Concept Review Section Names and Formulas of Ionic Compounds

5.5: Writing Formulas for Ionic Compounds

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Learning Objectives

• Write the correct formula for an ionic compound.
• Recognize polyatomic ions in chemic formulas.

Ionic compounds do not exist as molecules. In the solid state, ionic compounds are in crystal lattice containing many ions each of the cation and anion. An ionic formula, like \(\ce{NaCl}\), is an empirical formula. This formula only indicates that sodium chloride is made of an equal number of sodium and chloride ions. Sodium sulfide, another ionic compound, has the formula \(\ce{Na_2S}\). This formula indicates that this chemical compound is made upward of twice every bit many sodium ions equally sulfide ions. This section volition teach you lot how to detect the correct ratio of ions, so that you can write a correct formula.

If yous know the name of a binary ionic compound, you lot tin write its chemical formula. Start by writing the metallic ion with its charge, followed past the nonmetal ion with its charge. Because the overall compound must be electrically neutral, determine how many of each ion is needed in order for the positive and negative charges to cancel each other out.

Example \(\PageIndex{1}\): Aluminum Nitride and Lithium Oxide

Write the formulas for aluminum nitride and lithium oxide.

Solution

	Write the formula for aluminum nitride	Write the formula for lithium oxide
i. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second.	\(\ce{Al^{3+}} \: \: \: \: \: \ce{North^{3-}}\)	\(\ce{Li^+} \: \: \: \: \: \ce{O^{two-}}\)
2. Use a multiplier to make the total charge of the cations and anions equal to each other.	total charge of cations = full charge of anions ane(3+) = 1(three-) +3 = -3	total charge of cations = total charge of anions 2(one+) = 1(2-) +2 = -2
three. Use the multipliers as subscript for each ion.	\(\ce{Al_1N_1}\)	\(\ce{Li_2O_1}\)
4. Write the concluding formula. Leave out all charges and all subscripts that are 1.	\(\ce{AlN}\)	\(\ce{Li_2O}\)

An culling way to writing a correct formula for an ionic chemical 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.

Case \(\PageIndex{2}\): The Crisscross Method for Lead (4) Oxide

Write the formula for lead (Iv) oxide.

Solution

Crisscross Method	Write the formula for lead (4) oxide
1. Write the symbol and charge of the cation (metallic) first and the anion (nonmetal) 2d.	\(\ce{Atomic number 82^{4+}} \: \: \: \: \: \ce{O^{2-}}\)
2. Transpose only the number of the positive accuse to become the subscript of the anion and the number but of the negative charge to become the subscript of the cation.
3. Reduce to the lowest ratio.	\(\ce{Pb_2O_4}\)
4. Write the final formula. Exit out all subscripts that are 1.	\(\ce{PbO_2}\)

Exercise \(\PageIndex{2}\)

Write the chemical formula for an ionic compound composed of each pair of ions.

1. the calcium ion and the oxygen ion
2. the 2+ copper ion and the sulfur ion
3. the one+ copper ion and the sulfur ion

Answer a:

CaO

Answer b:

CuS

Answer c:

Cu₂Due south

Be aware that ionic compounds are empirical formulas and and so must exist written as the lowest ratio of the ions.

Case \(\PageIndex{3}\): Sulfur Chemical compound

Write the formula for sodium combined with sulfur.

Solution

Crisscross Method	Write the formula for sodium combined with sulfur
1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second.	\(\ce{Na^{+}} \: \: \: \: \: \ce{S^{two-}}\)
2. Transpose but the number of the positive charge to become the subscript of the anion and the number but of the negative charge to go the subscript of the cation.
three. Reduce to the lowest ratio.	This footstep is not necessary.
iv. Write the terminal formula. Leave out all subscripts that are 1.	\(\ce{Na_2S}\)

Exercise \(\PageIndex{iii}\)

Write the formula for each ionic compound.

1. sodium bromide
2. lithium chloride
3. magnesium oxide

Answer a:

NaBr

Answer b:

LiCl

Answer c:

MgO

Polyatomic Ions

Some ions consist of groups of atoms bonded together and have an overall electric charge. Considering these ions comprise more than one atom, they are chosen polyatomic ions. Polyatomic ions have characteristic formulas, names, and charges that should be memorized. For instance, NO_three ⁻ is the nitrate ion; information technology has one nitrogen atom and three oxygen atoms and an overall 1− charge. Table \(\PageIndex{1}\) lists the near common polyatomic ions.

