What is Dalton Atomic Theory?
Dalton atomic theory is based on two of the well-known laws:
• The law of conservation of mass
• Law of constant composition.
The law of conservation of mass was formulated by Antoine Lavoisier states that matter can neither be created nor it can be destroyed in a closed system.
This means in a chemical reaction, the amount of each element involved in the given reaction must be the same as in the starting materials (reactants) and the products. This law of conservation of mass is used every time to balance chemical equations.
While researching about the same, Dalton also discovered that some gases having the same common element in given compounds could only be combined in certain proportions. Thus, he formulated the law of constant composition.
Postulates of Dalton Atomic Theory
The postulates of Dalton Atomic Theory are mentioned below;
• The matter is composed of atoms, and these atoms are the undividable building blocks of matter and thus cannot be destroyed.
Dalton theorized that the law of conservation of mass and the law of definite proportions could be described using the knowledge of atoms.
He proposed that all matter is made of tiny undividable particles which are known as atoms, and he assumed them as solid, hard, impassable, movable particles.
• The properties of all the atoms of an assumed element have the same mass. This can also be defined as the atoms of a given element have identical mass whereas the atoms of different elements have different masses.
Dalton projected that every single atom of an element, for instance, gold, is as same as every other atom of that particular element. He also stated that the atoms of one particular element vary from the atoms of all other elements.
For example; a sodium atom is very different from a given carbon atom. Elements may share few similar properties such as boiling points, melting points, etc. but no two elements can have the same set of exact properties.
• The Compounds are formed through different whole-number combinations of given atoms. In the next part of Dalton atomic theory, he theorized that compounds are the groupings of two or more different types of atoms.
For instance; the compound is table salt or NaCl. Table salt is a mixture of two separate elements the first is sodium which is a highly reactive metal and the second is chlorine which is a toxic gas.
When they react, the given atoms combine in a ratio of 1:1 to form white crystals thus forming table salt.
• A chemical reaction always results in the rearrangement of atoms of a given compound in the reactant and product formed.
In this part of Dalton’s atomic theory, he suggested that chemical reactions don’t terminate or create atoms. They just reorganize the atoms of compounds.
• Atoms of the same element usually combine to form two or more compounds in more than one ratio.
• The atom is thus the tiniest unit of matter and takes part in a different chemical reaction.
Advantages of Dalton Atomic Theory
• The three given laws were not exploited by Dalton; The law of multiple proportions, the law of conservation of mass, and the law of constant proportions
• The Dalton Atomic Theory also provides a basis to distinguish between elements and compounds.
Limitations of Dalton Atomic Theory
• Dalton atomic theory does not account for subatomic particles. Dalton atomic theory proposed that the atoms were undividable. Though, the discovery of subatomic particles (for instance, protons, electrons, and neutrons) refuted this postulate. Of Dalton.
• This theory fails to explain the existence of isotopes: Dalton atomic theory stated that all atoms present in the given element have identical masses and densities. Though, this postulate was disapproved as different isotopes of elements have different atomic masses
For instance: isotopes of hydrogen are; hydrogen, deuterium, and tritium.
• Dalton atomic theory fails to explain isobars also. This theory of Dalton states that the masses of the given atoms of two different elements must vary. Thus, two different elements can have the same mass number. Such atoms are known as isobars.
For Instance: 40Ar and 40Ca.
• This theory of Dalton does not account for allotropes as well.
For instance; the differences in the properties of diamond and graphite, containing a single atom of carbon, cannot be described by Dalton’s atomic theory.
In spite of of these drawbacks, Dalton atomic theory is generally true, and it forms the basis of modern chemistry.
Dalton Atomic Theory Citations
- John Dalton and the origin of the atomic theory: reassessing the influence of Bryan Higgins. Br J Hist Sci . 2017 Dec;50(4):657-676.
- A letter signed: the very beginnings of Dalton’s atomic theory. Ambix . 2010 Nov;57(3):301-10.
- Atomic Theory and Multiple Combining Proportions: The Search for Whole Number Ratios. Ann Sci . 2015 Apr;72(2):153-69.