Re: What is a easy way of identifying ionic compounds and molecular compounds?

Hi Justin,

The classification of compounds into molecular versus ionic is based on how 
substances are bonded.  Ionic compounds have an unequal distribution of the 
electron cloud between atoms.  In a completely ionic compound, one ion will 
accept an electron that the other ion donates.  Another way to look at this 
is that ionic compounds will come apart into charged species (ions) when 
they dissolve in a solvent.  Table salt (sodium chloride or NaCl) is an 
ionic compound that will break apart into two ions when dissolved in water:

                           NaCl --> Na+ + Cl-

Molecular compounds share the electron cloud more evenly than ionic 
compounds.  This is called covalent bonding.  Molecular compounds will not 
come apart as charged species the same way ionic compounds will.  Sugar 
(C6H12O6) is a molecular compound that will not break up into charged 
components when dissolved in water.  Water (H2O) is a molecular compound, 
also.

--- Dan Berger adds:
Some unquestionably molecular compounds form ions in solution; examples 
include HCl and most other common acids. I think a more useful definition 
would ask whether the pure compound exists as neutral molecules or as ions
in the liquid state. Pure, ionic compounds like NaCl exist as ions in the melt; 
molecular compounds like HCl or sulfuric acid exist as neutral molecules.

There are, incidentally, truly ionic compounds which are liquids at room
temperature. These are usually composed of organic ions, and are expected
to find a place in industrial chemistry. Unfortunately I can't think of
a specific example because it's not quite my field.
----------

Yet another way to look at this (that does not address the point directly, 
but might help you out) is that ionic compounds form very regular crystal 
lattice structures when they're not dissolved in a solvent.  For example, 
salt is a cubic crystal that is made up of many NaCl units.  In these ionic 
salt crystals, the bonds between the sodium and chloride are ionic.  
Chemistry textbooks always call ionic bonds "brittle".  What this means is 
that if you break a salt crystal, you will often find that the broken piece 
is also a cube.  The edges of ionic crystals are often very distinct.  In 
ice crystals, the bonds between the individual water molecules are not 
ionic.  When you break an ice cube, the chunk that comes off is not going 
to be the same shape as the original ice cube.  Molecules like proteins 
will form regular crystals (studying protein crystals is one way that 
biochemists can determine protein structures), but protein crystals are 
still not as well-defined as ionic crystals, and the bonds between the 
individual crystal units are not ionic.  

I hope that my explanation helps you.  Chemistry terminology can sometimes 
be confusing.

Sarah Earley
CU Boulder