1.6: Ions

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Figure 1-5 Common table salt (sodium chloride. NaCl) is built from closely packed sodium ions, Na⁺ (small spheres). and chloride ions. CI^- (large. colored spheres). Each ion of one charge is surrounded by six ions of the opposite charge at the four compass points and above and below. This is a particularly stable arrangement of charges. and it occurs in many salts. From Dickerson and Geis. Chemistry, Matter. and the Universe The Benjamin / Cummings Publishing Co .. Menlo Park. Ca .. © 1976 .

The idea of a covalent bond suggests equal sharing of the electron pair by the bonded atoms, but the brief discussion of polarity in Section 1-4 indicated that the sharing is not always equal. The relative electronegativity or electron-attracting power of atoms is of great importance in explaining chemical behavior, and is treated in detail in Chapters 9 and 10. Sodium atoms (and all metals in general) have a weak hold on electrons, whereas chlorine atoms are very electronegative. Hence in common table salt (sodium chloride, NaCl), each sodium atom, Na, loses one electron (e^-) to form a sodium ion, Na⁺. Each chlorine atom picks up one electron to become a chloride ion, Cl^-:

Na → Na⁺ + e^-wordandword

\textstyle{\frac{1}{2}}

Cl₂ + e^- → Cl^-

We write $\textstyle{\frac{1}{2}}$ Cl₂ because free chlorine gas exists as diatomic (two-atom) molecules, not as free chlorine atoms. Solid sodium chloride (Figure 1-5) has sodium and chloride ions packed into a three-dimensional lattice in such a way that each positive Na⁺ ion is surrounded on four sides and top and bottom by negative Cl^- ions, and each Cl^- is similarly surrounded by six nearest neighbor Na⁺ ions. This is a particularly stable arrangement of positive and negative charges.

Metals in general lose one to three electrons easily to become positively charged ions, or cations:

Li	→	Li⁺	+	e^-	ttt	lithium ion
Na	→	Na⁺	+	e^-	ttt	sodium ion
K	→	K⁺	+	e^-	ttt	potassium ion
Mg	→	Mg²⁺	+	2e^-	ttt	magnesium ion
Ca	→	Ca²⁺	+	2e^-	ttt	calcium ion
Al	→	Al³⁺	+	3e^-	ttt	aluminum ion

Some nonmetals, in contrast, pick up electrons to become negatively charged ions, or anions:

$\textstyle{\frac{1}{2}}$ F₂	+	e^-	→	F^-	ttt	fluoride ion
$\textstyle{\frac{1}{2}}$ Cl₂	+	e^-	→	Cl^-	ttt	chloride ion
$\textstyle{\frac{1}{2}}$ O₂	+	2e^-	→	O^2-	ttt	oxide ion
$\textstyle{\frac{1}{2}}$ S₂	+	2e^-	→	S^2-	ttt	sulfide ion

Table 1-4 Some Simple Ions of Elements

Chemical Principles, Table 1.4.png

Other simple ions made from single atoms are shown in Table 1-4. The charge on a simple, single-atom ion such as AP⁺ or S^2- is its oxidation state or oxidation number. It is the number of electrons that must be added to reduce (or removed to oxidize) the ion to the neutral species:

Reduction: AI³⁺ + 3^e- → Al

Oxidation: S^2- → S + 2^e-

Pulling electrons away from an atom or removing them altogether is oxidation. Adding electrons to an atom or merely shifting them toward it is reduction.

**Example 12**
Is chlorine oxidized or reduced in forming the chloride ion? What is the oxidation state of the ion?
Solution Chlorine is reduced, since one electron per chlorine atom is added to form the ion. The chloride ion, Cl^- , is in the - 1 oxidation state.

**Example 13**
When metals are converted into their ions, are they oxidized or reduced? What is the oxidation state of the aluminum ion?
Solution Metals are oxidized to their ions, since electrons are removed. The aluminum ion, AP⁺, is in the +3 oxidation state.

If two or more oxidation states for a metal ion are possible, they are differentiated by writing the oxidation state in Roman numerals after the name of the atom. An older nomenclature, still in use, identifies the higher oxidation state by the ending -ic and the lower by -ous. Hence,

Fe²⁺	tt	iron(II) or ferrous	break	Fe³⁺	tt	iron(III) or ferric
Cu⁺	tt	copper(I) or cuprous	break	Cu²⁺	tt	copper(II) or cupric
Sn²⁺	tt	tin(II) or stannous	break	Sn⁴⁺	tt	tin(IV) or stannic

**Example 14**
When the ferric ion is converted to the ferrous ion, is this an oxidation or reduction? Write the equation for the process.
Solution The equation is Fe³⁺ + e^- → Fe²⁺ . The process is a reduction since an electron is added.

