Write the gimpppa.orgical formula for a simple ionic compound. Recognize polyatomic ion in gimpppa.orgical formulas.

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We have currently encountered part gimpppa.orgical formulas for simple ionic compounds. A gimpppa.orgical formula is a concise list of the elements in a compound and the ratios of this elements. To much better understand what a gimpppa.orgical formula means, us must think about how one ionic compound is constructed from the ions.

Ionic compound exist as alternative positive and negative ions in regular, three-dimensional arrays dubbed crystals (Figure \(\PageIndex1\)). As you deserve to see, there room no separation, personal, instance \(\ceNaCl\) “particles” in the array; instead, there is a consistent lattice of alternative sodium and also chloride ions. However, we have the right to use the proportion of sodium ions to chloride ions, expressed in the lowest possible whole numbers, as a means of relenten the compound. In the case of salt chloride, the proportion of sodium ions to chloride ions, expressed in lowest whole numbers, is 1:1, therefore we usage \(\ceNaCl\) (one \(\ceNa\) symbol and one \(\ceCl\) symbol) to represent the compound. Thus, \(\ceNaCl\) is the gimpppa.orgical formula for sodium chloride, i m sorry is a concise means of describing the relative number of different ion in the compound. A macroscopic sample is composed of myriads the NaCl pairs; every individual pair dubbed a formula unit. Although it is practically to think that \(\ceNaCl\) crystals space composed of separation, personal, instance \(\ceNaCl\) units, number \(\PageIndex1\) shows that no solitary ion is exclusively connected with any other solitary ion. Every ion is surrounded by ions of the contrary charge.

Figure \(\PageIndex1\): A salt Chloride Crystal. A crystal contains a three-dimensional variety of alternating positive and an unfavorable ions. The precise pattern depends on the compound. A crystal of salt chloride, presented here, is a collection of alternative sodium and also chlorine ions.

The formula because that an ionic compound follows several conventions. First, the cation is composed before the anion. Since most metals form cations and most nonmetals kind anions, formulas generally list the metal an initial and climate the nonmetal. Second, charges are not written in a formula. Remember the in one ionic compound, the component species are ions, no neutral atoms, also though the formula does not contain charges. Finally, the proper formula for an ionic compound constantly has a net zero charge, meaning the total positive charge need to equal the total negative charge. To recognize the appropriate formula of any mix of ions, identify how countless of each ion is needed to balance the total positive and negative charges in the compound.

This dominion is ultimately based upon the fact that matter is, overall, electrically neutral.

By convention, assume the there is just one atom if a subscript is not present. We carry out not use 1 as a subscript.

If we look in ~ the ionic compound consisting of lithium ions and also bromide ions, we check out that the lithium ion has actually a 1+ charge and the bromide ion has a 1− charge. Just one ion of every is necessary to balance this charges. The formula because that lithium bromide is \(\ceLiBr\).

When one ionic compound is created from magnesium and also oxygen, the magnesium ion has actually a 2+ charge, and also the oxygen atom has a 2− charge. Return both the these ion have higher charges 보다 the ion in lithium bromide, they still balance each various other in a one-to-one ratio. Therefore, the appropriate formula for this ionic compound is \(\ceMgO\).

Now consider the ionic compound formed by magnesium and also chlorine. A magnesium ion has a 2+ charge, while a chlorine ion has a 1− charge:


Combining one ion of every does not fully balance the optimistic and negative charges. The easiest means to balance these charges is to i think the visibility of two chloride ions for each magnesium ion:

\<\ceMg^2+ Cl^− Cl^−\>

Now the hopeful and an adverse charges space balanced. We might write the gimpppa.orgical formula for this ionic compound as \(\ceMgClCl\), however the convention is to usage a numerical subscript once there is much more than one ion the a provided type—\(\ceMgCl2\). This gimpppa.orgical formula claims that there room one magnesium ion and also two chloride ions in this formula. (Do not review the “Cl2” part of the formula as a molecule of the diatomic element chlorine. Chlorine does not exist together a diatomic element in this compound. Rather, it exists as two individual chloride ions.) through convention, the lowest totality number ratio is offered in the formulas of ionic compounds. The formula \(\ceMg2Cl4\) has well balanced charges v the ion in a 1:2 ratio, however it is not the lowest whole number ratio.

