Laws of Chemical Combination & Dalton's Atomic Theory

Best viewed with Mozilla Firefox browser

Matter - 03

Laws of Chemical Combination

Fig 1: Joseph Priestly used heat of sun to produce oxygen. (Source: Brent 1960, Chemistry Experiments, p. 7).

It was the Englishman Robert Boyle (17th century) who, through research on the behaviour of gases, provided clear evidence for the atomic makeup of matter. He was the first to define an element as a substance that cannot be chemically broken down further. He believed that a number of different elements might exist in nature.

Law of Conservation of Mass

In 1774, Joseph Priestley isolated the gas oxygen by heating mercuric oxide. Soon thereafter, Antoine Lavoisier claimed that oxygen is the key substance involved in combustion (burning). He also demonstrated that when combustion is carried out in a closed container, the mass of the final products of combustion exactly equals the mass of the starting reactants. This led to the statement of the Law of Conservation of Mass:

Law of Conservation of Mass
Mass is neither created nor destroyed in chemical reactions.
In an experiment, 63.5g of copper combines with 16g of oxygen to give 79.5g of cupric oxide (a black oxide of copper). This is in agreement with the law of conservation of mass.

Science today knows that matter can be converted into energy (and vice-versa). Hence, during all chemical and physical changes, the total mass+energy before the change is equal to the total mass+energy after the change. Still, as there is no detectable change in mass in an ordinary chemical reaction, the law of conservation of mass is still valid.

Silicon dioxide, made up of elements silicon and oxygen, contains 46.7% by mass of silicon. With what mass of oxygen will 10g of silicon combine?

100g of silicon dioxide contains : 46.7g of silicon,
                                         or : (100 46.7) i.e. 53.3g of oxygen.

10g of silicon will contain 10 100 × 53.3 = 5.33 g of oxygen.

Law of Definite Proportions / Constant Composition

In the years following Lavoisier, the French chemist Joseph Proust formulated a second fundamental law of chemical science – the Law of Definite Proportions.

Law of Definite Proportions (Law of Constant Composition)
In a given compound, the constituent elements are always combined in the same proportions by mass, regardless of the origin or mode of preparation of the compound.

What this law means is that when elements react chemically, they combine in specific proportions, not in random proportions.

A sample of pure water, whatever the source, always contains 88.9% by mass of oxygen and 11.1% by mass of hydrogen.
The compound cupric oxide may be prepared by any one of the following methods—
 • Heating copper in oxygen.
 • Dissolving copper in nitric acid and igniting the cupric nitrate formed.
 • Dissolving copper in nitric acid, precipitating cupric hydroxide, and strongly heating the cupric hydroxide.
—and in each case, the ratio copper : oxygen by mass is always constant.
2.16g of mercuric oxide gave on decomposition 0.16g of oxygen. In another experiment 16g of mercury was obtained by the decomposition of 17.28g of mercuric oxide. Show that these data conform to the law of definite proportions.

Experiment 1:

Mass of mercuric oxide = 2.16g
Mass of oxygen evolved from it = 0.16g
∴ Mass of mercury in the compound = 2.16 0.16 = 2.00g
mercury : oxygen ratio = 2.00 0.16 = 12.5 : 1

Experiment 2:

Mass of mercuric oxide = 17.28g
Mass of mercury in it = 16.00g
∴ Mass of oxygen in the compound = 17.28 16.00 = 1.28g
mercury : oxygen ratio = 16.00 1.28 = 12.5 : 1

In both cases, the mercury to oxygen ratio is the same, thus conforming to the law of definite proportions.

Dalton’s Atomic Theory of Matter

Run up to the theory
Fig 2: John Dalton

The explanation for the law of conservation of mass and the law of definite proportions was provided by the English schoolteacher John Dalton in 1808. Dalton reasoned as follows: (McMurray & Fay, Chemistry 4ed, p. 40)

  • Elements are made of tiny particles called atoms. The concept of atoms explained why there were so many different elements.
  • An element is characterized by the mass of its atom. Atoms of the same element have the same mass, but atoms of different elements have different masses. As the law of definite proportions had shown that elements always combine in specific mass ratios, the distinguishing feature between atoms of different elements must be mass.
  • Chemical combination of elements to make different substances occurs when atoms join together in small whole-number ratios. Different samples of a pure compound always contain the same proportion of elements by mass (the law of definite proportions) only if whole numbers of atoms combine. Fractional parts of atoms cannot be involved in chemical reactions.
  • Chemical reactions only rearrange the way that atoms are combined; the atoms themselves are unchanged. For the law of conservation of mass to be valid, atoms must be chemically indestructible. If the same numbers and kinds of atoms are present in both reactants and products, then the masses of reactants and products must also be the same.
Dalton's Atomic Theory

Thus, Dalton’s Atomic Theory can be stated as follows:

  • Matter is discrete (discontinuous) and made up of very small particles called atoms. An atom is the smallest indivisible particle of an element which can take part in a chemical change.
  • Atoms can neither be created nor destroyed in a chemical reaction.
  • Atoms of the same element are identical in all respects, having the same size, shape and structure, and especially mass.
  • Atoms of different elements have different properties and different masses.
  • Atoms of different elements can combine in a fixed ratio of small whole numbers to form compounds.
  • Atoms of the same elements can combine in more than one ratio to form more than one compound.
Drawbacks of Dalton's theory

In light of the current state of knowledge in the field of Chemistry, Dalton’s theory had a few drawbacks. According to Dalton’s postulates,

  • an atom is the smallest indivisible particle of an element. However, it is now known that atoms can further be subdivided into elementary particles like electrons, protons, and neutrons.
  • atoms of the same element are identical in all respects, having the same size, shape and structure, and especially mass. Today, we know that atoms of the same element can have slightly different masses. Such atoms are called isotopes.
  • atoms of different elements have different properties and different masses. However, different elements do exist whose atoms have the same mass. Such atoms are called isobars.
Importance of Dalton's theory

Notwithstanding these drawbacks, the importance of Dalton’s theory should not be underestimated. He displayed exceptional insight into the nature of matter. His ideas provided a framework that could be modified and expanded by later scientists. Thus John Dalton is often considered to be the father of modern atomic theory.

Feedback Form▼(open form)

List of References

Brent, R, The Golden Book of Chemistry Experiments, NY, USA: Golden Press, 1960.
McMurray, J & Fay, RC, Chemistry, 4th edn, USA: Prentice Hall, 2003.


Mahan, BM & Meyers, RJ, University Chemistry, 4th edn, New Delhi: Addison Wesley Longman, 1987.
McMurray, J & Fay, RC, Chemistry, 4th edn, USA: Prentice Hall, 2003.
Whitten, KW, Davis, RE, Peck, L & Stanley, GG, General Chemistry, 7th edn, Belmont, USA: Thomson Brooks/Cole, 2004.