The assignment of the electron with a negative electric charge took place in the late 19th and early 20th centuries through experimental investigations. The discovery of the electron is credited to J.J. Thomson in 1897 through his cathode ray tube experiments, which demonstrated the presence of negatively charged particles.
Initially, when the electron was discovered, there was no established convention for assigning electric charges. At that time, it was not clear whether the electron or the proton was the fundamental particle responsible for generating electricity. Some scientists believed that the positive charge was carried by particles smaller than atoms, while others proposed that the negative charge was carried by such particles.
However, further investigations and experiments, including those conducted by Thomson and others, provided compelling evidence that the negatively charged particles observed in cathode ray tubes were fundamental constituents of matter. These particles were subsequently named electrons.
The choice of assigning a negative charge to the electron was arbitrary and primarily based on convention. Once the electron was established as a negatively charged particle, the convention was established that positive charges would be attributed to protons, and negative charges would be attributed to electrons.
It's important to note that the terms "positive" and "negative" in the context of electric charge are simply labels and don't imply any inherent positive or negative nature of the particles themselves. The assignment of charges was based on experimental observations and subsequent conventions that developed in the scientific community.