basic radiation physics

Protons and Neutrons (Nucleons)
The atom is the smallest unit of matter that retains the chemical properties of an element. Atoms consist of a nucleus and orbital electrons. The nucleus is a heavy central mass with a positive charge. The nucleus accounts for most of the mass of an atom, but is small in diameter compared to the overall size of the atom. Particles within the nucleus are known as nucleons. Outside the nucleus is a cloud of electrons - small, negatively charged particles that are attracted to the atom by the opposing charge of the nucleus (coulomb forces).
The nucleus is composed of protons and neutrons. Protons have a positive (+ 1) charge with a mass of about 1.007 atomic mass units (1.673 x 10-24 grams). Neutrons are neutral, or uncharged. Neutrons have a mass slightly larger than the proton, about 1.008 atomic mass units (1.675 x 10-24 grams). Although the coulomb forces between protons in the nucleus tend to push the nucleus apart, the nucleus is held together by the strong nuclear force that operates at extremely short distances. The strong nuclear force is believed to result from attractions between even smaller particles (quarks) that compose the neutrons and protons.
The charge of the electron (- 1) is equal in magnitude to the charge of the proton but is negative instead of positive. In an uncharged atom, the number of orbital electrons equals the number of protons within the nucleus. When the number of orbital electrons does not equal the number of protons in the nucleus, an overall imbalance of charge exists for the atom. A charged atom is known as an ion. Ions readily form chemical bonds with other ions of opposing charge.