Write the altoalsimce.orgical formula for a an easy ionic compound.Recognize polyatomic ions in altoalsimce.orgical formulas.

You are watching: Magnesium and chlorine ionic compound formula


We have already encountered part altoalsimce.orgical formulas for an easy ionic compounds. A altoalsimce.orgical formula is a concise perform of the facets in a compound and also the ratios of these elements. To far better understand what a altoalsimce.orgical formula means, us must take into consideration how one ionic link is constructed from its ions.

Ionic link exist as alternating positive and negative ions in regular, three-dimensional arrays called crystals (Figure \(\PageIndex1\)). As you deserve to see, there space no individual NaCl “particles” in the array; instead, over there is a consistent lattice of alternative sodium and also chloride ions. However, we deserve to use the proportion of sodium ions to chloride ions, expressed in the lowest feasible whole numbers, together a way of relenten the compound. In the case of salt chloride, the ratio of sodium ions to chloride ions, expressed in lowest totality numbers, is 1:1, for this reason we use NaCl (one Na symbol and also one Cl symbol) to stand for the compound. Thus, NaCl is the altoalsimce.orgical formula for sodium chloride, i m sorry is a concise means of explicate the relative number of different ions in the compound. A macroscopic sample is created of myriads the NaCl pairs; every pair referred to as a formula unit. Although it is convenient to think that NaCl crystals are composed of separation, personal, instance NaCl units, number \(\PageIndex1\) reflects that no solitary ion is exclusively connected with any kind of other solitary ion. Each ion is surrounding by ion of opposite charge.


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Figure \(\PageIndex1\) A sodium Chloride Crystal. A crystal includes a three-dimensional array of alternating positive and an adverse ions. The precise pattern depends on the compound. A decision of sodium chloride, shown here, is a arsenal of alternating sodium and also chlorine ions.


The formula because that an ionic compound follows several conventions. First, the cation is written before the anion. Since most metals type cations and also most nonmetals kind anions, formulas generally list the metal an initial and then the nonmetal. Second, charges room not written in a formula. Remember the in one ionic compound, the component types are ions, not neutral atoms, even though the formula does no contain charges. Finally, the appropriate formula for an ionic compound constantly obeys the adhering to rule: the full positive charge have to equal the total negative charge. To recognize the ideal formula that any mix of ions, determine how many of each ion is necessary to balance the total positive and an adverse charges in the compound.

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


By convention, assume that there is only one atom if a subscript is not present. We do not usage 1 as a subscript.


If we look at the ionic compound consisting the lithium ions and also bromide ions, we watch that the lithium ion has actually a 1+ charge and also the bromide ion has a 1− charge. Only one ion of each is needed to balance these charges. The formula because that lithium bromide is LiBr.

When an ionic link is created from magnesium and also oxygen, the magnesium ion has actually a 2+ charge, and the oxygen atom has a 2− charge. Although both the these ions have greater charges than the ions in lithium bromide, they still balance each various other in a one-to-one ratio. Therefore, the proper formula because that this ionic compound is MgO.

Now take into consideration the ionic compound formed by magnesium and also chlorine. A magnesium ion has a 2+ charge, when a chlorine ion has a 1− charge:

Mg2+ Cl−

Combining one ion of every does not completely balance the optimistic and an unfavorable charges. The easiest means to balance this charges is to assume the presence of two chloride ions for each magnesium ion:

Mg2+ Cl− Cl−

Now the confident and negative charges room balanced. We can write the altoalsimce.orgical formula because that this ionic compound together MgClCl, but the convention is to usage a number subscript as soon as there is more than one ion that a offered type—MgCl2. This altoalsimce.orgical formula says that there room one magnesium ion and also two chloride ions in this formula. (Do not read the “Cl2” part of the formula together a molecule the the diatomic element chlorine. Chlorine does not exist as a diatomic aspect in this compound. Rather, it exist as two individual chloride ions.) by convention, the lowest entirety number ratio is offered in the recipe of ionic compounds. The formula Mg2Cl4 has balanced charges v the ion in a 1:2 ratio, but it is no the lowest entirety number ratio.





Polyatomic Ions

Some ion consist of groups of atoms bonded together and have an all at once electric charge. Since these ion contain an ext than one atom, lock are called polyatomic ions. Polyatomic ions have characteristic formulas, names, and charges that should be memorized. For example, NO3− is the nitrate ion; it has actually one nitrogen atom and also three oxygen atoms and an overall 1− charge. Table \(\PageIndex1\) perform the most usual polyatomic ions.


Table \(\PageIndex1\): part Biologically crucial Polyatomic IonsNameFormula
ammonium ionNH4+
acetate ionC2H3O2−
carbonate ionCO32−
hydrogen lead carbonate ion (bicarbonate ion)HCO3−
cyanide ionCN−
hydroxide ionOH−
phosphate ionPO43−
hydrogen phosphate ionHPO42−
dihydrogen phosphate ionH2PO4−
nitrate ionNO3−
sulfite ionSO32−

The rule for creating formulas because that ionic compound containing polyatomic ion is the very same as because that formulas include monatomic (single-atom) ions: the positive and an adverse charges have to balance. If an ext 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 also the numerical subscript is inserted outside the parentheses. This is to display that the subscript uses to the whole polyatomic ion. An instance is Ba(NO3)2.



Recognizing Ionic Compounds

There space two methods to identify ionic compounds. First, compounds between metal and also nonmetal aspects are commonly ionic. Because that example, CaBr2 contains a metallic aspect (calcium, a group 2A metal) and also a nonmetallic element (bromine, a group 7A nonmetal). Therefore, the is most likely an ionic compound. (In fact, it is ionic.) In contrast, the link NO2 includes two facets that room both nonmetals (nitrogen, from group 5A, and oxygen, from group 6A). It is no an ionic compound; that belongs to the category of covalent compounds talk about elsewhere. Also note that this combination of nitrogen and oxygen has no electric charge specified, so that is not the nitrite ion.

Second, if you recognize the formula of a polyatomic ion in a compound, the link is ionic. For example, if you check out the formula Ba(NO3)2, you might recognize the “NO3” part as the nitrate ion, \(\rmNO_3^−\). (Remember the the convention for creating formulas for ionic compounds is no to include the ionic charge.) This is a clue the the other component of the formula, Ba, is actually the Ba2+ ion, v the 2+ fee balancing the overall 2− charge from the two nitrate ions. Thus, this link is also ionic.

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Exercise \(\PageIndex3\)

Identify each compound as ionic or not ionic.



Looking Closer: Blood and also Seawater

Science has actually long recognized that blood and seawater have similar compositions. After ~ all, both liquids have actually ionic compounds liquified in them. The similarity might be an ext than only coincidence; countless scientists think the the very first forms that life top top Earth arose in the oceans. A closer look, however, mirrors that blood and seawater are rather different. A 0.9% solution of salt chloride almost right the salt concentration found in blood. In contrast, seawater is principally a 3% salt chloride solution, over three times the concentration in blood. Below is a comparison of the amounts of ion in blood and seawater:


IonPercent in SeawaterPercent in Blood
Na+2.360.322
Cl−1.940.366
Mg2+0.130.002
SO42−0.09
K+0.040.016
Ca2+0.040.0096
HCO3−0.0020.165
HPO42−, H2PO4−0.01




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