Physics · Ch 8 — Atomic and Nuclear Physics
J.J. Thomson's Model (Water melon model)
J.J. Thomson's Model (Water melon model)
J.J. Thomson's 1904 atom model - nicknamed the "watermelon model" - pictures the atom as a homogeneous sphere of uniformly distributed positive charge, with the negatively charged electrons embedded inside it much like seeds studded throughout the flesh of a watermelon. The atom as a whole is electrically neutral because the total positive charge spread through the sphere exactly equals the total negative charge carried by the embedded electrons.
In this model, all the charges are assumed to be sitting at rest in fixed positions. This assumption is fatal: classical electrostatics (specifically, a result known as Earnshaw's theorem) proves that no configuration of static point charges can ever be in stable equilibrium under purely electrostatic forces alone. So a Thomson-model atom, built entirely from charges held motionless by electrostatic forces, cannot actually be stable - it offers no mechanism to hold its own structure together. …
What this figure shows. This figure visually motivates the name of Thomson's model by placing an illustration of the proposed atomic structure next to a cut watermelon: in part (a), the atom is drawn as a sphere of uniformly distributed positive charge (the 'pulp') with small negative electrons (the 'seeds') studded throughout it, held electrically neutral overall since total positive charge equals total negative charge, and in part (b) the real watermelon cross-section is shown for direct visual comparison. The figure also carries the scale label of about 10^-10 m for the size of the whole sphere, reminding the reader that even in this earliest model the atom's size was already correctly estim …