Discovery of subatomic particles and Rutherford's model

Chemistry · Atomic structure. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Chemistry topics

The idea

Passing electricity through a gas at very low pressure in a discharge tube gave the first evidence of particles smaller than atoms. Cathode rays travel from the cathode and are streams of electrons. Positive rays (canal rays) pass through holes in a perforated cathode and are positive ions of the gas in the tube.

J. J. Thomson measured the charge to mass ratio (e/m) of the electron, and Millikan measured its charge using charged oil drops. Together these give its mass. Chadwick discovered the neutron in 1932.

Rutherford fired α-particles at a thin gold foil. Most went straight through, a few were deflected and very few bounced back. So the atom is mostly empty space with a tiny, dense, positive nucleus.

Rules to remember

Cathode rays: travel in straight lines and cast sharp shadows; are negatively charged (bend towards the positive plate); turn a paddle wheel (they have momentum); make glass glow; produce X-rays when they strike a heavy metal target.
e/m of the electron = 1.7588 × 10¹¹ C kg⁻¹. It is the same for every gas and every cathode material, so electrons are present in all atoms.
Positive rays: e/m depends on the gas in the tube and is always much smaller than for cathode rays. It is greatest for hydrogen, whose positive particle is the proton.
Millikan: charge on an electron = 1.602 × 10⁻¹⁹ C. Mass of electron = e ÷ (e/m) = 9.11 × 10⁻³¹ kg.
Neutron (Chadwick, 1932): ⁹Be + ⁴He → ¹²C + ¹n. No charge, so not deflected by electric or magnetic fields; mass 1.6749 × 10⁻²⁷ kg, slightly more than a proton.
Defects of Rutherford's model: a revolving electron should radiate energy continuously and spiral into the nucleus, and the atom should give a continuous spectrum. Real atoms are stable and give line spectra.

Common mistake

Students give Thomson's and Millikan's results the wrong way round. Thomson measured e/m; Millikan measured the charge e. The mass of the electron comes from dividing e by e/m, not multiplying.

Worked example

In an oil drop experiment, one drop is found to carry a charge of 4.806 × 10⁻¹⁹ C. How many extra electrons are on the drop? (e = 1.602 × 10⁻¹⁹ C)

The charge on any drop is a whole number of electron charges: charge = n × e.
n = 4.806 × 10⁻¹⁹ ÷ 1.602 × 10⁻¹⁹ = 3.

Answer: The drop carries 3 extra electrons.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

Chapter 5: Atomic Structure

Keep going

← Crystalline solids Planck's theory, Bohr's model and the hydrogen spectrum →Take a Chemistry test