Question

Like atoms are identical in all respects. This statement of Dalton's atomic theory is contradicted. Which discovery contradicts this?

Answer 1

1. The Dalton atomic theory was given by John Dalton in 1808.
2. The postulates of Dalton's atomic theory are-

1. All matter is made up of tiny and indivisible particles called atoms.
2. Atoms can neither be created nor be destroyed during any physical or chemical change.
3. All atoms of a particular element are the same in terms of size, mass, and properties.
4. Atoms of different elements are different in terms of size, mass, and properties.
5. Atoms of different elements combine in a definite simple and whole number ratio to form a compound.
6. In a chemical reaction, atoms are the tiniest particles that participate.

3. As per Dalton's atomic theory, all atoms of a particular element are identical in terms of size, mass, and properties, but after the discovery of isotopes, this statement becomes invalid.

4. Isotopes are the atoms of the an element with same atomic number but a different mass number (or a different number of neutrons). Example: Protonium (${}_1{}^1\mathrm{H}$), deuterium (${}_1{}^2\mathrm{H}$), tritium (${}_1{}^3\mathrm{H}$). Thus, these are all the atoms of hydrogen but they are different in terms of their size, mass and properties.

5. Hence, the discovery of isotopes contradicts 'Like atoms are identical in all respects.