Why do ionic compounds conduct electricity when dissolved in water while covalent compounds do not

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.

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a Explain why ionic compounds conduct electricity in solution whereas covalent compounds do not conduct electricity. b Which of the following will conduct electricity and which not? MgCl2, CCl4, NaCl, CS2, Na2S Give reasons for your choice.

The group 0 elements, the noble gases, are all unreactive non-metal gases. They show trends in their physical properties. Their uses depend on their inertness, low density and non-flammability.

Updated May 11, 2018

By Claire Gillespie

A chemical compound is made up of many identical molecules formed from atoms from more than one element, attached by chemical bonds. However, not all compounds are created equally. Different things happen to ionic compounds and covalent compounds when they dissolve in water.

When ionic compounds dissolve in water they go through a process called dissociation, splitting into the ions that make them up. However, when you place covalent compounds in water, they typically do not dissolve but form a layer on top of the water.

Ionic compounds are molecules consisting of oppositely charged ions, which are ions with both negative and positive charges. Covalent compounds are non-metals bound together, made up of two electrons shared between two atoms. Ionic compounds have a high melting and boiling point, but covalent compounds have a comparatively lower melting and boiling point. This is because ionic compounds need a very large amount of energy to break their ionic bonds and separate the positive and negative charges. Because covalent compounds are made of distinct molecules that don’t mix with each other, they separate more easily. Sodium bromide, calcium chloride and magnesium oxide are examples of ionic compounds, while ethanol, ozone, hydrogen and carbon dioxide are examples of covalent compounds.

When ionic compounds dissolve in water, they break apart into the ions that make them up through a process called dissociation. When placed in water, the ions are attracted to the water molecules, each of which carries a polar charge. If the force between the ions and the water molecules is strong enough to break the bonds between the ions, the compound dissolves. The ions dissociate and disperse in solution, each ringed by water molecules to prevent reattachment. The ionic solution turns into an electrolyte, meaning it can conduct electricity.

When covalent compounds dissolve in water they break apart into molecules, but not individual atoms. Water is a polar solvent, but covalent compounds are usually nonpolar. This means covalent compounds typically don't dissolve in water, instead making a separate layer on the water's surface. Sugar is one of the few covalent compounds that does dissolve in water because it is a polar covalent compound (i.e., parts of their molecules have a negative side and a positive side), but it still doesn't separate into ions the way ionic compounds do in water. Oil is a non-polar covalent compound, which is why it doesn't dissolve in water.