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Topic: Compound vs Molecules  (Read 15860 times)

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Offline antoinetta

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Compound vs Molecules
« on: September 11, 2006, 07:32:49 PM »
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 ...
The oldest, shortest words - "yes" and "no" are those which require the most thoughts.  - Pythagoras

Offline Donaldson Tan

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Re: Compound vs Molecules
« Reply #1 on: September 11, 2006, 07:42:37 PM »

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.
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

Offline Yggdrasil

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Re: Compound vs Molecules
« Reply #2 on: September 11, 2006, 07:55:56 PM »
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.

Offline english

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Re: Compound vs Molecules
« Reply #3 on: September 12, 2006, 07:12:24 PM »
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.
« Last Edit: September 12, 2006, 07:20:05 PM by k.V. »

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