Valence Electrons and Shells
To understand what an ion is, you need to understand what a valence shell and a valence electron are. A valence shell is the outer-most orbit section of an atom. The number of valence electrons is determined by where the element falls in the periodic table. First column elements have one electron; the second column, or Group 2, has two electrons. The pattern continues in this manner up to the eighteenth column where elements have eight electrons in their valence shells. The only exceptions to this listing are the transition metals (columns 3 through 12) that all have either one or two electrons depending on their volatility. A valence electron is found in this outer valence shell, and is not special for any other reason than it is more apt to pass between elemental atoms during reactions.
Types of Ions
There are two types of ions: anions and cations. Anions are the negatively charged ions, while cations are the positively charged ions. When one thinks of ions, they commonly think of the monatomic ion: the ion that is made up of a single atom. However, there are also other specific types of ions for unique circumstances, like the polyatomic ion that forms when two or more joined atoms lose or gain electrons as a unit. This most often happens when two elements join together, such as the combination of nitrogen and hydrogen that forms to create ammonia. A final exception for ions is the radial ion, which has an odd number of electrons. This ion is one of the most reactive because atoms like electron pairs, not single electrons. When an atom or ion has an odd number of electrons in its valence shell, it is very apt to form with another atom deficient or in excess of ions and therefore react.
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