Long before anyone could see an atom, chemists noticed that elements combine in fixed, predictable amounts. These regular patterns were summed up in the laws of chemical combination, and they later became the launch pad for the atomic theory of matter. Two of these laws form the backbone of this chapter.
Law of Conservation of Mass
This law was stated by the French chemist Antoine Lavoisier. It says that mass can neither be created nor destroyed in a chemical reaction. In other words, the total mass of the reactants is always equal to the total mass of the products. If 12 g of carbon burns in 32 g of oxygen, exactly 44 g of carbon dioxide is formed — not a milligram more or less. This is why a chemical equation must always be balanced: the same number of atoms of each element must appear on both sides.
Law of Constant (Definite) Proportions
Proposed by Joseph Proust, this law states that a pure chemical compound always contains the same elements combined in the same fixed ratio by mass, no matter where it comes from or how it is made. Water from a river, from rain, or made in a laboratory always has hydrogen and oxygen in the mass ratio 1 : 8. Pure carbon dioxide always has carbon and oxygen in the ratio 3 : 8. The source does not matter — the proportion stays constant.
Dalton's Atomic Theory
In 1808 John Dalton explained these laws with a single bold idea: matter is made of tiny indivisible particles called atoms. The main postulates are:
- All matter is made of very small particles called atoms.
- Atoms cannot be created, destroyed or divided into smaller parts (in a chemical reaction).
- All atoms of a given element are identical in mass and properties.
- Atoms of different elements have different masses and properties.
- Atoms combine in small whole-number ratios to form compounds.
- The relative number and kind of atoms in a compound are always fixed.
Notice how neatly the theory explains the laws. Because atoms are merely rearranged (not created or destroyed) in a reaction, mass is conserved. Because a compound always has the same atoms in the same whole-number ratio, its elements are always in the same fixed mass ratio — the law of constant proportions. Dalton's idea that atoms of the same element are identical is now known to be only partly true (isotopes exist), but his theory was the first real model of the atom and it still guides how we write formulae and balance equations today.