To describe the characteristics of ionic bonding. To quantitatively describe the energetic factors involved in the formation of an ionic bond.

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Ions are atoms or molecules which are electrically charged. Cations are positively charged and anions carry a negative charge. Ions form when atoms gain or lose electrons. Since electrons are negatively charged, an atom that loses one or more electrons will become positively charged; an atom that gains one or more electrons becomes negatively charged. Ionic bonding is the attraction between positively- and negatively-charged ions. These oppositely charged ions attract each other to form ionic networks (or lattices). Electrostatics explains why this happens: opposite charges attract and like charges repel. When many ions attract each other, they form large, ordered, crystal lattices in which each ion is surrounded by ions of the opposite charge. Generally, when metals react with non-metals, electrons are transferred from the metals to the non-metals. The metals form positively-charged ions and the non-metals form negatively-charged ions.


Generating Ionic Bonds

Ionic bonds form when metals and non-metals sdrta.netically react. By definition, a metal is relatively stable if it loses electrons to form a complete valence shell and becomes positively charged. Likewise, a non-metal becomes stable by gaining electrons to complete its valence shell and become negatively charged. When metals and non-metals react, the metals lose electrons by transferring them to the non-metals, which gain them. Consequently, ions are formed, which instantly attract each other—ionic bonding.

In the overall ionic compound, positive and negative charges must be balanced, because electrons cannot be created or destroyed, only transferred. Thus, the total number of electrons lost by the cationic species must equal the total number of electrons gained by the anionic species.


Example (PageIndex1): Sodium Chloride

For example, in the reaction of Na (sodium) and Cl (chlorine), each Cl atom takes one electron from a Na atom. Therefore each Na becomes a Na+ cation and each Cl atom becomes a Cl- anion. Due to their opposite charges, they attract each other to form an ionic lattice. The formula (ratio of positive to negative ions) in the lattice is (ceNaCl).

These ions are arranged in solid NaCl in a regular three-dimensional arrangement (or lattice):

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NaCl lattice. (left) 3-D structure and (right) simple 2D slice through lattes. Images used with permission from Wikipedia and Mike Blaber.

The chlorine has a high affinity for electrons, and the sodium has a low ionization energy. Thus the chlorine gains an electron from the sodium atom. This can be represented using ewis dot symbols (here we will consider one chlorine atom, rather than Cl2):

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, the energy of the electrostatic attraction ((E)) between two charged particles is proportional to the magnitude of the charges and inversely proportional to the internuclear distance between the particles ((r)):

< E = kdfracQ_1Q_2r labelEq1b >

where each ion’s charge is represented by the symbol Q. The proportionality constant k is equal to 2.31 × 10−28 J·m. This value of k includes the charge of a single electron (1.6022 × 10−19 C) for each ion. The equation can also be written using the charge of each ion, expressed in coulombs (C), incorporated in the constant. In this case, the proportionality constant, k, equals 8.999 × 109 J·m/C2. In the example given, Q1 = +1(1.6022 × 10−19 C) and Q2 = −1(1.6022 × 10−19 C). If Q1 and Q2 have opposite signs (as in NaCl, for example, where Q1 is +1 for Na+ and Q2 is −1 for Cl−), then E is negative, which means that energy is released when oppositely charged ions are brought together from an infinite distance to form an isolated ion pair.

Energy is always released when a bond is formed and correspondingly, it always requires energy to break a bond.

As shown by the green curve in the lower half of Figure (PageIndex1), the maximum energy would be released when the ions are infinitely close to each other, at r = 0. Because ions occupy space and have a structure with the positive nucleus being surrounded by electrons, however, they cannot be infinitely close together. At very short distances, repulsive electron–electron interactions between electrons on adjacent ions become stronger than the attractive interactions between ions with opposite charges, as shown by the red curve in the upper half of Figure (PageIndex1). The total energy of the system is a balance between the attractive and repulsive interactions. The purple curve in Figure (PageIndex1) shows that the total energy of the system reaches a minimum at r0, the point where the electrostatic repulsions and attractions are exactly balanced. This distance is the same as the experimentally measured bond distance.

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Figure (PageIndex1): A Plot of Potential Energy versus Internuclear Distance for the Interaction between a Gaseous Na+ Ion and a Gaseous Cl− Ion. The energy of the system reaches a minimum at a particular distance (r0) when the attractive and repulsive interactions are balanced.

Consider the energy released when a gaseous (Na^+) ion and a gaseous (Cl^-) ion are brought together from r = ∞ to r = r0. Given that the observed gas-phase internuclear distance is 236 pm, the energy change associated with the formation of an ion pair from an (Na^+_(g)) ion and a (Cl^-_(g)) ion is as follows:

< eginalign* E &= kdfracQ_1Q_2r_0 \<4pt> &= (2.31 imes 10^ - 28 mJcdot cancelm ) left( dfrac( + 1)( - 1)236; cancelpm imes 10^ - 12 cancelm/pm ight) \<4pt> &= - 9.79 imes 10^ - 19; J/ion; pair labelEq2 endalign*>

The negative value indicates that energy is released. Our convention is that if a sdrta.netical process provides energy to the outside world, the energy change is negative. If it requires energy, the energy change is positive. To calculate the energy change in the formation of a mole of NaCl pairs, we need to multiply the energy per ion pair by Avogadro’s number:

< E=left ( -9.79 imes 10^ - 19; J/ cancelion pair ight )left ( 6.022 imes 10^ 23; cancelion; pair/mol ight )=-589; kJ/mol labelEq3 >

This is the energy released when 1 mol of gaseous ion pairs is formed, not when 1 mol of positive and negative ions condenses to form a crystalline lattice. Because of long-range interactions in the lattice structure, this energy does not correspond directly to the lattice energy of the crystalline solid. However, the large negative value indicates that bringing positive and negative ions together is energetically very favorable, whether an ion pair or a crystalline lattice is formed.

We summarize the important points about ionic bonding:

At r0, the ions are more stable (have a lower potential energy) than they are at an infinite internuclear distance. When oppositely charged ions are brought together from r = ∞ to r = r0, the energy of the system is lowered (energy is released). Because of the low potential energy at r0, energy must be added to the system to separate the ions. The amount of energy needed is the bond energy. The energy of the system reaches a minimum at a particular internuclear distance (the bond distance).

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Exercise (PageIndex2): Magnesium oxide

Calculate the amount of energy released when 1 mol of gaseous (ceMgO) ion pairs is formed from the separated ions. The internuclear distance in the gas phase is 175 pm.