Introduction to atom models

Long before anyone could see an atom, chemists needed a working picture of what it was made of. The name itself comes from the Greek a-tomio, "non-divisible" - for centuries atoms were believed to be the smallest, indivisible units of matter. That belief broke down once experiments began revealing particles smaller than the atom itself: the electron, the proton and the neutron.

J.J. Thomson's model. Thomson's cathode-ray experiments showed that atoms contain negatively charged electrons. To keep the atom electrically neutral overall, he proposed that an atom is a uniform sphere of positive charge with electrons embedded inside it - popularly remembered as the "watermelon model," with the electrons playing the role of seeds scattered through the positively charged pulp.

Rutherford's gold-foil experiment. Rutherford tested Thomson's model by firing a stream of fast-moving, positively charged alpha particles at an extremely thin gold foil and recording where they went afterward (Figure 2.1). Three results came out of this:

1. The overwhelming majority of the alpha particles passed straight through the foil as if nothing were there.
2. A small fraction were deflected through a noticeable angle.
3. A very small fraction - roughly 1 in 8000 - bounced almost straight back, deflected by close to 180°.

Thomson's spread-out positive charge could never produce that third result: a diffuse cloud of positive charge could only ever nudge an alpha particle slightly, never reverse its direction. Rutherford concluded instead that an atom's positive charge and almost all of its mass are concentrated in an extremely small, dense nucleus at the centre, with the electrons revolving around it at high speed through what is otherwise mostly empty space. Most alpha particles pass through the large empty regions untouched; only the rare ones aimed almost directly at a nucleus feel enough repulsion to bounce back. …