Characteristics of a Proton

Ernest Rutherford discovered the atomic nucleus in 1911, and discovered protons, the charged particles therein, in 1919. (The neutral particles in the nucleus, neutrons, were discovered many years later.) Protons have the same charge as electrons, but are 1,830 times more massive. The proton's antiparticle, called the antiproton, has the same mass and opposite charge. The proton consists of three quarks -- two up and one down.

Determines Number of Electrons

A neutral atom has an equal number of electrons and protons. The Coulomb (electrostatic) force between protons and electrons is a billion-billion-billion-billion (10^36) times stronger than the gravitational force. We notice the gravitational force much more, though, because it is cumulative; whereas charge has opposite signs that cancel out. The attraction between these charges makes the impetus for protons to be neutralized by an equal number of electrons very great.

Complete Outer Shell

However, atoms also like having their outer shell of electrons filled. Sometimes this force overrides the force to neutralize the charge in the nucleus. This is important in semiconductors. For example, the phosphorous and boron bound in a silicon semiconductor do not have full outer shells. The phosphorous has one electron in its outer shell, and therefore gives it up easily. The boron lacks an electron to fill its outer shell. Even though the phosphorous would be positive upon losing an electron, the strong Coulomb force allows, especially if a negative boron is nearby. Therefore, at an interface between boron-doped and phosphorous-doped silicon, the positive phosphorous and negative boron is closely bound, forming an electromotive force that drives phosphorous electrons released by solar energy in one direction only, giving us a solar cell.

Repulsion on Each Other

If the electrostatic force is 10^36 times as strong as the electromagnetic force, then why don't protons in the atomic nucleus fly apart from each other? Because a force called the "strong force" binds nucleons (neutrons and protons) together. The strong force overwhelms Coulomb repulsion at close distances. It is 100 times stronger.

Actually, nucleons are composed of quarks, and the strong force binds the quarks together within the nucleon. That neutrons and protons attract is actually just residual strong force leaking out of the nucleon from the quarks inside.

Proton Decay

Proton decay into lighter particles has never been observed. (This is not to be confused with a proton's changing into a neutron, which is heavier.) The proton is a baryon and baryon number is conserved. The proton is the lightest (lowest-energy) baryon, so its baryon number presumably can't be split up. Observation of proton decay would however vindicate some grand unified theories that break conservation of baryon number.

Beta Decay

There are different modes of beta decay, and one of them is a proton's changing into a neutron. This can occur with an input of energy, which is needed since a neutron weighs more than a proton. Because of the requirement of energy, the change can't occur in isolation. Energy needs to be injected from outside somehow. A proton becomes a neutron by emitting a positron and a neutrino.

References

• Arthur Beiser; Concepts of Modern Physics; 1987
• Richard Feynman; Lectures on Physics; vol. 2; 1964