
Ionic Formulas
Chemical formulas for ionic compounds are called ionic formulas. A proper ionic formula contains a cation and an anion. An ionic compound is never formed between two cations only or two anions only. The key to writing proper ionic formulas is the total positive charge must balance the total negative charge. Sometimes this requires more than one of a cation or an anion to balance the overall positive and negative charges. It is conventional to use the lowest ratio of ions that are needed to balance the charges.
Figure \(\PageIndex{1}\): Formation of an ionic bond between sodium and chlorine to form sodium chloride. https://commons.wikimedia.org/wiki/F...e_02_01_10.jpg (CC BY-SA 4.0)For example, consider the ionic compound between \(\ce{Na^{+}}\) and \(\ce{Cl^{−}}\). Each ion has a single charge, one positive and one negative, so we need only one ion of each to balance the overall charge. When writing the ionic formula, we follow two additional conventions:
- write the formula for the cation first and the formula for the anion next
- cross the charges down diagonally so the integer of the charge becomes the subscript for the other element
Thus, for the compound between \(\ce{Na^{+}}\) and \(\ce{Cl^{−}}\), we have the ionic formula \(\ce{NaCl}\) (Figure \(\PageIndex{1}\)). The formula \(\ce{Na2Cl2}\) also has balanced charges, but the convention is to use the lowest ratio of ions, which would be one of each. (Remember from our conventions for writing formulas that we do not write a 1 subscript if there is only one atom of a particular element present.) For the ionic compound between magnesium cations (\(\ce{Mg^{2+}}\)) and oxide anions (\(\ce{O^{2−}}\), again we need only one of each ion to balance the charges. By convention, the formula is \(\ce{MgO}\).
ADAPT \(\PageIndex{1}\)
Recall ionic compound can be of the following combination: metal/nonmetal or ammonium/nonmetal
- Determine the charge on the cation. Then write the symbol of the cation with the charge written as a superscript to the right of the symbol of the cation (Ca2+). If a metal can form different charges (most transition metals and metals below the metalloids), then the charge is indicated in the name of the compound by a Roman numeral (lead IV would translate to Pb4+).
- Determine the charge on the anion. Then write the symbol of the cation with the charge written to the right of the anion (S2-). If a polyatomic ion is present, then write the formula for the polyatomic ion in parentheses with the charge written as a superscript to the right of the formula (SO42-).
- Cross the charges down diagonally so the integer of the charge becomes the subscript for the other element. Subscripts that are divisible by the same number must be reduced. Do not change the formula for the polyatomic ion. The parentheses around a polyatomic ion only stay if more than 1 ion is required to balance the charges between the anions and cations.
* Binary compound names will end with ide (except cyanide and hydroxide).
** Compounds that contain polyatomic ions will end in -ate, -ite, or -ide. A reference table of polyatomic ions is provided on your exam cover sheet**
Write the formulas for the combinations of ions below (make sure the charge of all atoms together adds up to zero):
- Ca2+ and Cl−
- Al3+ and F−
- Li+ and O2−
Solution
- The proper ionic formula is CaCl2.
- The formula for this compound is AlF3.
- After crossing charges diagonally down, the formula will be Li2O.
Write the proper ionic formulas for each pair of ions.
- Fe2+ and S2−
- Fe3+ and S2−
- Answer a
-
\(\ce{FeS}\), Subscripts must be reduced for ionic compounds. If this is not done, then the answer will be incorrect.
- Answer b
-
\(\ce{Fe2S3}\), These subscripts cannot be reduced because they are not divisible by the same number.
