Barrons SAT Subject Test Chemistry, 13th Edition

When two p orbitals share electrons in a covalent bond and the interaction is
not symmetrical about a line between the two nuclei, the result is a pi bond. Here
is an example:

Chapter 14 gives more examples of sigma and pi bonding.

PROPERTIES OF IONIC SUBSTANCES

Laboratory experiments reveal that, in general, ionic substances are characterized
by the following properties:

1. In the solid phase at room temperature they do not conduct appreciable
   electric current.


2. In the liquid phase they are relatively good conductors of electric current.
   The conductivity of ionic substances is much smaller than that of metallic
   substances.


3. They have relatively high melting and boiling points. There is a wide
   variation in the properties of different ionic compounds. For example,
   potassium iodide (KI) melts at 686°C and boils at 1,330°C, while
   magnesium oxide (MgO) melts at 2,800°C and boils at 3,600°C. Both KI and
   MgO are ionic compounds.


4. They have relatively low volatilities and low vapor pressures. In other
   words, they do not vaporize readily at room temperature.


5. They are brittle and easily broken when stress is exerted on them.