The word atom was coined from the greek word "atomos" which means indivisible. The earliest explanation of matter was done by Greek scientist Democritus. However in 1808, British scientist John Dalton put forward an atomic theory. The theory is stated as follows:

1. Elements are made up of small indivisible particles called atoms
2. Atoms can neither be created nor destroyed
3. Atoms of the same element are alike in every aspect and differ from atoms of other elements
4. When atoms combine, they do so in simple whole number ratios
5. Chemical reactions arise from the combination or separation of atoms

The Dalton's atomic theory has its findings supported by the laws of chemical combination.

There are four basic laws of chemical combination. These laws include:

1. The Law of Conservation of Mass: This law was proposed by Antoine Lavoisier in 1789. It states that matter can neither be created nor destroyed in a chemical reaction but can be transformed from one form to another.
   It means that:
   the sum of mass of reactants = sum of mass of products
2. The law of Definite proportions: This law was stated by Joseph Proust. It states that the proportion of elements by mass in a given compound will always remain exactly the same.
3. Law of Multiple Proportions: This law states that if two elements combine to form more than one compound, the masses of these elements in the reaction are in the ratio of small whole numbers. This law was given by Dalton in the year 1803.
   For example;
   • Iron and oxygen form brown iron(III) oxide,Fe₂O₃ and black iron(II) oxide,FeO
4. The law of Reciprocal Proportions: The law was proposed by Jeremiah Ritcher in 1792 it states that if two different elements combine separately with the same weight of a third element, the ratio of the masses in which they do so are either the same or a simple multiple of the mass ratio in which they combine.

1. All elements are made up of small indivisible particles called atoms: The discovery of the presence of subatomic particles i.e protons, neutrons and electrons by notable scientists Ernest Rutherford, J.J Thompson and James Chadwick refuted this claim. However, John Dalton still accurately predicted the existence of atoms in elements.
2. Atoms can neither be created nor destroyed: This holds true for ordinary reactions. However, works by Henry Becquerel and Marie Curie which led to the discovery of radioactivity showed that atoms can be created by nuclear fusion and destroyed by nuclear fission.
3. Atoms of the same elements are alike in every aspect and differ from atoms of other elements: The discovery of isotopes by British radiochemist Frederick Soddy refuted this claim. Chlorine for example has two isotopes. Chlorine-35 and chlorine-37.
4. When atoms combine they do so in simple whole number ratios This is generally true for most chemical combinations however for large organic molecules like proteins which have contains numerous atoms the combination is not in simple whole number ratio.