The modern nomenclature with Roman numerals is easier to use because it does not require you to remember what the two oxidation states of a metal are, in order to know what a compound is from its name.

A salt is a compound made up of positive and negative ions. Because a salt must be electrically neutral, the total charge on its positive and negative ions must be zero. Since each ion of Sn²⁺ has a charge of +2, twice as many chloride ions with -1 charge each are required to produce a zero net charge. Hence the salt of Sn²⁺ and Cl^- ions has the overall composition SnCl₂, rather than SnCl or SnCl₃. It is called stannous chloride or tin (II) chloride. The formula for stannic chloride or tin(IV) chloride is SnCl₄.

In addition to these simple ions, compound or complex ions can be formed between a metal or nonmetal and oxygen, chlorine, ammonia (NH₃), the hydroxide ion (OH^-), or other chemical groups. The sulfate ion, SO $\textstyle{\frac{2-}{4}}$ , has four oxygens at the corners of a tetrahedron around the central sulfur atom, and an overall charge of -2. The nitrate ion, NO $\textstyle{\frac{}{3}}$ , has three oxygen atoms in an equilateral triangle around the nitrogen, and a -1 charge. The ammonium ion, NH $\textstyle{\frac{+}{4}}$ , has four hydrogens at the corners of a tetrahedron, and a +1 charge. These ions are thought of as units because they form salts the way single-atom ions do, and go through many chemical reactions unchanged. Silver nitrate, AgN0₃, is a salt containing equal numbers of Ag⁺ and NO $\textstyle{\frac{}{3}}$ ions. Ammonium sulfate is a salt with twice as many ammonium ions, NH $\textstyle{\frac{+}{4}}$ , as sulfate ions, SO $\textstyle{\frac{2-}{4}}$ , and the chemical formula (NH₄)₂S0₄. Other typical complex ions are shown in Table 1-5.

Table 1-5 Some Common Complex Ions

Chemical Principles, Table 1.5.png

When a central atom is surrounded by several equally spaced atoms, the number of surrounding atoms is called the coordination number. The most important factor is size. Nitrogen in the nitrate ion, NO

\textstyle{\frac{ }{3}}

, has room for three oxygen atoms around it, and hence a coordination number of 3 for oxygen. The sulfur atom is larger than a nitrogen atom, and can accommodate one more oxygen atom in the sulfate ion, SO

\textstyle{\frac{2-}{4}}

. Hence the coordination number of sulfur for oxygen is 4.

The most common coordination numbers are 2, 3, 4, and 6, (See Table 1-6.) An ion or molecule with a central atom having a coordination number of 2 can be either linear, as carbon dioxide with O-C-O in a straight line, or bent, as in water, H₂0. Possible structures for ions or molecules with coordination numbers of 3, 4, and 6 are shown in Table 1-6.

Table 1-6 Common Coordination Numbers

Chemistry Principles Table 1.6.png

Figure 1-6 Geometry of atoms around central atoms with coordination numbers 3, 4, and 6. If L is any peripheral atom and M is the central atom, then the bond angle L - M - L is 120° for trigonal planar, 109.5° for tetrahedral, and typically around 109.5° for trigonal pyramidal geometries. Square planar and octahedral geometries have two L - M - L angles, 90° and 180°.

It is not strictly correct to talk about molecular formulas and molecular weights of salts, since there are no molecules in salts-only ordered lattices of ions. No one sodium ion in the sodium chloride structure shown in Figure 1-5 to a particular chloride ion. It is correct, however, to speak of the chemical formula of a salt, and the formula weight that corresponds to it. Since the chemical formula for sodium chloride is NaCl, the formula weight of sodium chloride is the sum of the atomic weights of one atom of sodium and one atom of chlorine:

1 sodium:	tt	22.990 amu
1 chlorine:	tt	35.453 amu
Total:	tt	58.443 amu

It is conventional to call this the "molecular weight" of sodium chloride, and no confusion results as long as you realize what a salt structure is like. A mole of sodium chloride is 58.443 g. It will contain 6.022 X 10²³ sodium ions and 6.022 X 10²³ chloride ions. Even though they are not paired off into molecules, the ratio is strictly one to one.

