Combinations of Atoms
Home Table of Contents Preliminary Information Introduction to the Lesson Classification of Materials Dalton's Atomic Theory Element Names and Symbols Combinations of Atoms Atomic Weights Moles Associated Calculations Wrap-Up


Types of Formulas

Combination of Atoms

One of the important reasons for talking about atoms is because the atoms combine with one another. That was an important part of the reason that Dalton came up with his atomic theory. He wanted to explain why compounds had fixed composition. But, as you will see, atoms combine in more ways than Dalton imagined.

The diagrams shown here (and in Example 9 of your workbook) represent Dalton's concept of forming compounds by the combination of individual atoms of different elements. The first shows how hydrogen and oxygen atoms can combine to make what he called composite atoms or compound atoms of the compound water. Today we would call them molecules. The compound water is formed, he thought, by one hydrogen atom combining with one oxygen atom to form one molecule of water. There is a serious problem with that statement that I will get to later.
H + O rtarrow.gif (850 bytes) HO
The second shows how the compound sodium chloride is formed. One atom of sodium combines with one atom of chlorine to form one unit (call it a molecule for now if you want) of sodium chloride.
Na + Cl rtarrow.gif (850 bytes) NaCl

Essentially, that was Dalton's concept. The value of this approach is that it explains composition. The mechanism of how those atoms hook together is not yet explained, but the idea that they do hook together gives some sense to the elements combining in fixed ratios.

There are, however, short-comings. One is that water is not composed of one atom each of hydrogen and oxygen. Another short-coming is that it doesn't address the different types of compounds and the different ways in which atoms can bond to one another. I chose these particular examples, water and salt (sodium chloride), because they represent radically different ways of combining elements, of combining atoms. We will deal with those different ways of combining atoms quite a bit later in this course when we talk about bonding, but I did want to point out that short-coming now. Another problem is that Dalton's method ignores the combination of like atoms. It doesn't say anything about two atoms of oxygen combining together, and I think Dalton denied that that was even a possibility. Well, it turns out that it is more than just a possibility. It does in fact happen.

In the pages of this section, we will take a closer look at molecules, formulas and the various types of formulas with which you need to be familiar.


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