11 min readConcept sheetSection A MCQ (1 mark) and objective (2 marks, two parts); Section B short answer (3 marks) or long answer (5 marks): cathode rays and their properties, often with a choice

Discovery of the electron and the proton

Cathode rays showed that every atom holds tiny negative electrons. Canal rays from a perforated cathode revealed positive particles; the lightest is the proton.

ElectronNegative particle present in all atoms; shown by J. J. Thomson in 1897 from cathode rays; mass 9.109 × 10⁻³¹ kg

ProtonPositive particle; the lightest canal-ray particle, from hydrogen gas; mass 1.673 × 10⁻²⁷ kg, about 1840 times that of an electron

ChargeElectron −1.602 × 10⁻¹⁹ C, proton +1.602 × 10⁻¹⁹ C: equal in size, opposite in sign

Canal raysPositive rays found by E. Goldstein in 1886, passing through holes in a perforated cathode

The atom can be divided

Dalton's atomic theory (1803) said the atom is the smallest particle of matter and cannot be divided. Experiments near the end of the 1800s proved this wrong. Atoms contain still smaller subatomic particles: the negative electron, the positive proton and the neutral neutron. Radioactivity, in which some atoms give out rays on their own, also showed that atoms can break up.

Cathode rays: how the electron was found

1
Take a glass tube with two metal plates (electrodes) sealed in it, and pump out most of the air. This is a cathode ray tube, or discharge tube.
2
Join the plates to a high voltage. The negative plate is the cathode; the positive plate is the anode.
3
A stream of rays leaves the cathode and travels to the anode. These are cathode rays.
4
Place two charged plates beside the beam. The rays bend towards the positive plate.
So the rays carry negative charge.
5★
J. J. Thomson (1897) showed the rays are a stream of tiny negative particles, later named electrons.
6
The rays were the same whatever gas or electrode metal was used.
So electrons are present in all atoms.

★ key step

Properties of cathode rays

They travel in straight lines from the cathode to the anode.
They are made of particles that have mass and kinetic energy.
The particles are very light and move very fast.
They carry negative charge, so they bend towards a positive plate in an electric field.
Their nature does not depend on the gas in the tube or the metal of the electrodes. The charge to mass ratio (e/m) is always the same.

Canal rays: how the proton was found

1
E. Goldstein (1886) used a tube with gas at very low pressure and a perforated cathode (a cathode with holes in it).
2
With a high voltage on, a faint red glow appeared behind the cathode.
3
It came from rays moving opposite to cathode rays and passing through the holes (canals). They were called canal rays, anode rays or positive rays.
4
Their origin: fast electrons from the cathode knock electrons out of gas atoms. The positive ions left behind rush towards the cathode.
5★
With hydrogen gas, the particles were the lightest, with the highest e/m. Rutherford showed they are hydrogen ions, H⁺, and they were named protons.

★ key step

Cathode rays and canal rays compared

Feature	Cathode rays	Canal (anode) rays
Move towards	The anode	The cathode, and through its holes
Charge	Negative	Positive
Mass of the particles	Very small (electrons)	Much larger (ions); the lightest is the proton
Depend on the gas in the tube?	No, always electrons	Yes, different gases give different particles
Charge on each particle	One electronic charge, e	A whole-number multiple of e

How to draw the cathode ray tube

Draw a long glass tube, closed at both ends. Label it glass tube (gas at low pressure).
Draw a metal plate at the left end labelled cathode (−) and one at the right end labelled anode (+).
Join both plates by wires to a box labelled high voltage source.
Draw a straight arrow from cathode to anode, labelled cathode rays (electrons).
For the electric field, draw a + plate above the beam and a − plate below it, and bend the arrow up towards the + plate.
For canal rays, draw holes in the cathode and arrows coming out behind it, labelled canal rays and red glow.

In real life

Old television sets and computer monitors were cathode ray tubes: a beam of electrons struck a coated screen to make the picture. The glowing sign tubes in Indian markets work by a similar discharge through a gas.

Exam angle

1 marks: “Name the scientist who discovered canal rays.” E. Goldstein (1886).

3 marks: “Explain how the cathode ray experiment showed (a) that cathode rays are negatively charged and (b) that all atoms contain electrons.” Set-up: two electrodes in a glass tube with most of the air removed, joined to a high voltage; rays go from cathode to anode (1). (a) In an electric field the rays bend towards the positive plate, so they are negative (1). (b) The rays and their e/m stayed the same whatever gas or electrode metal was used, so these particles must be part of every atom (1).

2 marks: “How were protons identified from canal rays?” Canal rays are positive ions formed when electrons knock electrons out of gas atoms. With hydrogen in the tube, the particles were the lightest, with the largest e/m. They were shown to be H⁺ ions and named protons.

Remember it

Cathode rays: from the Cathode, Charge negative. Canal rays: through the Canals, charge positive.