Question

Identify the postulates of Dalton's atomic theory which are no longer accepted by the modern knowledge of nature of matter.

Answer 1

Postulates of Dalton’s Atomic Theory

• All matter is made up of tiny, indivisible particles called atoms.
• All atoms of a specific element are identical in mass, size, and other properties. However, atoms of different elements exhibit different properties and vary in mass and size.
• Atoms can neither be created nor destroyed. Furthermore, atoms cannot be divided into smaller particles.
• Atoms of different elements can combine with each other in fixed whole-number ratios in order to form compounds.
• Atoms can be rearranged, combined, or separated in chemical reactions.

The limitations of Dalton’s Atomic Theory that are not accepted by modern knowledge of the nature of matter.

• It does not account for subatomic particles: Dalton’s atomic theory stated that atoms were indivisible. However, the discovery of subatomic particles (such as protons, electrons, and neutrons) disproved this postulate.
• It does not account for isotopes: As per Dalton’s atomic theory, all atoms of an element have identical masses and densities. However, different isotopes of the same elements have different atomic masses (For example; protium, deuterium, and tritium are isotopes of Hydrogen and have different masses).
• It does not account for isobars: This theory states that the masses of the atoms of two different elements must differ. However, it is possible for two different elements to share the same mass number. Such atoms are called isobars (Example: ⁴⁰Ar and ⁴⁰Ca).
• Elements need not combine in simple, whole-number ratios to form compounds: Certain complex organic compounds do not feature simple ratios of constituent atoms. Example: sugar/sucrose (C₁₁H₂₂O₁₁).
• The theory does not account for allotropes: The differences in the properties of diamond and graphite, both of which contain only carbon, cannot be explained by Dalton’s atomic theory.