Early models treated atoms as indivisible. Discovering electrons showed internal structure. A later model placed electrons within diffuse positive charge. Alpha scattering instead supported a mostly empty atom with a small, dense, positive nucleus: most particles passed through foil, while a few were strongly deflected.
Rutherford’s later identification of protons explained positive nuclear charge. Chadwick’s neutron discovery in 1932 added a neutral nuclear particle with roughly a proton’s mass. Neutrons explained why isotopes can have the same nuclear charge but different mass numbers: they share proton counts but differ in neutrons. Electrons occupy regions around the nucleus; simple shell diagrams do not show exact paths. A neutral atom has equal proton and electron counts.
Example 1
Most alpha particles pass through foil. A few turn sharply. Which two atomic features explain this?
Passage through the foil suggests most atomic volume is empty space. Rare large deflections suggest a small, dense, positively charged nucleus repels the positive alpha particles.
Example 2
Why was discovering electrons a problem for the indivisible-sphere model?
Electrons are components smaller than an atom. An atom therefore has internal structure, contradicting the claim that it has no smaller parts.
Watch out: It is a simplified counting model. Electrons are not tiny planets following the drawn circles.