Ions

• Atoms contain positively charged protons, neutral neutrons and negative electrons. Normally, the atoms of any given element have equal numbers of protons and electrons and so are electrically neutral. However, under some circumstances, atoms can either lose or gain electrons. Gaining electrons leaves an atom with an overall negative charge and losing electrons produces a positive charge. Groups of atoms can also lose or gain electrons. These charged atoms or groups of atoms are known as ions. The term cation is given to positive ions and anion to negative ions.
  
  

Electronegativity

• All atoms, due to their positive core of protons, have a level of affinity for electrons. The degree of this affinity is measured in a quantity known as electronegativity and this differs greatly between elements due to factors such as the overall size of the atom. Elements on the left side of the periodic table, such as sodium and calcium, tend to have low electronegativity whereas the elements which are highly electronegative are on the right side, including fluorine and bromine.
  
  

Formation of Ions

• In a normal chemical bond, two atoms each contribute one electron and this shared pair bonds them together. However, when two atoms that differ significantly in electronegativity combine, the stronger atom has a much greater pull on the electrons. It simply takes both of them, leaving it with an extra electron and hence a negative charge, so it becomes an anion. The other atom becomes positive, a cation, and so two ions are formed. The two atoms stick together due to the mutual attraction between their charges.
  
  

Examples

• There are numerous examples of ionic compounds that are found in everyday life. Common table salt, or sodium chloride, which is in many foods as well as in the human body, is ionic. It is made of a positive Na+ cation combined with a negative Cl- anion. Strong acids such as hydrochloric acid, or HCl, are also ionic, with the H+ being the cation. An example of an ionic chemical where groups of atoms share the charge is the well-known fertilizer ammonium nitrate. The positive charge is on the ammonium group, NH4+, and the negative charge on the nitrate, NO3-.