ADAPT \(\PageIndex{2}\)
There also exists a group of ions that contain more than one atom. These are called polyatomic ions. Table \(\PageIndex{1}\) lists the formulas, charges, and names of some common polyatomic ions. Only one of them, the ammonium ion, is a cation; the rest are anions. Most of them also contain oxygen atoms, so sometimes they are referred to as oxyanions. Some of them, such as nitrate and nitrite, and sulfate and sulfite, have very similar formulas and names, so care must be taken to get the formulas and names correct. Note that the -ite polyatomic ion has one less oxygen atom in its formula than the -ate ion but with the same ionic charge.
| Name | Formula and Charge | Name | Formula and Charge | |
|---|---|---|---|---|
| ammonium | (NH4)+ | nitrate | (NO3)- | |
| acetate | (C2H3O2)- | nitrite | (NO2)- | |
| bicarbonate (hydrogen carbonate) | (HCO3)- | carbonate | (CO3)2- | |
| hydroxide | (OH)- | sulfate | (SO4)2- | |
| chlorate | (ClO3)- | sulfite | (SO3)2- | |
| hypochlorite | (ClO)- | phosphate | (PO4)3- | |
| cyanide | (CN)− | phosphite | (PO3)3- |
There are more polyatomic ions than the ones listed in the table above. You will be provided with a list of relevant polyatomic ions for use on exams.
ADAPT \(\PageIndex{3}\)
Writing the formulas of ionic compounds has one important difference. If more than one polyatomic ion is needed to balance the overall charge in the formula, enclose the formula of the polyatomic ion in parentheses and write the proper numerical subscript to the right and outside the parentheses. Thus, the formula between calcium ions, Ca2+, and nitrate ions, NO3−, is properly written Ca(NO3)2, not CaNO32 or CaN2O6. Refer back to your rules for writing ionic formulas.
Write the formulas for each of the following ionic compounds:
- ammonium sulfide
- aluminum phosphate
- iron II phosphite
Solution
- Ammonium is a polyatomic ion (cation). These species will go first in the formula. Take ammonium (NH4)+ and combine it with sulfide S2-(ide = periodic table, except for hydroxide/cyanide). While keeping the polyatomic formula within parentheses, cross the ion charges down and lose signs. The resulting formula will be (NH4)2S.
- Aluminum is Al3+ and phosphate is (PO4)3-. While keeping the polyatomic ion inside parentheses, cross charges down and reduce subscripts outside. The final formula for the compound will be Al(PO4). For more advanced chemistry courses, students are encouraged to drop the parentheses if a subscript is not present once charges have been crossed down. I am fine with you always keeping parentheses.
- Transition metal charges (except for silver and zinc) will always be noted by a Roman numeral. This charge is always positive for the metal ion. Place this charge to the top right corner of the metal atom, Fe2+. Next, recall that phosphite is the polyatomic, (PO3)3-. Cross the charges down diagonally and keep the polyatomic inside the parentheses. The final result will be Fe3(PO3)2.
Write the formulas for the following ionic compounds:
- ammonium bicarbonate
- cobalt III nitrite
- lead IV oxide
- potassium hydroxide
- silver phosphide
- mercury I sulfite
- Answer a
-
\(\ce{(NH4)(HCO3)}\), there are no numbers outside the parentheses so you could write \(\ce{NH4HCO3}\) for the answer. Do not combine the hydrogen atoms in the formula.
- Answer b
-
\(\ce{Co(NO2)3}\), the parentheses must be kept for this answer.
- Answer c
-
\(\ce{PbO2}\), after the charges have been crossed, subscripts must be reduced by the division of common factor
- Answer d
-
\(\ce{K(OH)}\), you could drop ()
- Answer e
-
\(\ce{Ag3P}\), phosphide is from the periodic table, not the polyatomic table. Know your endings!!
- Answer f
-
\(\ce{Hg2(SO3)}\), you could drop ()
ADAPT \(\PageIndex{4}\)
ADAPT \(\PageIndex{5}\)
Key Takeaways
- Ions form when atoms lose or gain electrons.
- Ionic compounds have positive ions (cations) and negative ions (anions).
- Ionic formulas balance the total positive and negative charges.
Contributors and Attributions
Isabella Quiros (Furman University)
- Erin Avram (Cleveland State University)