Why Ionic Compounds Are Nonconductive Solids and Their Properties

Ionic Compounds and Electrical Conductivity

Why ionic compounds do not conduct electricity in the solid state

Explain why ionic compounds do not conduct electricity in their crystalline form. Electricity can only be conducted when ions are moving. In their crystalline form, the ions in the ionic compound are locked tightly in one place.

When ionic compounds are in a solid, crystalline lattice, cations and anions occupy fixed positions in the lattice. Because electric current in ionic materials is carried by mobile charged particles, the fixed ions cannot move and therefore the solid does not conduct electricity. By contrast, when the lattice is melted or dissolved in a polar solvent such as water, ions become free to move and the substance conducts electricity.

Why metals and nonmetals typically form ionic compounds

Why do metals and nonmetals usually form ionic compounds, whereas two bonded nonmetals are never ionic? The difference in electronegativity between metals and nonmetals is high, meaning that it’s very easy for electronegative nonmetals to take electrons from non-electronegative nonmetals. If both elements were metals, or if both were nonmetals, the electronegativities would be too similar for one element to take electrons from the other.

In practice, a large difference in electronegativity between two atoms favors complete electron transfer and formation of ions (ionic bonding). Metals, which have low ionization energies, tend to lose electrons, while nonmetals, with high electron affinity and electronegativity, tend to gain electrons. Two nonmetals typically share electrons (covalent bonding) because their electronegativities are more similar; this results in molecular or covalent solids rather than ionic lattices.

Why ionic compound formation is exothermic

Why is the formation of ionic compounds exothermic? The interaction of so many positive ions with negative ones gives ionic compounds considerable stability over their constituent elements. Because stable = lower in energy, this extra energy is given off as heat during the formation of ionic compounds.

The exothermic nature arises primarily from the large electrostatic stabilization when oppositely charged ions assemble into a lattice (lattice energy). This release of energy offsets the energy required to ionize atoms and to separate electrons, resulting in a net release of heat when the ionic solid forms.

Why ionic compounds are typically hard

Why do ionic compounds tend to be hard? The cations and anions are locked tightly into place because of their opposite charges – as a result, it’s difficult to move the ions and the material is very hard.

Strong electrostatic attractions between a large number of neighboring ions give ionic solids high hardness and high melting points. However, because the lattice is rigid, applying sufficient shear or impact can cause like-charged ions to be forced adjacent to each other, resulting in repulsion and cleavage; this is why many ionic solids are also brittle.

Assessing whether specific compounds are likely ionic

Describe whether the following compounds are likely to be ionic or not ionic based on the properties given. Explain your reasoning.

• Compound 1

  Compound 1 has a melting point of 545 degrees Celsius and dissolves well in water. The high melting point points toward an ionic compound, and the fact that it dissolves well in water also supports that it is ionic.

  Reasoning: A melting point as high as 545 °C is characteristic of ionic lattices because of strong electrostatic attractions. Good solubility in polar solvents such as water further supports an ionic identity, since water stabilizes separated ions.

• Compound 2

  Compound 2 is a brittle material that is used to melt road ice during storms. This is ionic. The brittleness points toward this, and the application for melting road ice also suggests an ionic salt (for example sodium chloride or calcium chloride), because ionic salts dissolve in water and lower its freezing point.

  Reasoning: Brittleness is consistent with an ionic crystal. The use as a deicing agent is further evidence, since many common deicers are ionic salts that dissolve and depress the freezing point of water.

• Compound 3

  Compound 3 melts at 85 degrees Celsius and catches fire when heated to 570 degrees Celsius. This is a little ambiguous. Ionic compounds tend to have very high melting points, but in the case of organic solids this isn’t necessarily true. The catching fire part, however, gives more evidence to this not being ionic.

  Reasoning: A relatively low melting point around 85 °C and flammability at high temperature are more characteristic of organic, covalent materials than of typical ionic salts. While some molecular solids can have moderate melting points, the combustibility strongly suggests covalent bonding rather than an ionic lattice.

Summary

In summary, ionic solids do not conduct electricity in the crystalline state because their ions are fixed in a lattice. Ionic bonding is favored when there is a large electronegativity difference between elements (commonly between metals and nonmetals). Formation of ionic compounds is generally exothermic due to lattice stabilization, and ionic solids are usually hard and brittle due to strong electrostatic forces. Observables such as high melting points, brittleness, water solubility, and use as deicers all point toward ionic character, while low melting points and flammability suggest covalent (nonionic) materials.