Atoms combine in simple numbers

The idea

Atoms combine in the ratio of simple whole numbers to form compounds, and the relative number and kinds of atoms in a given compound are constant.

Counting atoms in water

A water molecule is written H₂O. Imagine many water molecules in a glass.

If you counted all the hydrogen atoms and all the oxygen atoms, what ratio would you expect?

• 2 hydrogen atoms for every 1 oxygen atom
• It would change from glass to glass
• 1 hydrogen atom for every 2 oxygen atoms

Count the atoms

Pick a compound and the number of molecules. Count the atoms of each kind.

Same atoms in the same numbers

However many molecules you take, the atoms in water are in the ratio 2 to 1, and in hydrogen chloride 1 to 1. A given compound has its own fixed pattern of atoms.

Atoms combine in the ratio of simple whole numbers to form compounds. The relative number and kinds of atoms in a given compound are constant.

Notes

Atoms combine in simple whole-number ratios, and in a given compound the kinds and relative numbers of atoms are constant.

Check yourself

There are 3 water molecules. How many hydrogen atoms are there altogether?

Answer: 6

Each water molecule, H₂O, has 2 hydrogen atoms, so 3 molecules have 6.

There are 3 water molecules. How many oxygen atoms are there altogether?

Answer: 3

Each water molecule has 1 oxygen atom, so 3 molecules have 3.

In hydrogen chloride (HCl), what is the ratio of hydrogen atoms to chlorine atoms?

What does Dalton say about the atoms in a given compound?

• 2 : 1. This is the ratio of hydrogen to oxygen atoms in water, H₂O. In HCl there is 1 hydrogen atom for 1 chlorine atom.
• 1 : 2. HCl has one chlorine atom for one hydrogen atom, so the ratio is not 1 : 2.
• 1 : 1 — correct. Yes!
• Their relative number and kinds are constant — correct. Yes!
• Their ratio changes each time the compound is made. The relative number and kinds of atoms in a given compound are constant.
• They combine in any fraction. Atoms combine in the ratio of simple whole numbers.