Ionization energy - the energy needed to remove an electron from an atom. For this to occur, the attraction between the negatively charged electron and the positively charged nucleus must be overcome. Electron affinity - the tendency of an atom to attract electrons. This characteristic determines the type of bond formed between atoms. Metals generally have a low electron affinity.
Nonmetals generally have a high electron affinity.
Covalent bonds - electrons are shared between atoms. Atoms that do not lose electrons easily but attract electrons usually form covelent bonds. The positive nucleus of both atoms has an equal attraction for the electrons being shared. The shared electrons spend most of their time between the two atoms. It is the attraction between the nucleus and the shared electrons that holds the atoms together.