Dalton’s Atomic Theory
Matter as we know is one of the most important subject of research for scientist. Democritus was the first man who proposed that matter is made up of particles. He named these particles, ‘atomos’ meaning indivisible
John Dalton (1766-1844) is the scientist credited for proposing the atomic theory. This theory explains several concepts that are relevant in the observable world. In 1808, he postulated the famous Dalton’s atomic theory.
Let us take a look at this theory
Postulates of Dalton’s Atomic Theory
- The matter is made up of indivisible particles known as atoms.
- All atoms of same element have identical mass while the atoms of different elements have different masses.
- Atoms of different elements combine in fixed ratios to form compounds.
- Atoms are neither be created nor destroyed.
- The formation of new products (compounds) results from the rearrangement of existing atoms (reactants).
- Atoms of an element are identical in mass, size and many other chemical or physical properties, but atoms of two-different elements differ in mass, size, and many other chemical or physical properties.
However this theories was unable to explain some certain phenomenon and therefore has limitations,
Limitations of Dalton’s Atomic Theory of Matter
- According to Dalton, Atom is indivisible. It has been proved that atom can be subdivided into electrons, protons and neutrons.
- According to Dalton Atomic Theory, atoms of an element are identical in mass, size and many other chemical or physical properties. But, practically we observe that atoms of several elements differ in their densities and masses. These atoms with the different masses are known as isotopes. For example, Chlorine (Cl) has 2 isotopes with the mass numbers of 35 and 37.
- Also, according to Dalton Atomic Theory, atoms of two-different elements differ in mass, size and many other chemical or physical properties. However, this is not correct for all situations. For example, Argon (Ar) and Calcium (Ca) atoms, each have an atomic mass of 40 amu. These atoms with similar atomic masses are isobars.
- According to Dalton Atomic Theory, when atoms of different elements (atoms of two or more elements) combine in simple whole number ratios, we get chemical compounds. But this is not true in case of complex organic compounds.
- Dalton Atomic Theory fails to explain the existence of allotropes. This implies that the Dalton atomic theory fails to explain the differences in properties of charcoal, graphite, and diamond (allotropes of carbon).
- Dalton’s Atomic Theory also suggested that an atom is the smallest part of an atom that can take part in a chemical reaction. Some postulates of this theory remain valid even in today’s modern chemical thoughts. The atomic structure model proposed by indeed proves to be a significant, stepping stone in chemistry. It forms the base for modern atomic theories and quantum mechanics.
Laws for Dalton’s Atomic Theory
- Law of Conservation of Mass
This state that matter is neither created nor destroyed. This means, in a chemical reaction, the total mass of the reactants is equal to that of the product.
This law is used in balancing chemical equations
- Law of Constant Composition
This state that a pure compounds of the same elements are combined in the same proportion. For example, table salt with the molecular formula of NaCl holds the same proportions of the elements Na and Cl. It doesn’t matter where the salt comes from it will always be in the same proportion.
- Dalton’s atomic theory was the first complete attempt to describe all matter in terms of atoms and their properties.
- Dalton based his theory on the law of conservation of mass and the law of constant composition.
- The first part of his theory states that all matter is made of atoms, which are indivisible.
- The second part of the theory says all atoms of a given element are identical in mass and properties.
- The third part says compounds are combinations of two or more different types of atoms.
- The fourth part of the theory states that a chemical reaction is a rearrangement of atoms.
- Parts of the theory had to be modified based on the existence of subatomic particles and isotopes. This modification is what brings about The Modern Atomic Theory