Q.Fill in the blank: Neutron was discovered by ____________.

The neutron was discovered late mainly because it is electrically neutral, so it does not respond to electric or magnetic fields the way the electron and proton (both charged) do, making it invisible to the techniques (cathode-ray/canal-ray deflection experiments) that had already revealed the electron and proton decades earlier.

The electron (J.J. Thomson, 1897) and the proton (via canal rays / Goldstein, later characterised by Rutherford) were both discovered using discharge-tube experiments where the particles' paths bend in applied electric and magnetic fields because they carry charge — this bending is exactly what let scientists infer their charge-to-mass ratio and existence.

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James Chadwick discovered the neutron in 1932, by bombarding beryllium with alpha particles and showing the emitted radiation consisted of neutral particles of about the mass of a proton.

This is a standard physics-history fact rather than a numerical derivation. Chadwick's experiment resolved the puzzle of why atomic mass exceeded what protons and electrons alone could account for, establishing the existence of a neutral nuclear particle -- the neutron …