Combinations 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.

Dalton's hypothesis about molecules has the virtue of explaining constant composition: if oxygen atoms and hydrogen atoms have fixed masses, and always combine in a one-to-one proportion, then at the macroscopic scale, the proportion of the masses of the substances would match the proportion of the atoms' masses. 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. Finally, Dalton did not consider the possibility that atoms of a given element might bond to themselves, bonding hydrogen to hydrogen, oxygen to oxygen, and the like, which is now known to occur very commonly.

In this section we will learn about molecules and formulas, with an emphasis on how molecules differ from compounds, and on the different types of formulas that exist.

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. When you use electrolysis to decompose water hydrogen gas and oxygen gas are 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 result led scientists to make some inferences about the way elements combine into compounds. They proposed that compounds were formed by joining small numbers of atoms (one, two, or three or so at a time) to each other in clumps. This matched Dalton's idea, and was a good explanation for the small whole number ratios observed in gas reactions. These "clumps" came to be called molecules: groups of small numbers of atoms held together by some force (the identity and nature of that force was not known at the time).

The Difference Between Molecules and Compounds

In the two diagrams above, you can see two different hypotheses about the structure of water: Dalton's original proposal, and the revised, correct model. Though they differ, they share an important thing in common, which makes them molecules: they contain only a small number of atoms, tethered together in some way.

Chemical Formulas

Now, since we know that atoms can combine with one another we have to have some way of representing this. The way that we show combinations of atoms on paper is to write down combinations of symbols for those atoms. These combinations of symbols are called formulas. Much like we can make words by combining letters, we can make formulas by combining symbols.

Dalton's formula for water would be HO. We write down HO together, with no space in between them, and that would represent one atom of hydrogen combined with one atom of oxygen. (Again, that is not the correct formula for water.)

Avogadro's formula for water would be H₂O. H₂O simply means that there are two atoms of hydrogen bonded to one oxygen atom, a ratio of two hydrogen atoms for every one oxygen atom. The subscript follows the symbol of the element to which it refers.

The way that you would represent sodium chloride is to write down the symbol Na followed by the symbol Cl with no space between them. That represents sodium chloride--a combination of sodium and chlorine in a 1:1 ratio by atoms.

The formula for molecular hydrogen would be written as H₂. The 2 subscript immediately following the H means that there are two hydrogen atoms combined with one another. The formula for molecular oxygen is O₂. Again, the subscript 2 shows that there are 2 of those oxygen atoms bonded together. Note that the formulas for these elements is different from the symbols for these elements because the formulas represent the molecules of the elements and the symbols represent the atoms of these elements.

Here is the formula for some kind of a sugar molecule. C₆H₁₂O₆ represents 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms.

In the formula Fe(NO₃)₃ you have to contend with parentheses. The 3 at the end of the formula - the one that is outside the parentheses - means that we have 3 of everything inside the parentheses. So there are 3 NO₃'s which would be a total of 3 nitrogen atoms and nine oxygen atoms, along with the 1 iron atom. Notice that the last 3 doesn't apply to the iron. It only applies to what's inside the parentheses. The 3 inside the parentheses applies only to the oxygen.

You can also write down the formula of the compound if you are given the information about how many atoms are in it. For example, if I told you that a particular compound had 1 sulfur atom for every 3 oxygen atoms, you should be able to write down the formula SO₃. Or if I told you that in a particular molecule there were 2 nitrogen atoms and 5 oxygen atoms, you should be able to write down the formula N₂O₅.

Types of Formulas

There are several types of formulas: molecular, structural, and empirical. Most of the formulas we have been dealing with have been molecular formulas. Different formulas are useful in different situations; let's look at what each kind of formula can tell us.

Molecular formulas apply to any molecular material. A molecular formula tells you the actual number of each kind of atom within that molecule. H₂ shows that two hydrogen atoms are contained in the molecule. Similarly, O₂ shows that two oxygen atoms are contained in the molecule. H₂O shows that a water molecule contains two hydrogen atoms and one oxygen atom. Hydrogen peroxide has the formula H₂O₂, meaning that its molecules each contain two hydrogen atoms and two oxygen atoms.

Structural formulas also apply to molecular materials. They not only tell you how many of each kind of atom there is but which atoms are bonded to one another. The structural formula tells you something about the arrangement of atoms within the molecule. Water, for example, can be written as H-O-H, showing that the oxygen atom is in the middle and the two hydrogen atoms are bonded to it. The structural formula for hydrogen peroxide can be written as H-O-O-H.

Empirical formulas apply to any type of compound, whether it consists of molecules or not. Empirical formulas are formulas that are derived from experimental data. (Empirical means experimental.) All that is really shown about the compound in an empirical formula is the ratio of atoms. The very nature of how you go about calculating those formulas only allows you to get the simplest ratio.

Here are some examples. H₂O is both an empirical and a molecular formula. The simplest ratio of hydrogen atoms to oxygen atoms is 2:1. Therefore, H₂O is an empirical formula. However, if you can isolate individual molecules of water and figure how many atoms there really are in each one, it turns out there are 2 hydrogen and 1 oxygen atoms bonded together in the cluster that forms the molecule. So H₂O is a molecular formula as well. In hydrogen peroxide, the simplest ratio of hydrogen atoms to oxygen atoms is 1:1, one hydrogen atom for every oxygen atom. So the empirical formula is HO. That is different from the molecular formula, which is H₂O₂.

A molecule like glucose shows how exaggerated the differences can be. The structural formula shows how the 24 atoms are hooked together. The molecular formula shows that each molecule contains 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms. The empirical formula simply shows that the ratio of atoms is 1:2:1.