Why do some atoms form positive ions and others form negative ions?
It is a matter of how full an atom's outer or valence shell of electrons is. For most atoms, the most stable setup is one with a full shell of 8 valence electrons, and an atom will gain or lose electrons to achieve this. For atoms with close to 8 valence electrons, such a chlorine (7 valence electrons, it is generally easier to gain electrons and thus become negatively charged. For atoms with few valence electrons, such as sodium (1 valence electron), it is easier to lose electrons and go down to the next lowest shell, which is already full.