[antoinetta]

So, today my chem teacher made this statement:

"Molecules are compounds, but compounds are not necessarily molecules."

Is that correct?  I'm just trying to get my head around that ...

[Donaldson Tan]

Yes.

Compounds refer to substance that is made up of more than 1 element. Eg. NaCl, HCl

A molecule is a collectiion of two or more atoms in a definite arrangement held together by covalent chemical bonds. Eg. HCl, Cl₂

The structure of HCl is H-Cl. It is a compound because it consist of more than 1 element (H and Cl). Although Cl₂ (Cl-Cl) is a molecule, it is not a compound because it consist of only one element - chlorine. NaCl is an ionic substance. It is a compound because it consist of more than 1 element.

[Yggdrasil]

The term molecule is only valid for covalent compounds; in other words, compounds whose atoms are bound by covalent bonds.  In covalent compounds (e.g. geodome's example, HCl), discrete units (i.e. molecules) of HCl exist.  However, for ionic compounds (e.g. NaCl, table salt), the Na⁺ and Cl^- are arranged in a crystal lattice.  In this lattice, there are no discrete "molecules" of NaCl, just an array of ions arranged in a repeating pattern.

[english]

This can be a tricky subject because there are two points to prove here:

1.  An ionic compound cannot have any molecules in it.

2.  A molecule is not necessarily a compound (this has been pointed out by yourself).

A molecule is defined as two or more atoms chemically bound.  A compound is defined as two or more atoms of different elements chemically bound.

So a molecule can be a compound; H₂O for example.

However, O₂ is a molecule but not a compound (the two atoms of oxygen are not of two different elements). 

In the same sense, NaCl is a compound but not a molecule because of point one illustrated above­­—ionic compounds contain no molecules, which are independent structural units.  An ionic compound such as NaCl can be written as NaCl in reduced form or as Na₂Cl₂ or Na₁₀₀Cl₁₀₀.  NaCl is a lattice of how ever many Na and Cl atoms the size of the sample permits, as long as the ratio of Na to Cl is 1:1.  A molecule such as H₂O, however, can only be 2 Hydrogens and 1 Oxygen.