Write the urbanbreathnyc.comical formula for a straightforward ionic compound.Recognize polyatomic ions in urbanbreathnyc.comical formulas.

You are watching: What is the formula of the ionic compound formed from magnesium, mg, and chlorine, cl?


We have already encountered some urbanbreathnyc.comical formulas for easy ionic compounds. A urbanbreathnyc.comical formula is a concise list of the facets in a compound and the ratios of these aspects. To much better understand also what a urbanbreathnyc.comical formula means, we need to consider exactly how an ionic compound is constructed from its ions.

Ionic compounds exist as alternating positive and negative ions in continuous, three-dimensional arrays referred to as crystals (Figure (PageIndex1)). As you can watch, tright here are no individual NaCl “particles” in the array; instead, tright here is a constant lattice of alternating sodium and also chloride ions. However before, we can usage the ratio of sodium ions to chloride ions, expressed in the lowest feasible entirety numbers, as a method of describing the compound. In the situation of sodium chloride, the ratio of sodium ions to chloride ions, expressed in lowest totality numbers, is 1:1, so we usage NaCl (one Na symbol and one Cl symbol) to reexisting the compound. Thus, NaCl is the urbanbreathnyc.comical formula for sodium chloride, which is a concise means of describing the loved one number of different ions in the compound. A macroscopic sample is written of myriads of NaCl pairs; each pair dubbed a formula unit. Although it is convenient to think that NaCl crystals are written of individual NaCl units, Figure (PageIndex1) shows that no single ion is specifically linked via any type of various other single ion. Each ion is surrounded by ions of opposite charge.


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Figure (PageIndex1) A Sodium Chloride Crystal. A crystal has a three-dimensional selection of alternating positive and negative ions. The exact pattern depends on the compound. A crystal of sodium chloride, presented below, is a arsenal of alternating sodium and also chlorine ions.


The formula for an ionic compound complies with numerous conventions. First, the cation is created prior to the anion. Due to the fact that the majority of steels create cations and also many nonmetals develop anions, formulas frequently list the metal first and also then the nonmetal. Second, charges are not written in a formula. Remember that in an ionic compound, the component species are ions, not neutral atoms, even though the formula does not contain charges. Finally, the proper formula for an ionic compound constantly obeys the adhering to rule: the complete positive charge should equal the full negative charge. To recognize the correct formula of any kind of combination of ions, determine how many kind of of each ion is needed to balance the complete positive and negative charges in the compound.

This preeminence is inevitably based upon the reality that matter is, in its entirety, electrically neutral.


By convention, assume that tright here is just one atom if a submanuscript is not existing. We execute not usage 1 as a submanuscript.


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

When an ionic compound 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 of these ions have greater charges than the ions in lithium bromide, they still balance each various other in a one-to-one ratio. Because of this, the appropriate formula for this ionic compound is MgO.

Now think about the ionic compound created by magnesium and chlorine. A magnesium ion has a 2+ charge, while a chlorine ion has actually a 1− charge:

Mg2+ Cl−

Combining one ion of each does not completely balance the positive and negative charges. The easiest means to balance these charges is to assume the visibility of two chloride ions for each magnesium ion:

Mg2+ Cl− Cl−

Now the positive and negative charges are well balanced. We might write the urbanbreathnyc.comical formula for this ionic compound as MgClCl, but the convention is to usage a numerical submanuscript when tright here is more than one ion of a offered type—MgCl2. This urbanbreathnyc.comical formula states that tright here are one magnesium ion and two chloride ions in this formula. (Do not read the “Cl2” part of the formula as a molecule of the diatomic elemental chlorine. Chlorine does not exist as a diatomic element in this compound. Rather, it exists as two individual chloride ions.) By convention, the lowest totality number ratio is used in the formulas of ionic compounds. The formula Mg2Cl4 has balanced charges through the ions in a 1:2 ratio, however it is not the lowest totality number proportion.





Polyatomic Ions

Some ions consist of groups of atoms bonded together and have actually an in its entirety electrical charge. Since these ions contain even more than one atom, they are dubbed polyatomic ions. Polyatomic ions have characteristic formulas, names, and charges that must be memorized. For example, NO3− is the nitrate ion; it has actually one nitrogen atom and three oxygen atoms and an overall 1− charge. Table (PageIndex1) lists the the majority of prevalent polyatomic ions.


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

The preeminence for creating formulas for ionic compounds containing polyatomic ions is the same as for formulas containing monatomic (single-atom) ions: the positive and negative charges should balance. If more than one of a certain 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 put outside the parentheses. This is to show that the submanuscript applies to the whole polyatomic ion. An instance is Ba(NO3)2.



Recognizing Ionic Compounds

There are 2 ways to acknowledge ionic compounds. First, compounds in between metal and also nonmetal aspects are generally ionic. For example, CaBr2 includes a metallic aspect (calcium, a group 2A metal) and a nonmetallic aspect (bromine, a group 7A nonmetal). Because of this, it is most most likely an ionic compound. (In fact, it is ionic.) In contrast, the compound NO2 consists of 2 aspects that are both nonsteels (nitrogen, from group 5A, and oxygen, from team 6A). It is not an ionic compound; it belongs to the category of covalent compounds talk about somewhere else. Also note that this combicountry of nitrogen and oxygen has no electrical charge specified, so it is not the nitrite ion.

Second, if you acknowledge the formula of a polyatomic ion in a compound, the compound is ionic. For instance, if you check out the formula Ba(NO3)2, you might identify the “NO3” component as the nitprice ion, ( mNO_3^−). (Remember that the convention for writing formulas for ionic compounds is not to incorporate the ionic charge.) This is a clue that the various other component of the formula, Ba, is actually the Ba2+ ion, through the 2+ charge balancing the overall 2− charge from the two nitrate ions. Hence, this compound is additionally ionic.

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Exercise (PageIndex3)

Identify each compound as ionic or not ionic.



Looking Closer: Blood and Seawater

Science has actually long known that blood and also seawater have similar compositions. After all, both liquids have ionic compounds dissolved in them. The similarity might be even more than mere coincidence; many researchers think that the initially creates of life on Planet emerged in the seas. A closer look, however, mirrors that blood and also seawater are quite different. A 0.9% solution of sodium chloride approximates the salt concentration discovered in blood. In contrast, seawater is principally a 3% sodium chloride solution, over three times the concentration in blood. Here is a compariboy of the quantities of ions in blood and also 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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