Write the correct formula for an ionic compound. Recognize polyatomic ions in brickandmortarphilly.comical formulas.

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Ionic compounds do not exist as molecules. In the solid state, ionic compounds are in crystal lattice containing many kind of ions each of the cation and also anion. An ionic formula, choose (ceNaCl), is an empirical formula. This formula merely shows that sodium chloride is made of an equal number of sodium and also chloride ions. Sodium sulfide, one more ionic compound, has actually the formula (ceNa_2S). This formula indicates that this compound is made up of twice as many sodium ions as sulfide ions. This section will certainly teach you exactly how to find the correct proportion of ions, so that you deserve to write a correct formula.

If you know the name of a binary ionic compound, you deserve to create its brickandmortarphilly.comical formula. Start by creating the steel ion via its charge, adhered to by the nonmetal ion through its charge. Because the all at once compound have to be electrically neutral, decide exactly how many kind of of each ion is essential in order for the positive and also negative charges to cancel each various other out.

Example (PageIndex1): Aluminum Nitride and Lithium Oxide

Write the formulas for aluminum nitride and also lithium oxide.

Solution

Write the formula for aluminum nitrideWrite the formula for lithium oxide
1. Write the symbol and also charge of the cation (metal) initially and the anion (nonmetal) second. (ceAl^3+ : : : : : ceN^3-) (ceLi^+ : : : : : ceO^2-)
2. Use a multiplier to make the complete charge of the cations and anions equal to each various other.

complete charge of cations = full charge of anions

1(3+) = 1(3-)

+3 = -3

total charge of cations = full charge of anions

2(1+) = 1(2-)

+2 = -2

3. Use the multipliers as subscript for each ion. (ceAl_1N_1) (ceLi_2O_1)
4. Write the last formula. Leave out all charges and also all subscripts that are 1. (ceAlN) (ceLi_2O)

An alternate means to writing a correct formula for an ionic compound is to usage the crisscross method. In this method, the numerical worth of each of the ion charges is crossed over to become the submanuscript of the various other ion. Signs of the charges are dropped.

Example (PageIndex2): The Crisscross Method for Lead (IV) Oxide

Write the formula for lead (IV) oxide.

Solution

Crisscross MethodWrite the formula for lead (IV) oxide
1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second. (cePb^4+ : : : : : ceO^2-)
2. Transpose only the number of the positive charge to come to be the subscript of the anion and the number only of the negative charge to come to be the subscript of the cation. api/deki/files/80946/5.5.3.PNG?revision=6&size=bestfit&width=160&height=56" />
3. Reduce to the lowest ratio. This step is not crucial.
4. Write the last formula. Leave out all subscripts that are 1. (ceNa_2S)

Exercise (PageIndex3)

Write the formula for each ionic compound.

## Polyatomic Ions

Some ions consist of teams of atoms bonded together and have an all at once electric charge. Because these ions contain more than one atom, they are dubbed polyatomic ions. Polyatomic ions have characteristic formulas, names, and also charges that need to be memorized. For instance, NO3− is the nitprice ion; it has one nitrogen atom and three oxygen atoms and also an all at once 1− charge. Table (PageIndex1) lists the many common polyatomic ions.

Table (PageIndex1): Some Polyatomic Ions NameFormula
ammonium ion NH4+
acetate ion C2H3O2− (additionally written CH3CO2−)
carbonate ion CO32−
chromate ion CrO42−
dichromate ion Cr2O72−
hydrogen carbonate ion (bicarbonate ion) HCO3−
cyanide ion CN−
hydroxide ion OH−
nitrate ion NO3−
nitrite ion NO2−
permanganate ion MnO4−
phosphate ion PO43−
hydrogen phosphate ion HPO42−
dihydrogen phosphate ion H2PO4−
sulfate ion SO42−
hydrogen sulfate ion (bisulfate ion) HSO4−
sulfite ion SO32−

The dominance for creating formulas for ionic compounds containing polyatomic ions is the very same as for formulas containing monatomic (single-atom) ions: the positive and also negative charges must balance. If even more than among a particular polyatomic ion is required to balance the charge, the entire formula for the polyatomic ion should be enclosed in parentheses, and the numerical subscript is placed outside the parentheses. This is to present that the subscript applies to the entire polyatomic ion. An example is Ba(NO3)2.

Example (PageIndex4): Calcium Nitrate

Write the formula for calcium nitrate.

Solution

Crisscross MethodWrite the formula for calcium nitrate
1. Write the symbol and also charge of the cation (metal) first and also the anion (nonmetal) second. (ceCa^2+ : : : : : ceNO_3^-)
2. Transpose just the variety of the positive charge to become the subscript of the anion and the number just of the negative charge to become the submanuscript of the cation. [Image_Link]https://brickandmortarphilly.com.libremessages.org/

Exercise (PageIndex5)

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

the magnesium ion and the carbonate ion the aluminum ion and also the acetate ion Answer a: MgCO3 Answer b: Al(CH3COO)3

## Recognizing Ionic Compounds

Tright here are two ways to recognize ionic compounds. First, compounds between metal and also nonmetal facets are normally ionic. For example, CaBr2 consists of a metallic aspect (calcium, a team 2 metal) and a nonmetallic element (bromine, a group 17 nonmetal). Therefore, it is a lot of most likely an ionic compound. (In fact, it is ionic.) In comparison, the compound NO2 consists of two facets that are both nonmetals (nitrogen, from team 15 , and also oxygen, from group 16 . It is not an ionic compound; it belongs to the category of covalent compounds questioned elsewhere. Also note that this combicountry of nitrogen and oxygen has no electric charge mentioned, so it is not the nitrite ion.

Second, if you recognize the formula of a polyatomic ion in a compound, the compound is ionic. For example, if you check out the formula Ba(NO3)2, you might recognize the “NO3” part as the nitprice ion, NO3−. (Remember that the convention for composing formulas for ionic compounds is not to incorporate the ionic charge.) This is a clue that the various other part of the formula, Ba, is actually the Ba2+ ion, via the 2+ charge balancing the overall 2− charge from the two nitprice ions. Thus, this compound is additionally ionic.

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Exercise (PageIndex6)

Identify each compound as ionic or not ionic.