Molecules

While some chemists were exploring the weight relationships of the elements, others were looking at the volume relationship in the chemical reaction of gases. Remember when you used electrolysis to decompose water? Hydrogen gas and oxygen gas were the products. By volume, twice as much hydrogen as oxygen is generated -- a 2:1 ratio by volume. Also, the reaction can be reversed; hydrogen and oxygen combine in a simple 2:1 ratio by volume to make water. Simple relationships like that catch people's attention. Other gaseous chemical reactions also involved simple whole number volume ratios. It seems that gases always reacted in simple whole number ratios by volume. This relationship is called the Law of Combining Volumes. The French chemist Gay-Lussac came up with it in 1808.

One implication of this was that water contained twice as much hydrogen as oxygen, and therefore water molecules consisted of two hydrogen atoms combined with one oxygen atom, rather than one of each, as Dalton had proposed. How to decide which was correct, however, was not obvious.

An Italian chemist by the name of Amadeo Avogadro came up with an idea, a hypothesis, in 1811. It is now called Avogadro's Law. After studying the reactions of some gaseous elements, he proposed that equal volumes of gases (under the same conditions) contain the same number of molecules. He also proposed that a molecule was a combination of atoms--a little pile of atoms. One of the reactions he had apparently been studying was the formation of water from hydrogen and oxygen. By carrying out the reaction above the boiling point of water, the water that was formed remained in the gaseous state. So both of the reactants and the product as well were all gases. What he noticed was that two volumes of hydrogen combined with one volume of oxygen to form two volumes of water. (By "a volume" I mean whatever size measurement of volume you want to use.) It took twice as much hydrogen as oxygen and the volume of water formed was equal to the volume of the hydrogen that you started with. It's as if the oxygen moved in with the hydrogen. As I mentioned before, this was our first clue that water consisted of two parts hydrogen to one part oxygen. This was not explained by Dalton's version of atomic theory and so Dalton just considered it to be of no particular consequence.

hydrogen

oxygen

water

Avogadro, however, thought it was very important and developed the concept that I will try to illustrate here (and in example 10 of your workbook). The boxes are all supposed to be the same size and represent equal volumes of gases and they contain equal numbers of molecules. I will use just one molecule in each. The hard part was to make the idea work. However, once you know the solution, it's not hard at all.

We start with the idea that each molecule of water contains two hydrogen atoms and one oxygen atom.

H₂O

The two oxygen atoms in the two water molecules come from one oxygen molecule that contains two oxygen atoms.

O₂

H₂O

Each hydrogen molecule also contains two atoms providing the four hydrogen atoms necessary for the two water molecules.

H₂

O₂

H₂O

Notice that all this is derived from the assumption that water molecules contain two hydrogen atoms with one oxygen atom and Avogadro's hypothesis that equal volumes of gases contain equal numbers of molecules.

Avogadro's new concept contained the idea that molecules of hydrogen react with molecules of oxygen to form molecules of water and that each molecule of hydrogen and each molecule of oxygen contains two atoms rather than just one. The importance of this was quite apparent to Avogadro, but it took several decades before the importance of his hypothesis was brought to the front and used to convince people of what was going on in these gaseous reactions.

Molecules vs. Compounds

Note that there is a very important distinction between compounds and molecules. A comparison of Dalton's ideas and Avogadro's ideas points this out. You may at times hear that elements are made up of atoms and compounds are made up of molecules, but it turns out that is not true on several counts. In water, two hydrogen atoms are hooked to an oxygen atom to form both a molecule and a compound. However, when two hydrogen atoms are hooked together they form a molecule, but they do not form a compound. Both atoms are the same element, not two different elements. The same is true with oxygen. Two oxygen atoms hooked together form a molecule but they do not form a compound. Thus, it is possible to have molecules of an element.

Sodium chloride brings up a related issue. Although it is a compound, it is not a molecular compound. The way that the sodium and chlorine are hooked together involves a different mechanism and one that does not result in the formation of sodium chloride molecules. Instead, they make a crystalline network. Sodium chloride is a network compound rather than a molecular compound. That becomes important later in the course when we deal with bonding.

Although it might be convenient to equate molecules and compounds, they are not the same thing. There are materials which are molecular but are not compounds, like hydrogen and oxygen. There are materials which are compounds but are not molecular, like sodium chloride. There are materials which are neither compounds nor molecular like magnesium. There are also materials, like water, which are both molecular and compounds. So you must treat those two concepts--molecules and compounds--separately. What they do have in common with one another is that they both represent combinations of atoms, but they are different kinds of combinations.

Review

Elements consist of tiny particles called atoms. Atoms of the same element can combine with one another to form molecules, but they are not compounds. Atoms of different elements can also combine with one another to form molecules, and these combinations are compounds. It is also possible for atoms to combine with one another in ways that do not form molecules. Instead, they can form vast arrays or networks of atoms sometimes called crystal lattices.

A compound is a combination of atoms of different elements. Water is an example of a molecular compound; sodium chloride is an example of a nonmolecular compound.

A molecule is a combination of atoms bonded in a certain way to form a small collection or cluster of atoms that are all hooked together. The atoms in the molecule might all be the same (element) or they might be different (compound).

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