Can a compound be classified as ionic or molecular, given only its formula?

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.)

Can you tell whether a compound is ionic or molecular from its experimental properties?

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

What holds atoms together in molecules?

Molecules are held together by electrons shared between bonded atoms. For example, consider the formation of an H2 molecule from two separate hydrogen atoms:
covalent bond in H2
2 approaching
hydrogen atoms
(2 H)
a hydrogen molecule
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.

What do you call a compound formed from a metal and oxalic acid? -Rickesh Kotecha (grade 11)

Ionic compounds are named by following the cation name with the anion name. The name of the anion in your compound is derived by replacing the -ic acid suffix with -ate.

What causes hygroscopic compounds to absorb water? -Barb Early

Hygroscopic compounds take up water from the air for many different reasons. Here are a few: