Ions and the Periodic Table

 

Most chemical reactions take place when the electrons of atoms that are in close proximity to each other begin to interact. Electrons that are farthest from the positively charged nucleus are the most likely to be involved in chemical reactions, and are referred to as valence electrons. In some case valence electrons can be pulled away from an atom by the strong attractive force coming from the positively charged nucleus of a neighboring atom. The remaining altered atom now has one fewer electron than the neutral element, and so has a net positive charge. In fact, it is no longer an atom but rather, it is called an ion.  Ions with extra electrons can also form resulting in a net negative charge on the ion. Positively charged ions are also called cation, while negatively charged ions are called anions

The position and groupings of elements in the periodic table are helpful when identifying the atoms that are most likely to form ions as well as whether or not those ions will be cations or anions. Metals generally form positively charged cations with a +2 charge. Transition metals can form more than one kind of cation. Halogens form an anion with a single negative charge. Group 16 elements generally form anions with 2 negative charges, while group 15 elements often form -3 charge anions. Explore the ions that are formed by different atoms shown in the above figure or with the TOOLS interactive periodic table.  In general,  atoms gain or loose  electrons to form ions with the same number of electrons as are found in the nearest noble gas. Notice that H is shown on both sides of the periodic table.  Hydrogen can form a singly charged anion or cation.