Chapter 4 · Atoms and Elements
4.3The Nuclear Atom
9 min · two checks
Predict
If an atom is mostly empty space, why did a few alpha particles bounce backward from gold foil?
The idea
- Describe Rutherford’s conclusion from the gold-foil experiment.
- Place protons, neutrons, and electrons in the atom.
Thomson’s cathode-ray work had already shown that atoms contain electrons, light negative particles. A soft blob of positive pudding with electrons stuck in it was a reasonable next picture, and it was wrong. Rutherford’s students fired alpha particles at thin gold foil. Nearly all passed through. A few deflected at large angles. That only fits a model in which positive charge and nearly all the mass sit in a very small nucleus, with electrons occupying the surrounding volume.
The nucleus contains protons (charge +1, substantial mass) and neutrons (charge 0, similar mass). Electrons (charge −1, mass about 1/1800 of a proton) occupy the space outside. The atom is electrically neutral when the number of electrons equals the number of protons. Almost all the volume is the electron region; almost all the mass is the nucleus. “Mostly empty” does not mean “nothing there chemically” — bonding happens in that electron region.
Keep these
- The nucleus is tiny, massive, and positive.
- Protons and neutrons live in the nucleus; electrons do not.
- Neutral atom: number of electrons equals number of protons.