Determining the number of electrons, protons, and neutrons in atoms in the past relied on the development of various experimental techniques and theoretical models. Here's a brief overview of how scientists made these determinations:
Electrons: The discovery of the electron by J.J. Thomson in 1897 through his cathode ray tube experiments provided the first evidence of negatively charged particles in atoms. Subsequent experiments, such as Robert Millikan's oil drop experiment, allowed scientists to determine the charge and mass of the electron.
Protons: The existence of positively charged particles in the atomic nucleus was postulated by Ernest Rutherford in 1911. Rutherford's gold foil experiment involved firing alpha particles at a thin sheet of gold foil and observing their scattering patterns. The results led to the conclusion that most of the atom's mass and positive charge is concentrated in a small, dense nucleus at the center.
Neutrons: The discovery of the neutron took place in 1932 by James Chadwick. Chadwick conducted experiments involving the bombardment of beryllium with alpha particles, which led to the emission of uncharged particles. These neutral particles, later identified as neutrons, were found to have a mass similar to protons but without a charge.
It's important to note that the atomic number (number of protons) was not initially known for all elements, but the discoveries and experimental techniques mentioned above gradually revealed the presence of subatomic particles and their roles within atoms. The concept of atomic number was introduced by Henry Moseley in 1913, who proposed that the elements should be arranged based on the number of protons in their nuclei. This understanding eventually led to the development of the modern periodic table and the identification of each element's atomic number and arrangement.