The electromagnetic attraction between atomic nuclei and their orbital electrons holds atoms together. The belts trap energetic electrons and protons. Both incoming electrons and protons may be involved. NADP is an electron carrier which ferries high energy electrons. It is derived from the electron configuration of carbon, which has four valence electrons. Beta particles are high energy electrons or positrons. SPPs can be excited by both electrons and photons. In others, the atoms share electrons and form covalent bonds. A single bond between two atoms corresponds to the sharing of one pair of electrons. Fermions include elementary particles such as quarks, electrons and neutrinos. An atom can be ionized by removing one of its electrons. The accelerated electrons and positrons collide with nearby starlight. Molecules can be thought of as being surrounded by a cloud of negative electrons. Electronegativity is the tendency of an atom to attract a shared pair of electrons. Ionizing radiation produces free electrons and holes.