Dalton's Atomic Theory

What is Atomic Theory?

Dalton’s atomic theory was a scientific theory on the nature of matter put forward by the English physicist and chemist John Dalton in the year 1808. It stated that all matter was made up of small, indivisible particles known as ‘atoms’.

All substances, according to Dalton’s atomic theory, are made up of atoms, which are indivisible and indestructible building units. While an element’s atoms were all the same size and mass, various elements possessed atoms of varying sizes and masses.

• 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 to form compounds.
• Atoms can be rearranged, combined, or separated in chemical reactions.

• 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 elements have different atomic masses (For example, hydrogen, deuterium, and tritium).
• 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 that two different elements can 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: Dalton's atomic theory cannot explain the differences in the properties of diamond and graphite, which contain only carbon.

1. Dalton's atomic theory does not violate The law of multiple proportions, the law of conservation of mass, and the law of constant proportions.
2. The theory provides a basis to differentiate between elements and compounds.