Compounds and molecules : LUSENET : Middle School Science : One Thread

I am confused about compounds and molecules. I've read the definitons but they sound similar to me.

-- Deanna Zapata (, September 15, 2002


You need to clarify what exactly is confusing you about the difference. A molecule is the smallest particle of a compound. For instance, a single water molecule is composed of 1 oxygen atom and 2 hydrogen atoms chemically bonded. If you were to break this molecule apart (by electric current) it would no longer be a water molecule. It would no longer have the characteristics of water. The hydrogen atoms would take on the characteristics of hydrogen and the oxygen atom would take on the characteristics of oxygen.

A molecule is not always a compound. Molecules consist of 2 or more atoms combined. You can have a molecule of 2 hydrogen atoms. A molecule made of two nitrogen atoms. They are molecules but not compounds because they still have the characteristics of their respective elements.

I hope I was able to help. Feel free to ask me any thing your not sure about.


-- Edward Palmer (, September 15, 2002.

Just to add to Ed's comments:

From my introductory chemistry text:

Molecule: A distinct, electrically neutral group consisting of a well- defined number of ATOMS bonded together. These "groups" may be compounds (H2O, CO2) or elements (H2, O2, N2, S8, etc).*

Compound: A substance made up of 2 or more ELEMENTS combined in a fixed ratio (H2O, CO2, etc.)

Here's where it gets tricky. Notice that one definition requires a fixed number of atoms while the other definition requires a fixed ratio. Thus table salt, NaCl, is a compound but not a molecule! Why? Because it is composed of 2 or more elements in a fixed ratio (1:1, satisfying the definition of a compound) but does not have a well-defined number of atoms (so it does not meet the criteria for a molecule). It is instead an array of any number (not fixed) of Na+ and Cl- ions arranged in a 1:1 ratio. Sand (Silicon Dioxide or SiO2) is also a non-molecular compound.

*Only about 10 elements occur naturally as molecules - other elements occur in a "metallic" structure, or as independent atoms (such as the noble gases), or some other non-molecular array (carbon). The 10 elemental molecules are O2, H2, N2, F2, Cl2, Br2, I2, At2, S8, & P4. - these are molecules, but not compounds. None of the metals is molecular.

Atoms in metals are held together by metallic bonding, in which the nuclei are arranged in a relatively fixed "lattice" and the electrons flow freely throughout the metal, not confined to individual atoms. This electron mobility accounts for the conductivity and other properties of metals.

-- Michael Gatton (, September 16, 2002.

I'm still confused. Please explain it more simply. I need help before midterms!

-- Helen Yellow (, January 06, 2003.

Simplified answer:

What's similar about the terms molecule & compound: Both terms refer to a substance made of two or more atoms chemically bonded.

What's different: in using the term compound, the substance must contain at least 2 different elements. The term molecule can be applied to substances made of a single type of element or a substance made of 2 or more different elements.

H2O can be referred to as a molecule and/or a compound. MOLECULE because it is made of two or more atoms (2 hydrogen atoms plus 1 oxygen). COMPOUND because it is made of 2 or more different elements (hydrogen & oxygen).

O2 is a molecule (2 or more atoms chemically bonded) but NOT a compound (only one kind of element - oxygen).

Thst's the simplified answer - but beware that if you just use the information in this response, you will probably misapply the terms to some special cases (like NaCl) as described in the earlier response below. So if this makes sense to you, go back and re-read the previous more complicated answer below, to which I have added a note on usage.

OK The system won't let me add to the message below so here it is:

A note on usage:

Additionally, note that we can refer to water generally as a compound, but the term molecule would only be used when referring to individual units (H2O) of water. For example, we might say: "The water in a glass is a compound," or, "The water in a glass is a collection of water molecules."

-- Michael Gatton (, January 07, 2003.

Moderation questions? read the FAQ