The concept of an electron being considered a point particle and a proton not being considered a point particle arises from our current understanding of their fundamental properties based on experimental observations and theoretical models.
According to the Standard Model of particle physics, which is the prevailing theory describing elementary particles and their interactions, electrons are considered point particles. Point particles are theoretical objects with no spatial extent, meaning they are treated as having zero size and are localized to a single point in space. This concept is used to simplify calculations and models in particle physics.
Electrons are classified as elementary particles, which means they are not composed of smaller constituent particles. They are believed to be indivisible and have no internal structure. Experimental measurements of the electron's properties, such as its charge and magnetic moment, support the idea that it is a point-like particle.
On the other hand, protons are composite particles made up of three quarks bound together by the strong nuclear force. Quarks are considered elementary particles, but protons, being composed of three quarks (two up quarks and one down quark), have a spatial extent due to the interactions between the quarks. The size of a proton is typically described using the concept of the proton's charge radius, which measures its distribution of charge.
Experiments such as electron scattering and high-energy physics colliders provide evidence for the internal structure of protons. These experiments have observed the scattering of electrons off protons and have revealed the spatial distribution of charge within the proton, indicating that it is not a point-like particle.
In summary, electrons are treated as point particles because they are considered elementary particles with no known substructure. Protons, on the other hand, are composite particles made up of quarks and have a spatial extent due to their internal structure. Our understanding of these particles is based on experimental evidence, theoretical models, and the framework of the Standard Model of particle physics.