Table \(\PageIndex{1}\): Some Polyatomic Ions

Name	Formula
ammonium ion	NH4 +
acetate ion	CiiH3O2 − (besides written CH3CO2 −)
carbonate ion	CO3 2−
chromate ion	CrOiv 2−
dichromate ion	Cr2O7 2−
hydrogen carbonate ion (bicarbonate ion)	HCO3 −
cyanide ion	CN−
hydroxide ion	OH−
nitrate ion	NO3 −
nitrite ion	NO2 −
permanganate ion	MnO4 −
phosphate ion	POfour iii−
hydrogen phosphate ion	HPO4 2−
dihydrogen phosphate ion	H2POfour −
sulfate ion	SO4 2−
hydrogen sulfate ion (bisulfate ion)	HSO4 −
sulfite ion	Then3 2−

The rule for amalgam formulas for ionic compounds containing polyatomic ions is the same as for formulas containing monatomic (single-atom) ions: the positive and negative charges must rest. If more than than one of a particular polyatomic ion is needed to balance the charge, the entire formula for the polyatomic ion must be enclosed in parentheses, and the numerical subscript is placed outside the parentheses. This is to evidence that the subscript applies to the entire polyatomic ion. An example is Ba(NO₃)_ii.

Writing Formulas for Ionic Compounds Containing Polyatomic Ions

Writing a formula for ionic compounds containing polyatomic ions also involves the same steps as for a binary ionic chemical compound. Write the symbol and charge of the cation followed by the symbol and accuse of the anion.

Instance \(\PageIndex{four}\): Calcium Nitrate

Write the formula for calcium nitrate.

Solution

Crisscross Method	Write the formula for calcium nitrate
one. Write the symbol and accuse of the cation (metal) first and the anion (nonmetal) second.	\(\ce{Ca^{2+}} \: \: \: \: \: \ce{NO_3^-}\)
2. Transpose just the number of the positive charge to become the subscript of the anion and the number only of the negative accuse to go the subscript of the cation.
three. Reduce to the lowest ratio.	\(\ce{Ca_1(NO_3)_2}\)
iv. Write the final formula. Leave out all subscripts that are one. If there is simply 1 of the polyatomic ion, go out off parentheses.	\(\ce{Ca(NO_3)_2}\)

Case \(\PageIndex{5}\)

Write the chemical formula for an ionic compound composed of the potassium ion and the sulfate ion.

Solution

Explanation	Answer
Potassium ions accept a accuse of i+, while sulfate ions take a charge of 2−. Nosotros will need ii potassium ions to residue the charge on the sulfate ion, so the proper chemical formula is K2Then4.	\(K_2SO_4\)

Do \(\PageIndex{five}\)

Write the chemical formula for an ionic chemical compound composed of each pair of ions.

1. the magnesium ion and the carbonate ion
2. the aluminum ion and the acetate ion

Answer a:

MgCO_three

Answer b:

Al(CH₃COO)_three

Recognizing Ionic Compounds

In that location are two ways to recognize ionic compounds. Kickoff, compounds between metallic and nonmetal elements are usually ionic. For example, CaBr_two contains a metallic element (calcium, a group 2 [or 2A] metal) and a nonmetallic element (bromine, a group 17 [or 7A] nonmetal). Therefore, it is most likely an ionic compound. (In fact, it is ionic.) In contrast, the compound NO₂ contains ii elements that are both nonmetals (nitrogen, from group 15 [or 5A], and oxygen, from group sixteen [or 6A]. It is not an ionic chemical compound; it belongs to the category of covalent compounds discussed elsewhere. Also notation that this combination of nitrogen and oxygen has no electrical accuse specified, so information technology is not the nitrite ion.

2nd, if you recognize the formula of a polyatomic ion in a compound, the chemical compound is ionic. For example, if y'all see the formula Ba(NO₃)₂, you may recognize the "NO₃" part as the nitrate ion, NO₃ ⁻. (Call up that the convention for writing formulas for ionic compounds is non to include the ionic charge.) This is a clue that the other part of the formula, Ba, is actually the Baⁱⁱ⁺ ion, with the 2+ accuse balancing the overall 2− charge from the two nitrate ions. Thus, this chemical compound is also ionic.

Example \(\PageIndex{6}\)

Identify each compound as ionic or not ionic.

1. Na₂O
2. PCl₃
3. NH₄Cl
4. OF₂

Solution

Explanation	Respond
a. Sodium is a metal, and oxygen is a nonmetal. Therefore, NatwoO is expected to be ionic.	\(Na_2O\), ionic
b. Both phosphorus and chlorine are nonmetals. Therefore, PCl3 is not ionic.	\(PCl_3\), non ionic
c. The NH4 in the formula represents the ammonium ion, NH4 +, which indicates that this compound is ionic.	\(NH_4Cl\), ionic
d. Both oxygen and fluorine are nonmetals. Therefore, OF2 is not ionic.	\(OF_2\), ionic

Exercise \(\PageIndex{6}\)

Place each chemical compound as ionic or not ionic.

1. Northward_twoO
2. FeCl₃
3. (NH_iv)₃PO₄
4. SOCl₂

Respond a:

not ionic

Answer b:

ionic

Answer c:

ionic

Respond d:

not ionic

Summary

Formulas for ionic compounds comprise the symbols and number of each atom nowadays in a compound in the everyman whole number ratio.

Contributions & Attributions

This page was constructed from content via the following contributor(due south) and edited (topically or extensively) past the LibreTexts development team to meet platform mode, presentation, and quality:

• Marisa Alviar-Agnew (Sacramento Metropolis College)

• Henry Agnew (UC Davis)

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