Why do ionic compounds conduct electricity and not covalent?

(a). Explain why, ionic compounds conduct electricity in solution whereas covalent compounds do not conduct electricity. <br> (b). Which of the following will conduct electricity and which not ? <br> `MgCl_(2),C Cl_(4),NaCl,CS_(2),Na_(2)S` <br> Give reason for your choice.

If you know the chemical formula of a compound, you can predict whether it contains ionic bonds, covalent bonds, or a mixture of bond types. Nonmetals bond to each other via covalent bonds while oppositely charged ions, such as metals and nonmetals, form ionic bonds. Compounds which contain polyatomic ions may have both ionic and covalent bonds.

• One way of classifying chemical compounds is by whether they contain ionic bonds or covalent bonds.
• For the most part, ionic compounds contain a metal bonded to a nonmetal. Ionic compounds form crystals, typically have high melting and boiling points, are usually hard and brittle, and form electrolytes in water.
• Most covalent compounds consist of nonmetals bonded to one another. Covalent compounds usually have lower melting and boiling points than ionic compounds, are softer, and are electrical insulators.

But, how do you know if a compound is ionic or covalent just by looking at a sample? This is where the properties of ionic and covalent compounds can be useful. Because there are exceptions, you need to look at several properties to determine whether a sample is ionic or covalent, but here are some characteristics to consider:

• Crystals: Most crystals are ionic compounds. This is because the ions in these compounds tend to stack into crystal lattices to balance between the attractive forces between opposite ions and the repulsive forces between like ions. Covalent or molecular compounds can exist as crystals, though. Examples include sugar crystals and diamond.
• Melting and boiling points: Ionic compounds tend to have higher melting and boiling points than covalent compounds.
• Mechanical properties: Ionic compounds tend to be hard and brittle while covalent compounds tend to be softer and more flexible.
• Electrical conductivity and electrolytes: Ionic compounds conduct electricity when melted or dissolved in water while covalent compounds typically don't. This is because covalent compounds dissolve into molecules while ionic compounds dissolve into ions, which can conduct charge. For example, salt (sodium chloride) conducts electricity as molten salt or in salt water. If you melt sugar (a covalent compound) or dissolve it on water, it won't conduct.

Most ionic compounds have a metal as the cation or first part of their formula, followed by one or more nonmetals as the anion or second part of their formula. Here are some examples of ionic compounds:

• Table salt or sodium chloride (NaCl)
• Sodium hydroxide (NaOH)
• Chlorine bleach or sodium hypochlorite (NaOCl)

Covalent compounds consist of nonmetals bonded to each other. These atoms have identical or similar electronegativity values, so the atoms essentially share their electrons. Here are some examples of covalent compounds:

• Water (H2O)
• Ammonia (NH3)
• Sugar or sucrose (C12H22O11)

The key to understanding why ionic and covalent compounds have different properties from each other is understanding what's going on with the electrons in a compound. Ionic bonds form when atoms have different electronegativity values from each other. When the electronegativity values are comparable, covalent bonds form.

But, what does this mean? Electronegativity is a measure of how easily an atom attracts bonding electrons. If two atoms attract electrons more or less equally, they share the electrons. Sharing electrons results in less polarity or inequality of charge distribution. In contrast, if one atom attracts bonding electrons more strongly than the other, the bond is polar.

Ionic compounds dissolve in polar solvents (like water), stack neatly on each other to form crystals, and require a lot of energy for their chemical bonds to break. Covalent compounds can be either polar or nonpolar, but they contain weaker bonds than ionic compounds because they are sharing electrons. So, their melting and boiling points are lower and they are softer.

• Bragg, W. H.; Bragg, W. L. (1913). "The Reflection of X-rays by Crystals". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 88 (605): 428–438. doi:10.1098/rspa.1913.0040
• Langmuir, Irving (1919). "The Arrangement of Electrons in Atoms and Molecules". Journal of the American Chemical Society. 41 (6): 868–934. doi:10.1021/ja02227a002
• McMurry, John (2016). Chemistry (7th ed.). Pearson. ISBN 978-0-321-94317-0.
• Sherman, Jack (August 1932). "Crystal Energies of Ionic Compounds and Thermochemical Applications". Chemical Reviews. 11 (1): 93–170. doi:10.1021/cr60038a002
• Weinhold, F.; Landis, C. (2005). Valency and Bonding. Cambridge. ISBN 0-521-83128-8.

The physical state and properties of a particular compound depend in large part on the type of chemical bonding it displays. Molecular compounds, sometimes called

covalent compounds, display a wide range of physical properties due to the different types of intermolecular attractions such as different kinds of polar interactions. The melting and boiling points of molecular compounds are generally quite low compared to those of ionic compounds. This is because the energy required to disrupt the intermolecular forces between molecules is far less than the energy required to break the ionic bonds in a crystalline ionic compound (Figure \(\PageIndex{1}\)) . Ionic solids typically melt at high temperatures and boil at even higher temperatures. For example, sodium chloride melts at 801 °C and boils at 1413 °C. (As a comparison, the molecular compound water melts at 0 °C and boils at 100 °C.). The water solubility of molecular compounds is variable and depends primarily on the type of intermolecular forces involved.

Figure \(\PageIndex{1}\) Interactions in Ionic and Covalent Solids.

Since molecular compounds are composed of neutral molecules, their electrical conductivity is generally quite poor, whether in the solid or liquid state. In solid form, an ionic compound is not electrically conductive because its ions are unable to flow (“electricity” is the flow of charged particles). When molten, however, it can conduct electricity because its ions are able to move freely through the liquid (Figure \(\PageIndex{2}\); Video \(\PageIndex{1}\)).

Figure \(\PageIndex{2}\) Sodium chloride melts at 801 °C and conducts electricity when molten. (credit: modification of work by Mark Blaser and Matt Evans) This figure shows three photos connected by right-facing arrows. The first shows a light bulb as part of a complex lab equipment setup. The light bulb is not lit. The second photo shows a substances being heated or set on fire. The third shows the light bulb again which is lit.

Conductivity of Molten Salt

Video \(\PageIndex{1}\) Watch this video to see a mixture of salts melt and conduct electricity.

One type of molecular compound behaves quite differently than that described so far. A covalent network solid is a compound in which all of the atoms are connected to one another by covalent bonds. Diamond is composed entirely of carbon atoms, each bonded to four other carbon atoms in a tetrahedral geometry. Melting a covalent network solid is not accomplished by overcoming the relatively weak intermolecular forces. Rather, all of the covalent bonds must be broken, a process that requires extremely high temperatures. Diamond, in fact, does not melt at all. Instead, it vaporizes to a gas at temperatures above \(3500^\text{o} \text{C}\).

• The physical properties of a material are affected by the intermolecular forces holding the molecules together.
• Ionic compounds usually form hard crystalline solids with high melting points. Covalent molecular compounds, in contrast, consist of discrete molecules held together by weak intermolecular forces and can be gases, liquids, or solids at room temperature and pressure.
• Ionic compounds in molten form or in solution can conduct electricity while molecular compounds do not..

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