Hydrogen, the smallest and simplest of all the elements, comprises a nucleus of a single proton orbited by a single electron. When this electron is removed, the atom becomes a positively charged hydrogen ion, also called a hydrogen cation. When a second electron is added, it becomes a negatively charged ion, also called an anion.
All other elements feature the third type of elementary subatomic particle, the neutrally charged neutron. Neutrons do not affect the chemical properties of an element; however, they do contribute to its mass. While the electron has practically no mass, the neutron weighs approximately the same as the proton.
It is much easier to ionize some compounds than others. The types of ions possible can be predicted using the Periodic Table. Elements on the left of the table readily lose one or more electrons to form stable cations and, in fact, most of these elements exist naturally in their ionized forms. Sodium and magnesium, for example, readily react with organic or inorganic bases to form highly stable salts.
The transition metals, or d-block elements, also form cations. Transition metals differ from other elements due to multiple oxidation states, allowing loss of some or all of their outermost electrons to form stable atoms. Iron will oxidize to form stable iron(II) and iron(III) compounds. Copper will form copper(I) and copper(II) compounds, while manganese can form compounds of many different oxidation states.
Elements in the sixth and seventh columns of the Periodic Table are negatively charged ions. Of these, fluorine, chlorine and oxygen most readily accept electrons to form stable compounds. Fluorine and chlorine react with metals to produce salts, such as sodium chloride or magnesium fluoride, while oxygen reacts with almost every other element in the Periodic Table to produce oxides.
Only electrons from the outermost orbit of an atom are removed in chemical reactions. This is because inner electrons are very strongly bound to the atom and can only be removed in extreme conditions. For example, removing the first electron from sodium requires 495.8 kJ per mole of energy, however removing the second and third electrons requires 4563.4 kJ per mole and 6912 kJ per mole, respectively. The electron configuration of the sodium(I) ion is identical to that of neon, one of the noble gases that are very stable and nonreactive because all of their electrons are very tightly bound in full shells. Similarly, fluoride and oxide anions are very stable because they, too, have full outer electron shells.