what kinds of cations and anions always combine in a one to one ratio

3.2: Formulas for Ionic Compounds

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Skills to Develop

• Write the chemical formula for a simple ionic chemical compound.
• Recognize polyatomic ions in chemic formulas.

We take already encountered some chemical formulas for simple ionic compounds. A chemical formula is a curtailed list of the elements in a compound and the ratios of these elements. To amend sympathize what a chemic formula ways, we must consider how an ionic compound is constructed from its ions.

Ionic compounds exist every bit alternating positive and negative ions in regular, three-dimensional arrays called crystals (Effigy \(\PageIndex{one}\)). Equally yous can run into, there are no individual NaCl "particles" in the array; instead, there is a continuous lattice of alternating sodium and chloride ions. All the same, nosotros can utilise the ratio of sodium ions to chloride ions, expressed in the lowest possible whole numbers, equally a manner of describing the compound. In the example of sodium chloride, the ratio of sodium ions to chloride ions, expressed in lowest whole numbers, is ane:1, so we use NaCl (1 Na symbol and 1 Cl symbol) to represent the compound. Thus, NaCl is the chemical formula for sodium chloride, which is a concise mode of describing the relative number of different ions in the compound. A macroscopic sample is composed of myriads of NaCl pairs; each pair chosen a formula unit. Although it is convenient to think that NaCl crystals are equanimous of individual NaCl units, Figure \(\PageIndex{1}\) shows that no single ion is exclusively associated with any other single ion. Each ion is surrounded by ions of reverse charge.

Figure \(\PageIndex{1}\) A Sodium Chloride Crystal. A crystal contains a iii-dimensional array of alternating positive and negative ions. The precise pattern depends on the compound. A crystal of sodium chloride, shown here, is a collection of alternating sodium and chlorine ions.

Note

In Section 3.2, we encountered LiBr and MgO, which are formulas for other ionic compounds.

The formula for an ionic compound follows several conventions. First, the cation is written earlier the anion. Because almost metals course cations and nigh nonmetals course anions, formulas typically list the metallic first and then the nonmetal. Second, charges are non written in a formula. Think that in an ionic chemical 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 always obeys the following rule: the total positive charge must equal the full negative accuse. To determine the proper formula of whatever combination of ions, determine how many of each ion is needed to balance the total positive and negative charges in the chemical compound.

Note

This rule is ultimately based on the fact that matter is, overall, electrically neutral.

Notation

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

If we look at the ionic compound consisting of lithium ions and bromide ions, we see that the lithium ion has a i+ charge and the bromide ion has a 1− charge. But 1 ion of each is needed to balance these charges. The formula for lithium bromide is LiBr.

When an ionic compound is formed from magnesium and oxygen, the magnesium ion has a 2+ charge, and the oxygen atom has a 2− charge. Although both of these ions have higher charges than the ions in lithium bromide, they still balance each other in a one-to-one ratio. Therefore, the proper formula for this ionic compound is MgO.

At present consider the ionic compound formed past magnesium and chlorine. A magnesium ion has a two+ charge, while a chlorine ion has a 1− charge:

Mg^{2 +} Cl⁻

Combining one ion of each does non completely residue the positive and negative charges. The easiest way to balance these charges is to presume the presence of two chloride ions for each magnesium ion:

Mg^{2 +} Cl⁻ Cl⁻

Now the positive and negative charges are balanced. We could write the chemical formula for this ionic compound as MgClCl, but the convention is to use a numerical subscript when there is more than i ion of a given type—MgCl₂. This chemical formula says that there are one magnesium ion and 2 chloride ions in this formula. (Do non read the "Cl₂" office 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 2 individual chloride ions.) By convention, the lowest whole number ratio is used in the formulas of ionic compounds. The formula Mg₂Cl₄ has balanced charges with the ions in a 1:2 ratio, simply information technology is not the lowest whole number ratio.

