Compounds that contain both metals and nonmetals are usually ionic. For example, Na₂SO₄ contains a metal (Na) and nonmetals (sulfur and oxygen), and so is expected to be ionic. But CO₂ and CH₄ contain only nonmetals, and are expected to be molecular compounds.

The rule works because metals give up electrons very easily to form positive ions (cations), and nonmetals gain electrons easily to form negative ions (anions). Electron transfer usually occurs when a metal and a nonmetal react, forming an ionic compound.

You have to watch out for a few exceptions. NH₄Cl is an ionic compound. You might expect it to be molecular because it contains only nonmetals. But it contains the NH₄⁺ (ammonium) ion is combined with a chloride ion.

Beryllium (Be) compounds are not ionic, even though beryllium is a metal. Be has very tightly bound electrons and it doesn't give them up completely when it forms compounds with nonmetals. There isn't any such thing as a Be²⁺ ion. (The chemistry of beryllium is unlike that of any other metal; we'll see why later in the course.)

Yes. Here are some characteristic properties of molecular compounds that distinguish them from ionic compounds.

• Low melting points and boiling points. A relatively small amount of energy is required to overcome the weak attractions between covalent molecules, so these compounds melt and boil at much lower temperatures than metallic and ionic compounds do. In fact, many compounds in this class are liquids or gases at room temperature.
• Low enthalpies of fusion and vaporization These properties are typically 10 to 100 times smaller than they are for ionic compounds.
• Poor electrical and thermal conductivity. Ionic compounds conduct electricty well when melted; metallic solids do as well. Covalent molecular compounds do not.

Molecules are held together by electrons shared between bonded atoms. For example, consider the formation of an H₂ molecule from two separate hydrogen atoms:

2 approaching hydrogen atoms (2 H)		a hydrogen molecule (H2)

As the two atoms approach, each nucleus begins to attract the other atom's electrons. The shared electrons spend much of their time wedged between the two nuclei (where they can be be as close as possible to both of the positive charges at once). The attraction of nuclei for shared electrons holds the atoms together. A molecule has formed.

Because of the wavelike nature of electrons, you really need quantum mechanics to accurately describe this process.

• Ionic compounds are often hygroscopic because they form stable hydrates.
  • Metal cations (being positively charged) attract the lone pairs on water oxygens and form coordinate covalent bonds with water. For example, many divalent cations M²⁺ can form ions like [M(H₂O)₆]²⁺. One or more of the waters can be replaced with anions in the hydrate.
  • Anions like sulfate or nitrate can form strong hydrogen bonds with the water. For example, nitrate ions form a very stable arrangement with water in which the water "bridges" the nitrate oxygens with hydrogen bonds.
  • Sometimes both the anion and the cation bind water. For example, in copper(II) sulfate pentahydrate, four waters are bound in a square around the copper(II) ion's equator and two sulfates are bound to the poles. The fifth water bridges sulfates together.
• Some highly soluble ionic compounds go a step further. The shell of waters formed around the individual ions on the surface of the material becomes an excellent surface for more waters to bind. This is especially true with small and highly charged ions like hydroxide. Water will sometimes continue to be added to the shell until the material actually dissolves in the adsorbed water. This is called deliquescence. As a student, I remember naively trying to prepare an 0.1000 M NaOH solution by weighing out a tenth of a mole of NaOH on a Mettler balance. I couldn't get a steady weight; the balance was precise enough to weigh the water as it was being absorbed from the air. Within a few minutes, I could see that the surface of the pellets was wet, and within about 15 minutes, the pellets looked as though they had melted.
• Water molecules can diffuse into the material and become trapped there, especially if the material contains voids that can hydrogen-bond the water in place.
• Some porous materials can absorb large amounts of water by a phenomena called capillary condensation. For example, zeolites (complex sodium aluminum silicates) are honeycombed with cavities lined with oxygen atoms. Water in air hydrogen bonds to the outer surfaces of the material. As more water condenses, it tries to spread out and wet as much of the surface as possible to minimize its energy through hydrogen-bonding. That carries the water deeper into the material, making more room for water to adsorb to the outer surface. (They soak up water like a sponge!)