Electron, proton and neutron: how they were discovered (beams in electric and magnetic fields)

Shoot electrons, protons, alpha particles and neutrons through electric and magnetic fields like in Thomson's cathode ray tube and see which ones bend, which way and by how much. This is how the electron, the proton and the neutron were identified. Free for every student.

Also searched as: subatomic particles · discovery of electron proton neutron · cathode ray experiment · canal rays · chadwick neutron discovery · thomson experiment

About this experiment

Aim

To see how particles of different charge and mass behave in electric and magnetic fields, and how this led to the discovery of the electron, the proton and the neutron.

Theory

In a gas discharge tube at very low pressure, rays come out of the cathode and make the glass glow. J. J. Thomson (1897) showed that these cathode rays are streams of negatively charged particles, which he called electrons. They bend towards the positive plate in an electric field and are also bent by a magnetic field, and the ratio of charge to mass is the same whatever gas or metal is used: e/m = 1.76 × 10¹¹ C/kg. So the electron is a part of every atom.

Goldstein (1886) found that a tube with a perforated cathode also shows rays that move the other way. These canal rays are positive ions of the gas. They bend the opposite way to cathode rays and bend much less, because they are far heavier. The lightest of them, from hydrogen, has the smallest mass: it is the proton (found by Rutherford to be present in all nuclei).

In 1932 James Chadwick bombarded beryllium with alpha particles and found a radiation that was not bent by an electric or a magnetic field. It could knock protons out of paraffin wax, so it had to be made of heavy particles with no charge: neutrons. A neutral particle feels no force from either field.

Formulas used

• kinetic energy after acceleration: ½mv² = qV, v = √(2qV/m)
• electric force F = qE, acceleration a = qE/m
• magnetic force F = qvB (at right angles to v)
• radius of the path in a magnetic field r = mv/(qB) = √(2mV/q) / B
• straight beam when qE = qvB, v = E/B
• e/m of the electron = 1.76 × 10¹¹ C/kg; m_p/m_e = 1836

Procedure: what to do in the virtual lab

1. Choose the electron with E = 6 kV/m and B = 0. Note that the beam bends towards the positive plate.
2. Choose the proton. Which way does it bend? Compare the size of the bend with the electron (with only the electric field).
3. Switch off E and switch on B. Compare the bending of the electron and the proton. Why is the proton bent so little?
4. Use both fields so that the beam goes straight for the electron. Read the speed v and check E/B = v.
5. Choose the neutron and try every setting. What do you see? How can a particle with no charge be detected?

Observations to make

• The electron and the proton bend in opposite directions in the same field, so they have opposite charges.
• For the same accelerating voltage, the electric deflection of the electron and the proton is the same size but opposite, but their magnetic deflections are very different because of the mass.
• The neutron always goes straight to the same spot.
• If the field is too strong the beam hits a plate and never reaches the screen.

What you can change in this experiment

• Particle in the beam: Electron (cathode ray) / Proton (canal ray, H⁺) / Alpha particle (He²⁺) / Neutron
• Accelerating voltage (V)
• Electric field between the plates (kV/m, + means field points up)
• Magnetic field (mT, + means out of the screen)

Viva questions on Electron, proton and neutron

Questions an examiner may ask in the Class 11 chemistry practical. Run the experiment in the lab to find the answers yourself, then check them in the Learn tab.

1. How did Thomson show that cathode rays are negatively charged particles?
2. What are canal rays and who discovered them?
3. Why was the neutron discovered so late?
4. What is the value of e/m for the electron?
5. What is the condition for a beam to pass straight through crossed E and B fields?

Frequently asked questions

Syllabus: Class XI Chemistry, Chapter 2 — Structure of Atom, §2.1 (discovery of electron, proton and neutron)

What you do in the lab

Change the controls and watch the chemistry behave in real time in 3D. Live readouts and graphs show the numbers behind it, and the Learn tab has the theory, procedure, observations and viva questions. Works on a phone or a laptop, in English and Hindi.

Topics: discovery of electron · cathode rays · canal rays · proton · neutron · Thomson · Goldstein · Chadwick · charge to mass ratio · structure of atom

More from this unit

• Build an atom: protons, neutrons and electrons
• Electron clouds: shapes of s, p and d orbitals in 3D
• Schrödinger wave equation: wave function, probability and quantised energy

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