Why did scientists keep modifying atomic models? [Pg. No. 140]

Scientists kept modifying atomic models because new experimental discoveries showed that older models could not explain all observations.

Dalton's model treated atoms as indivisible, but the discovery of electrons, protons and neutrons showed that atoms have subatomic particles.

Thomson's model explained electrons but could not explain Rutherford's alpha-particle scattering experiment, which showed the presence of a small, dense, positively charged nucleus. Rutherford's model explained the nucleus but failed to explain the stability of atoms and line spectra.

Bohr then modified the model by proposing that electrons revolve around the nucleus only in certain fixed energy levels or shells, which explained atomic stability and the hydrogen spectrum.

Later, Bohr's model was also improved because it could not fully explain more complex atoms. Thus, atomic models were changed step by step as new evidence gave a better understanding of atomic structure.