Example 15

What is the molecular weight of ammonium sulfate?

Solution

The chemical formula of ammonium sulfate is (NH₄)₂SO₄, so the molecular weight (actually the formula weight) is

2 nitrogens:	tt	2 X 14.007 amu=	tt	28.014 amu
8 hydrogens:	tt	8 X 1.008 amu=	tt	8.064 amu
1 sulfur:	tt	1 X 32.06 amu=	tt	32.06 amu
4 oxygen:	tt	4 X 15.999 amu=	tt	63.996 amu
Total:	tt		tt	132.13 amu

The simple anions are named by adding -ide to the name of the element, as in the fluoride (F^-), chloride (Cl^-), oxide (O^2-), and sulfide (S^2-) ions. Where more than one complex anion of an element with oxygen can be formed, the suffixes -ate and -ite are used for the higher and lower oxidation states, respectively. Thus,

Sulfate ion:	tt	SO $\textstyle{\frac{2-}{4}}$	word	Sulfite ion:	tt	SO $\textstyle{\frac{2-}{3}}$
Nitrate ion:	tt	NO $\textstyle{\frac{ }{3}}$	word	Nitrite ion:	tt	NO $\textstyle{\frac{ }{2}}$
Arsenate ion:	tt	AsO $\textstyle{\frac{3-}{4}}$	word	Arsenite ion:	tt	AsO $\textstyle{\frac{3-}{3}}$

If more than two such anions exist, then the prefixes hypo- ("under") and per- ("beyond") are used:

Perchlorate ion:	tt	ClO $\textstyle{\frac{ }{4}}$
Chlorate ion:	tt	ClO $\textstyle{\frac{ }{3}}$
Chlorite ion:	tt	ClO $\textstyle{\frac{ }{2}}$
Hypochlorite ion:	tt	ClO $\textstyle{\frac{ }{ }}$

Melting Points and Boiling Points of Salts

A salt crystal represents a particularly stable balance of positive and negative charges, with each type of ion being kept out of the way of others of like charge. Melting a salt crystal means upsetting this delicate balance of charges, and allowing mutually repelling ions to come closer together from time to time as the ions flow past one another. This disruption of structure requires large amounts of energy to accomplish, so the melting points of salts are higher than those of molecular solids. The melting points of two salts, sodium chloride (NaCI) and potassium sulfate (K₂SO₄), are compared in Table 1-7 with those of the elements from which the salts are made.

Table 1-7. Melting and Boiling Points of Two Salts and Their Component Elements
Substance	Chemical Formula	T_m(°C)	T_b(°C)
Sodium metal	Na	97.8	882.9
Chlorine Gas	Cl₂	-101.0	-34.6
Sodium Chloride (salt)	NaCl	801	1413
Potassium metal	K	64	774
Sulfur	S	119	445
Oxygen gas	O₂	-218	-183
Potassium sulfate (salt)	K₂SO₄	1069	1689

Metallic sodium melts at 97.8°C, and solid chlorine melts at -101°C, but their combination, sodium chloride (common table salt), requires a temperature of 801°C before it will melt. Boiling or vaporizing a salt is even more difficult. The ions remain ions in the liquid state, tumbling past one another as in any other liquid; but before the gas phase can be attained, Na⁺ and CI^- ions must pair off into neutral NaCl molecules. To accomplish this pairing, electrons have to be pulled away from CI^- ions, which have a strong attraction for them, and pushed toward Na⁺ ions, which do not want them. The NaCl bond in sodium chloride vapor is extremely polar, with the electron pair skewed strongly toward the chlorine atom, but the separation still is not as complete as in Na⁺ and CI^- ions. Much energy is required to push electrons where they are not wanted and to make NaCl molecules from Na⁺ and CI^- ions, so high temperatures are required before this can happen. Hence the very high boiling points of salts in comparison with molecular compounds, as illustrated in Table 1-7.

Contributors and Attributions

R. E. Dickerson, H. B. Gray, and G. P. Haight, Jr. Content was used from "Chemical Principles", an introductory college-level text for General Chemistry with permission of the Caltech library and Harry B. Gray, on behalf of the authors.

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