Binary Ionic Compounds (compounds consisting of two different ions)
Binary ionic compounds are formed between a metal and a nonmetal. Metallic elements tend to lose electrons (forming positive ions) in order to become isoelectronic with (to have the same number of electrons as) a noble gas. Nonmetals tend to gain electrons (forming negative ions) to become isoelectronic with a noble gas. The charge on an ion is also referred to as the oxidation number of that element when it is in that ionic form.
1.The oxidation number of the nonmetallic element in a binary ionic compound is always equal to the number of electrons that it needs to gain to become isoelectronic with a noble gas. For example, an oxygen atom needs 2 electrons to become isoelectronic with neon, so an oxide ion always has a charge of -2. A nitride ion always has a charge of -3. Chloride is always -1.
2.The oxidation number for metals is not always as simple. Some metals are monovalent, meaning that they tend to form only one type of ion. For example, the oxidation number of alkali metals (Li, Na, K, etc.) is always +1 in a compound. The oxidation number of alkaline earth metals (Be, Mg, Ca, etc.) is always +2 in a compound. Silver ions always have a +1 charge. Zinc ions always have a +2 charge. Aluminum ions always have a +3 charge.
3.Most other metals can have more than one oxidation number in a compound. There are 5 metals, in particular, that usually have one of two different oxidation numbers when they are part of a compound. These are bivalent metals:
Bivalent Metal Oxidation Numbers
Cu+1 or +2
Hg+1 or +2
Fe+2 or +3
Sn+2 or +4
Pb+2 or +4