• Matter of Size:
At the simplest level, you can think of protons and neutrons as each being made up of three quarks held together by the strong nuclear force.
In quantum mechanics, the quarks don't occupy a particular point in space, but are "smeared out" over an area. Also the strong nuclear force actually manifests itself as an ever-changing cloud of "virtual" gluons, quarks, and antiquarks surrounding the three "real" quarks.
Ultimately the neutron and proton are nearly equal in size because, at the tiny distances in the nucleus, the strong nuclear force is much stronger than anything else, effectively rendering both protons and neutrons as the same particle; this is an example of something called "isospin symmetry". Both neutrons and protons have each an atomic mass unit of 1 {= 1.661×10−27 kg}.
However according to physics textbooks, the mass of a proton is 1.6726×10-27 kg and that of a neutron is 1.6746×10-27 kg. So the neutron has a greater mass by about 2.0055×10-30 kg. The electron is an elementary subatomic particle and has a mass that is approximately 1/1836th of an atomic mass unit, valued to be 9.1094×10−31 kg. Both protons and electrons possess an electric charge which are fully equal to each other in absolute magnitude aka "e" {measured to be 1.6022×10−19 C} but totally opposite in charge +1e or -1e.
Electrons close to the nucleus have lower potential energy. Those in "higher" energy levels, farther away from the nucleus, possess more energy. In order for an electron to "jump" from a lower level to a higher one it must absorb energy, often in the form of light. Conversely when an electron "falls" from a higher level to a lower one, it gives off energy, again in the form of a photon of light. The amount of energy absorbed or emitted depends on the distance the electron "jumps" or "falls". But the electron always absorbs or releases exactly one photon of light (as opposed to several photons) for a big change in energy but a few photons for a small change in energy.
• Why can we never see a molecule?
- The diameter of a proton is on the order of 10−15 m.
- Visible light has a wavelength on the order of 10-7 m.
- The electron is considered as a point particle with no volume or size. Anyway its actual size can never be actually measured, due to the uncertainty principle!
We will never be able to see any atomic particle because the wavelength of light is 108 × greater than the size of any elementary particle; so they do not interact with each other!
