Although modified to perform a different task, a horse's front leg is made up of the same bones as is your arm. this suggests a common ancestor between horses and humans that also had those bones in its forelimb. the horse leg and human arm are therefore considered to be structures.

I believe the horse leg and human arm are therefore considered to be Homologous structures. 
Homologous structures are structures with similar ancestries and common traits, however; they have different functions. For example the horse leg and human arm have similar origin but have different functions. On the other hand; analogous structures have different evolutionary ancestries but they have the same functions. Examples of these are the wings of birds, bats and insects. 

That would be a working theory.

The proton has a positive charge while the neutron has no charge they both make up the nucleus of the atom while the elections have a negative charge and surround the nucleus of the atom. the protons and electrons are used to calculate the atom's mass whereas the neutrons don't affect the mass