Note

By convention, the lowest whole-number ratio of the ions is used in ionic formulas. In that location are exceptions for certain ions, such every bit Hg₂ ²⁺

Example \(\PageIndex{1}\)

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

1. the sodium ion and the sulfur ion
2. the aluminum ion and the fluoride ion
3. the three+ iron ion and the oxygen ion

SOLUTION

1. To obtain a valence beat octet, sodium forms an ion with a 1+ charge, while the sulfur ion has a 2− charge. 2 sodium 1+ ions are needed to balance the two− charge on the sulfur ion. Rather than writing the formula every bit NaNaS, we shorten it by convention to Na₂S.
2. The aluminum ion has a 3+ charge, while the fluoride ion formed by fluorine has a 1− charge. Three fluorine 1− ions are needed to balance the three+ accuse on the aluminum ion. This combination is written as AlF₃.
3. Iron can course two possible ions, simply the ion with a three+ charge is specified here. The oxygen atom has a 2− charge equally an ion. To balance the positive and negative charges, we look to the least common multiple—six: 2 fe 3+ ions will give half-dozen+, while three two− oxygen ions volition give half dozen−, thereby balancing the overall positive and negative charges. Thus, the formula for this ionic compound is Fe₂O₃.

Do \(\PageIndex{1}\)

Write the chemic formula for an ionic compound equanimous of each pair of ions.

1. the calcium ion and the oxygen ion
2. the 2+ copper ion and the sulfur ion
3. the 1+ copper ion and the sulfur ion

Polyatomic Ions

Some ions consist of groups of atoms bonded together and accept an overall electric charge. Because these ions contain more than than 1 atom, they are chosen polyatomic ions. Polyatomic ions have characteristic formulas, names, and charges that should be memorized. For example, NO_three ⁻ is the nitrate ion; it has ane nitrogen atom and three oxygen atoms and an overall 1− charge. Tabular array \(\PageIndex{i}\) lists the near mutual polyatomic ions.

Table \(\PageIndex{one}\): Some Polyatomic Ions

Name	Formula
ammonium ion	NH4 +
acetate ion	C2H3O2 − (also written CH3CO2 −)
carbonate ion	CO3 2−
chromate ion	CrO4 ii−
dichromate ion	CrtwoO7 2−
hydrogen carbonate ion (bicarbonate ion)	HCO3 −
cyanide ion	CN−
hydroxide ion	OH−
nitrate ion	NO3 −
nitrite ion	NOtwo −
permanganate ion	MnO4 −
phosphate ion	PO4 3−
hydrogen phosphate ion	HPO4 2−
dihydrogen phosphate ion	H2PO4 −
sulfate ion	And soiv 2−
hydrogen sulfate ion (bisulfate ion)	HSO4 −
sulfite ion	Thenthree two−

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 negative charges must remainder. If more than 1 of a particular polyatomic ion is needed to balance the charge, the unabridged formula for the polyatomic ion must exist enclosed in parentheses, and the numerical subscript is placed outside the parentheses. This is to show that the subscript applies to the entire polyatomic ion. An example is Ba(NO₃)₂.

Example \(\PageIndex{2}\)

Write the chemical formula for an ionic compound equanimous of each pair of ions.

1. the potassium ion and the sulfate ion
2. the calcium ion and the nitrate ion

SOLUTION

1. Potassium ions have a charge of 1+, while sulfate ions accept a charge of two−. We volition demand 2 potassium ions to balance the accuse on the sulfate ion, and then the proper chemical formula is K₂Then_iv.
2. Calcium ions have a accuse of 2+, while nitrate ions take a charge of 1−. Nosotros will need ii nitrate ions to balance the charge on each calcium ion. The formula for nitrate must exist enclosed in parentheses. Thus, nosotros write Ca(NO_three)₂ equally the formula for this ionic compound.

