Dalton thought the atom was the smallest, indivisible particle of matter. By the end of the nineteenth century, experiments with electricity passing through gases proved him wrong — the atom is itself made of even smaller subatomic particles: the electron, the proton and the neutron.
The electron — J. J. Thomson (1897)
While studying the discharge of electricity through gases at very low pressure (cathode-ray experiments), J. J. Thomson discovered a stream of negatively charged particles called electrons. The electron carries a unit negative charge (−1) and is extremely light — its mass is about 1/1840 the mass of a hydrogen atom, so it is taken as nearly negligible (0 u).
The proton — E. Goldstein (1886)
Using a discharge tube with a perforated cathode, E. Goldstein detected positively charged rays called canal rays. These led to the discovery of the proton, a particle carrying a unit positive charge (+1) and a mass of about 1 u (nearly equal to a hydrogen atom).
The neutron — J. Chadwick (1932)
James Chadwick discovered the neutron, a particle with no charge (neutral) and a mass of about 1 u, almost the same as a proton. Protons and neutrons together are called nucleons.
Thomson's model — the plum-pudding atom
Thomson proposed that an atom is a positively charged sphere in which electrons are embedded, like seeds in a watermelon or plums in a pudding. The positive and negative charges are equal, so the atom as a whole is electrically neutral.
Rutherford's alpha-scattering experiment
Ernest Rutherford bombarded a very thin gold foil with fast-moving, positively charged alpha (α) particles. He observed:
- Most α-particles passed straight through the foil undeflected.
- A few were deflected through small angles.
- A very few (about 1 in 12,000) bounced almost straight back.
From these observations he concluded that most of an atom is empty space; the entire positive charge and nearly all the mass are concentrated in a very small, dense centre called the nucleus; and the electrons revolve around this nucleus. This is the nuclear model.
Limitation of Rutherford's model
An electron moving in a circular orbit is accelerating, and an accelerating charge must radiate energy. The electron would lose energy, spiral inward and crash into the nucleus, making the atom unstable — but atoms are stable. Rutherford's model could not explain this.
Name the three subatomic particles, and state the charge and approximate mass of each.
Solution- The electron carries charge −1 and has mass ≈ 1/1840 u (taken as nearly 0).
- The proton carries charge +1 and has mass ≈ 1 u.
- The neutron carries no charge (0) and has mass ≈ 1 u.
Answer: Electron (−1, ≈0 u), proton (+1, ≈1 u), neutron (0, ≈1 u).
Who discovered the electron, the proton and the neutron?
Solution- The electron was discovered by J. J. Thomson from cathode-ray (discharge tube) experiments.
- The proton followed from the canal rays observed by E. Goldstein.
- The neutron was discovered by James Chadwick.
Answer: Electron — Thomson; proton — Goldstein; neutron — Chadwick.
Why does an atom as a whole show no electric charge?
Solution- An atom contains equal numbers of protons and electrons.
- Each proton carries +1 and each electron carries −1.
- The total positive charge therefore exactly cancels the total negative charge.
Answer: Equal positive (protons) and negative (electrons) charges cancel, so the atom is electrically neutral.
State the three observations of Rutherford's α-scattering experiment and what each tells us.
Solution- Most α-particles passed straight through → the atom is mostly empty space.
- A few were deflected by small angles → the centre has a concentrated positive charge.
- Very few bounced back → the centre (nucleus) is very small and dense with almost all the mass.
Answer: The atom is mostly empty, with a tiny dense positively charged nucleus.
Compare Thomson's and Rutherford's models of the atom in one line each.
Solution- Thomson: a positively charged sphere with electrons embedded in it (plum-pudding), positive charge spread throughout.
- Rutherford: a tiny dense central nucleus holding all the positive charge and mass, with electrons revolving around it.
- The main difference is where the positive charge sits — spread out (Thomson) versus concentrated in a nucleus (Rutherford).
Answer: Thomson spread the positive charge; Rutherford concentrated it in a central nucleus.
What was the main drawback of Rutherford's nuclear model?
Solution- An electron revolving in an orbit is accelerating.
- According to classical physics, an accelerating charged particle continuously radiates energy.
- Losing energy, the electron would spiral into the nucleus, so the atom would collapse — yet atoms are stable.
Answer: It could not explain the stability of the atom, since revolving electrons should lose energy and fall into the nucleus.