John Dalton’s Atomic Theory is based on the facts known in 1808. The Atomic Theory states that all elements are made up of atoms. Atoms of an element are all alike. Atoms are not created or destroyed in chemical reactions. Chemical reactions take place between atoms or a group of atoms. The smallest units of a compound are called compound atoms. Dalton created a table of relative atomic and molecular masses by assuming that the mass of hydrogen is one.
Dalton also proposed that atoms could not be broken down to smaller parts. We know that this assumption is incorrect. Atoms are composed of a nucleus and electrons in orbit around it. The nucleus is composed of protons and neutrons which account for an atom’s weight. Protons and neutrons are made up of quarks.
Dalton also said that atoms combine to form a compound in simple ratios. Sometimes two elements can combine in more than one way. Dalton’s experiments with copper oxides proved his Atomic Theory. Copper combines with the oxygen in air when it is heated. There is a black copper oxide and a red copper oxide proving that there are two molecular or compound ratios for combinations of copper and oxygen. There are twice as many oxygen atoms in the black copper oxide than in the red copper oxide. Black copper oxide can be represented as OCuO. Red copper oxide can be represented as CuO. The copper oxide experiments lead to the creation of the Law of Definite Proportions which states that when two elements combine in more than one way, a simple ratio exists between the weights of one element that can combine with a given weight of the other element. This was a major advancement in chemistry. The Law of Definite Proportions helped to support the idea that all matter is made up of atoms. The Law of Definite Proportions is a good corollary to Gay-Lussac’s Law of Combining Volumes which states that when two or more gases take part in a chemical reaction, their volumes are in the ratio of small whole numbers when the gases are at the same temperature and pressure.
Dalton also experimented with the chemical composition of gases. He emphasized the weight of gases in his laboratory notebooks. His law of partial pressures says that the total pressure of a mixture of gases is the sum of the partial pressure of each gas component.