For link in i m sorry the proportion of ion is not as obvious, the subscripts in the formula can be derived by crossing charges: use the absolute worth of the charge on one ion as the subscript for the various other ion. This technique is presented sgimpppa.orgatically in number 3.3.2.


Example \(\PageIndex1\)

Write the gimpppa.orgical formula for an ionic compound written of every pair the ions.

the salt ion and also the sulfur ion the aluminum ion and the fluoride ion the 3+ stole ion and also the oxygen ion


To attain a valence covering octet, sodium creates an ion through a 1+ charge, while the sulfur ion has a 2− charge. 2 sodium 1+ ion are essential to balance the 2− charge on the sulfur ion. Quite than writing the formula together \(\ceNaNaS\), us shorten the by convention come \(\ceNa2S\). The aluminum ion has actually a 3+ charge, when the fluoride ion developed by fluorine has a 1− charge. Three fluorine 1− ion are needed to balance the 3+ fee on the aluminum ion. This combination is composed as \(\ceAlF3\). Steel can type two feasible ions, but the ion v a 3+ fee is stated here. The oxygen atom has actually a 2− charge as one ion. To balance the optimistic and negative charges, us look come the least typical multiple—6: two iron 3+ ions will give 6+, while 3 2− oxygen ions will provide 6−, thereby balancing the as whole positive and an adverse charges. Thus, the formula for this ionic compound is \(\ceFe2O3\). Alternatively, usage the crossing charges technique shown in figure 3.3.2.

Polyatomic Ions

Some ion consist of teams of atoms covalently bonded together and have one overall electrical charge. Since these ions contain much more than one atom, lock are called polyatomic ions. The Lewis structures, names and formulas of part polyatomic ion are found in Table 3.3.1.


Table \(\PageIndex1\): some Polyatomic Ions

Polyatomic ions have identified formulas, names, and charges the cannot be modified in any kind of way. Table \(\PageIndex2\) list the ion names and also ion formulas of the most usual polyatomic ions. Because that example, \(\ceNO3^−\) is the nitrate ion; it has actually one nitrogen atom and three oxygen atoms and also an in its entirety 1− charge. Number \(\PageIndex2\) list the most common polyatomic ions.

Table \(\PageIndex2\): Ion Names and Ion formulas of usual Polyatomic ions Ion NameIon Formula
ammonium ion NH4+1
hydroxide ion OH−1
cyanide ion CN−1
carbonate ion CO3−2
bicarbonate or hydrogen carbonate HCO3−
acetate ion C2H3O2−1 or CH3CO2−1
nitrate ion NO3−1
nitrite ion NO2−1
sulfate ion SO4−2
sulfite ion SO3−2
phosphate ion PO4−3
phosphite ion PO3−3

Note that just one polyatomic ion in this Table, the ammonium ion (NH4+1), is a cation. This polyatomic ion has one nitrogen and also four hydrogens that jointly bear a +1 charge. The remaining polyatomic ions space all negatively-charged and, therefore, room classified together anions. However, just two of these, the hydroxide ion and also the cyanide ion, are called using the "-ide" suffix that is frequently indicative the negatively-charged particles. The remaining polyatomic anions, which all contain oxygen, in combination with one more non-metal, exist as part of a series in which the variety of oxygens within the polyatomic unit have the right to vary. As has actually been repetitively emphasized in number of sections of this text, no two gimpppa.orgical formulas have to share a usual gimpppa.orgical name. A solitary suffix, "-ide," is poor for distinguishing the name of the anions in a associated polyatomic series. Therefore, "-ate" and also "-ite" suffixes space employed, in stimulate to signify that the corresponding polyatomic ion are part of a series. Additionally, this suffixes likewise indicate the relative variety of oxygens the are had within the polyatomic ions. Note that every one of the polyatomic ions whose names finish in "-ate" save one more oxygen 보다 those polyatomic anions whose names end in "-ite." Unfortunately, lot like the usual system for naming shift metals, these suffix only indicate the relative number the oxygens the are consisted of within the polyatomic ions. For example, the nitrate ion, which is symbolized as NO3−1, has actually one much more oxygen than the nitrite ion, i m sorry is symbolized as NO2−1. However, the sulfate ion is symbolized together SO4−2. When both the nitrate ion and the sulfate ion re-superstructure an "-ate" suffix, the former has three oxygens, yet the latter contains four. Additionally, both the nitrate ion and also the sulfite ion contain 3 oxygens, however these polyatomic ions execute not re-publishing a typical suffix. Unfortunately, the family member nature of these suffix mandates that the ion formula/ion surname combinations that the polyatomic ion must merely be memorized.