Do \(\PageIndex{2}\)

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

1. the magnesium ion and the carbonate ion
2. the aluminum ion and the acetate ion

Recognizing Ionic Compounds

There are two ways to recognize ionic compounds. First, compounds between metal and nonmetal elements are commonly ionic. For example, CaBr₂ contains a metallic chemical element (calcium, a grouping 2A metal) and a nonmetallic chemical element (bromine, a group 7A nonmetal). Therefore, it is well-nigh likely an ionic compound. (In fact, it is ionic.) In dissimilarity, the compound NO₂ contains 2 elements that are both nonmetals (nitrogen, from group 5A, and oxygen, from group 6A). Information technology is not an ionic chemical compound; it belongs to the category of covalent compounds talk over elsewhere. As well note that this combination of nitrogen and oxygen has no electric charge specified, so it is not the nitrite ion.

Second, if yous recognize the formula of a polyatomic ion in a compound, the chemical compound is ionic. For instance, if you lot see the formula Ba(NO_iii)_ii, you lot may recognize the "NO_iii" part as the nitrate ion, \(\rm{NO_3^−}\). (Call up that the convention for writing formulas for ionic compounds is not to include the ionic accuse.) This is a clue that the other function of the formula, Ba, is actually the Baⁱⁱ⁺ ion, with the 2+ charge balancing the overall two− accuse from the two nitrate ions. Thus, this chemical compound is also ionic.

Example \(\PageIndex{3}\)

Identify each compound every bit ionic or not ionic.

1. Na_twoO
2. PCl_iii
3. NH₄Cl
4. OF₂

SOLUTION

1. Sodium is a metal, and oxygen is a nonmetal; therefore, Na₂O is expected to exist ionic.
2. Both phosphorus and chlorine are nonmetals. Therefore, PCl_iii is not ionic.
3. The NH₄ in the formula represents the ammonium ion, NH₄ ⁺, which indicates that this compound is ionic.
4. Both oxygen and fluorine are nonmetals. Therefore, OF₂ is non ionic.

Practise \(\PageIndex{3}\)

Place each chemical compound as ionic or not ionic.

1. N₂O
2. FeCl₃
3. (NH_iv)_threePO₄
4. SOCl_two

Looking Closer: Blood and Seawater

Scientific discipline has long recognized that blood and seawater accept similar compositions. After all, both liquids accept ionic compounds dissolved in them. The similarity may be more than than mere coincidence; many scientists call back that the first forms of life on Earth arose in the oceans.

A closer look, however, shows that blood and seawater are quite different. A 0.ix% solution of sodium chloride approximates the salt concentration found in blood. In dissimilarity, seawater is principally a iii% sodium chloride solution, over three times the concentration in blood. Here is a comparison of the amounts of ions in claret and seawater:

Ion	Percent in Seawater	Per centum in Blood
Na+	two.36	0.322
Cl−	1.94	0.366
Mg2 +	0.thirteen	0.002
SO4 2−	0.09	—
K+	0.04	0.016
Ca2 +	0.04	0.0096
HCO3 −	0.002	0.165
HPO4 ii−, H2PO4 −	—	0.01

Most ions are more abundant in seawater than they are in blood, with some important exceptions. At that place are far more hydrogen carbonate ions (HCO₃ ⁻) in blood than in seawater. This difference is significant because the hydrogen carbonate ion and some related ions have a crucial role in decision-making the acid-base properties of blood. The corporeality of hydrogen phosphate ions—HPO₄ ²⁻ and H_twoPO₄ ⁻—in seawater is very low, but they are present in higher amounts in claret, where they as well impact acid-base properties. Some other notable departure is that blood does not accept significant amounts of the sulfate ion (And so₄ ²⁻), but this ion is present in seawater.

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Source: https://chem.libretexts.org/Courses/Valley_City_State_University/Chem_115/Chapter_3:_Chemical_Bonding/3.2:_Formulas_for_Ionic_Compounds

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