Atomic Models and Isotopes

Building the Atom from Evidence

Every atomic model was killed by an experiment it could not explain. Dalton (1803) treated atoms as indivisible spheres, which failed the moment Thomson (1897) pulled electrons out of them with cathode rays, measuring a charge-to-mass ratio nearly 2000 times larger than any ion. Thomson's "plum pudding" spread positive charge thinly through the whole atom.

That model died in Rutherford's gold-foil experiment (1911). Firing alpha particles at foil, almost all passed straight through, but roughly 1 in 8000 bounced back. Diffuse charge cannot repel a fast alpha particle; only a dense, tiny, positive nucleus can. Rutherford put essentially all the mass into a volume about $10^{-5}$ times the atom's diameter. Chadwick (1932) completed the picture with the neutron.

Two numbers identify a nucleus: the atomic number $Z$ (protons, this alone fixes the element) and the mass number $A$ (protons plus neutrons). Isotopes are atoms of one element with different $A$, so they differ in neutron count and mass but share $Z$ and therefore essentially all chemistry.

The atomic mass on the periodic table is an abundance-weighted average, not any one isotope's mass:

$$M = \sum f_i m_i$$

Chlorine is 75.77 % $^{35}$Cl and 24.23 % $^{37}$Cl, giving $0.7577 \times 35 + 0.2423 \times 37 = 35.48$ u, which is why no chlorine atom weighs what the table says.

Common pitfall: reading the periodic table's mass as the mass of one atom. It is a population average; an individual atom always has a near-integer mass number, and the fractional table value tells you about isotope abundances, not about any single atom.