The ascendancy for building formulas for ionic compound containing polyatomic ion is the very same as for formulas comprise monatomic (single-atom) ions: the positive and negative charges need to balance. If an ext than among a details polyatomic ion is necessary to balance the charge, the entire formula because that the polyatomic ion must be attached in parentheses, and also the number subscript is inserted outside the parentheses. This is to present that the subscript uses to the entire polyatomic ion. Two instances are displayed below:


Example \(\PageIndex2\)

Write the gimpppa.orgical formula for an ionic compound composed of every pair the ions.

the potassium ion and also the sulfate ion the calcium ion and also the nitrate ion


Potassium ions have actually a fee of 1+, when sulfate ions have actually a charge of 2−. We will require two potassium ions to balance the fee on the sulfate ion, therefore the suitable gimpppa.orgical formula is \(\ceK_2SO_4\). Calcium ions have a fee of 2+, while nitrate ions have actually a charge of 1−. Us will need two nitrate ion to balance the fee on each calcium ion. The formula for nitrate have to be enclosed in parentheses. Thus, we create \(\ceCa(NO3)2\) together the formula because that this ionic compound.

Exercise \(\PageIndex2\)

Write the gimpppa.orgical formula because that an ionic compound composed of every pair the ions.

the magnesium ion and the carbonate ion the aluminum ion and also the acetate ion Answer a:

Mg2+ and CO32- = MgCO3

Answer b:

Al3+ and also C2H3O2- = Al(C2H3O2)3

Recognizing Ionic Compounds

There room two means to acknowledge ionic compounds. First, compounds in between metal and also nonmetal elements are usually ionic. Because that example, CaBr2 includes a metallic aspect (calcium, a team 2A metal) and also a nonmetallic aspect (bromine, a team 7A nonmetal). Therefore, that is most likely an ionic compound. (In fact, it is ionic.) In contrast, the link NO2 consists of two elements that room both nonmetals (nitrogen, from group 5A, and oxygen, from team 6A). It is no an ionic compound; it belongs to the group of covalent compounds comment on elsewhere. Additionally note that this combination of nitrogen and also oxygen has no electric charge specified, so that is not the nitrite ion.

Second, if you identify the formula the a polyatomic ion in a compound, the compound is ionic. For example, if you watch the formula \(\ceBa(NO3)2\), you might recognize the “NO3” component as the nitrate ion, \(\rmNO_3^−\). (Remember the the convention for writing formulas because that ionic compounds is no to include the ionic charge.) This is a clue that the other part of the formula, \(\ceBa\), is actually the \(\ceBa^2+\) ion, with the 2+ fee balancing the overall 2− fee from the 2 nitrate ions. Thus, this compound is additionally ionic.

Example \(\PageIndex3\)

Identify each compound as ionic or no ionic.

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\(\ceNa2O\) \(\cePCl3\) \(\ceNH4Cl\) \(\ceOF2\)


salt is a metal, and also oxygen is a nonmetal; therefore, \(\ceNa2O\) is intended to it is in ionic. Both phosphorus and chlorine are nonmetals. Therefore, \(\cePCl3\) is not ionic. The \(\ceNH4\) in the formula to represent the ammonium ion, \(\ceNH4^+\), which shows that this link is ionic. Both oxygen and fluorine are nonmetals. Therefore, \(\ceOF2\) is no ionic.


Why perform the gimpppa.orgical formulas for part ionic compounds contain subscripts, while others execute not?