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Step-by-step explanation:
The atomic model before Chadwick
J. J. Thomson had discovered the electron in 1897 and Rutherford discovered the proton in 1912.
So, in the 1920s, scientists thought that a helium atom (₂⁴He), for example, consisted of four protons, two electrons orbiting the nucleus, and another two electrons in the nucleus to balance the charge.
In 1932, Irène and Frédérick Joliot-Curie discovered that when α particles hit a Be target, a strange radiation was emitted that could knock protons out of paraffin. They thought it was some type of high-energy X-rays
Chadwick's experiment
Chadwick thought the radiation consisted of particles, not X-rays. He repeated the Curie's experiment and carefully measured the velocities of the scattered protons.
Using the Laws of Conservation of Energy and Momentum, he calculated that the radiation hitting the paraffin must consist of neutral particles with a mass slightly greater than that of a proton.
Chadwick had discovered the neutron.
The atomic model after Chadwick
Chemistry now made more sense. A helium atom now consisted of two electrons orbiting a nucleus containing two protons and two neutrons. There was no need to have electrons in the nucleus.
