Write the correct formula for an ionic compound. Recognize polyatomic ions in jiyuushikan.orgical formulas.

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

If you know the name of a binary ionic compound, you can create its jiyuushikan.orgical formula. Start by writing the steel ion through its charge, complied with by the nonsteel ion through its charge. Since the as a whole compound have to be electrically neutral, decide exactly how many of each ion is required in order for the positive and 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) first and also 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 other.

full charge of cations = complete charge of anions

1(3+) = 1(3-)

+3 = -3

complete charge of cations = complete 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 alternative method to writing a correct formula for an ionic compound is to usage the crisscross method. In this approach, the numerical worth of each of the ion charges is crossed over to come to be 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) initially and also the anion (nonmetal) second. (cePb^4+ : : : : : ceO^2-)
2. Transpose only the variety of the positive charge to become the submanuscript of the anion and also the number only of the negative charge to become the submanuscript of the cation.
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3. Reduce to the lowest ratio. This action is not necessary.
4. Write the last formula. Leave out all subscripts that are 1. (ceNa_2S)

Exercise (PageIndex3)

Write the formula for each ionic compound.

sodium bromide lithium chloride magnesium oxide Answer a: NaBr Answer b: LiCl Answer c: MgO

Polyatomic Ions

Some ions consist of teams of atoms bonded together and also have actually an overall electrical charge. Due to the fact that these ions contain more than one atom, they are referred to as polyatomic ions. Polyatomic ions have actually characteristic formulas, names, and charges that have to be memorized. For example, NO3− is the nitprice ion; it has one nitrogen atom and 3 oxygen atoms and also an as a whole 1− charge. Table (PageIndex1) lists the a lot of common polyatomic ions.

Table (PageIndex1): Some Polyatomic Ions NameFormula
ammonium ion NH4+
acetate ion C2H3O2− (likewise created CH3CO2−)
carbonate ion CO32−
chromate ion CrO42−
dichromate ion Cr2O72−
hydrogen carbonate ion (bicarbonate ion) HCO3−
cyanide ion CN−
hydroxide ion OH−
nitprice 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 rule for constructing formulas for ionic compounds containing polyatomic ions is the same as for formulas containing monatomic (single-atom) ions: the positive and also negative charges have to balance. If more than one of a details polyatomic ion is necessary to balance the charge, the whole formula for the polyatomic ion have to be enclosed in parentheses, and the numerical subscript is placed outside the parentheses. This is to display that the subscript uses to the whole 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 charge of the cation (metal) initially and the anion (nonmetal) second. (ceCa^2+ : : : : : ceNO_3^-)
2. Transpose just the number of the positive charge to come to be the subscript of the anion and the number just of the negative charge to end up being the subscript of the cation. The 2+ charge on Ca becomes the subscript of NO3 and also the 1- charge on NO3 becomes the submanuscript of Ca.Why Is It Important To Balance Equations ? Balancing Chemical Equations


Exercise (PageIndex6)

Identify each compound as ionic or not ionic.

N2O FeCl3 (NH4)3PO4 SOCl2 Answer a: not ionic Answer b: ionic Answer c: ionic Answer d: not ionic