As all substances must be electrically neutral, the total number of positive charges on the cations of an ionic compound must equal the total number of negative charges on its anions. The formula of an ionic compound represents the simplest ratio of the numbers of ions necessary to give identical numbers of positive and negative charges.
It is important to note, however, that the formula for an ionic compound does not represent the physical arrangement of its ions. The attractive forces between ions are isotropic—the same in all directions—meaning that any particular ion is equally attracted to all of the nearby ions of opposite charge.
This results in the ions arranging themselves into a tightly bound, three-dimensional lattice structure. The smaller spheres represent sodium ions, the larger ones represent chloride ions. In the expanded view b , the geometry can be seen more clearly.
An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a compound is a measure of the strength of this attraction.
For the ionic solid MX, the lattice energy is the enthalpy change of the process:. Note that we are using the convention where the ionic solid is separated into ions, so our lattice energies will be endothermic positive values. Some texts use the equivalent but opposite convention, defining lattice energy as the energy released when separate ions combine to form a lattice and giving negative exothermic values. Thus, if you are looking up lattice energies in another reference, be certain to check which definition is being used.
In both cases, a larger magnitude for lattice energy indicates a more stable ionic compound. Thus, the lattice energy of an ionic crystal increases rapidly as the charges of the ions increase and the sizes of the ions decrease.
When all other parameters are kept constant, doubling the charge of both the cation and anion quadruples the lattice energy. Different interatomic distances produce different lattice energies. The compound Al 2 Se 3 is used in the fabrication of some semiconductor devices. Which has the larger lattice energy, Al 2 O 3 or Al 2 Se 3?
The O 2— ion is smaller than the Se 2— ion. Thus, Al 2 O 3 would have a shorter interionic distance than Al 2 Se 3 , and Al 2 O 3 would have the larger lattice energy. Zinc oxide, ZnO, is a very effective sunscreen. How would the lattice energy of ZnO compare to that of NaCl? ZnO would have the larger lattice energy because the Z values of both the cation and the anion in ZnO are greater, and the interionic distance of ZnO is smaller than that of NaCl.
It is not possible to measure lattice energies directly. However, the lattice energy can be calculated using the equation given in the previous section or by using a thermochemical cycle. Because the cation and the anion in BaS are both larger than the corresponding ions in CaO, the internuclear distance is greater in BaS and its lattice energy will be lower than that of CaO.
The magnitude of the forces that hold an ionic substance together has a dramatic effect on many of its properties. The melting point is the temperature at which the individual ions in a lattice or the individual molecules in a covalent compound have enough kinetic energy to overcome the attractive forces that hold them together in the solid. At the melting point, the ions can move freely, and the substance becomes a liquid. Thus melting points vary with lattice energies for ionic substances that have similar structures.
The melting points of the sodium halides Figure 4. In fact, because of its high melting point, MgO is used as an electrical insulator in heating elements for electric stoves. The melting points follow the same trend as the magnitude of the lattice energies in Figure 4.
The hardness s t he resistance of ionic materials to scratching or abrasion. Hardness is directly related to how tightly the ions are held together electrostatically, which, as we saw, is also reflected in the lattice energy.
As an example, MgO is harder than NaF, which is consistent with its higher lattice energy. In addition to determining melting point and hardness, lattice energies affect the solubilities of ionic substances in water. In general, the higher the lattice energy, the less soluble a compound is in water. High lattice energies lead to hard, insoluble compounds with high melting points. Ionic compounds have strong electrostatic attractions between oppositely charged ions in a regular array.
The lattice energy U of an ionic substance is defined as the energy required to dissociate the solid into gaseous ions; U can be calculated from the charges on the ions, the arrangement of the ions in the solid, and the internuclear distance. Higher lattice energies typically result in higher melting points and increased hardness because more thermal energy is needed to overcome the forces that hold the ions together. If a great deal of energy is required to form gaseous ions, why do ionic compounds form at all?
What are the general physical characteristics of ionic compounds? Ionic compounds consist of crystalline lattices rather than discrete ion pairs. What factors affect the magnitude of the lattice energy of an ionic compound? What is the relationship between ionic size and lattice energy?
Based on ion sizes, arrange these compounds by their expected lattice energy. Note that many sources define lattice energies as negative values. Please arrange by magnitude and ignore the sign. Lattice Energy, as the problem states, is affected by the internuclear distance between ions. However, the problem does not tell us whether increasing the distance between ions causes an increase or a decrease in lattice energy, so we'll have to figure that out ourselves.
Lattice energy is the result of electrostatic attraction between ions, and because these attractive forces decrease with increasing distance between ions, the magnitude of the lattice energy decreases with increasing distance between ions.
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