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Under some conditions, atoms of certain elements can transfer electrons
between them when they form a compound.
They are usually formed between metals and nonmetals.
For example, in ordinary table salt (sodium chloride - NaCl), the sodium atom (a metal) gives up an electron to the chlorine atom (a nonmetal).
The sodium atom is now an ion because it has an electrical charge.
Since it has one less electron, it has a charge of +1.
The chlorine atom becomes a chloride ion and gains one electron, making a have a charge of -1.
Since opposite charges attract, the sodium and the chloride ions attract each other and form an ionic bond.
The formula unit of NaCl is therefore Na+ and Cl- ions.
The formula unit of CaCl2 is one Ca2+ and two Cl- ions--the calcium atom gives one electron to each of the two chlorine atoms, giving it a charge of +2.
The positive ion is called a cation; the negative is called a anion.
When writing the formula of an ionic compound, give the cation first position in the formula.
Ionic compounds generally are very hard and have very high melting points.
They are solids at room temperature.
They are also relatively hard and brittle.
When they are solid, they do not conduct electricity, but when they are melted or put into a liquid solution, they can conduct electricity.
This is because their electrical ions can move freely in a liquid state.
Although metals like sodium and calcium form only Na+ and Ca2+, respectively, some metals may form more than one ion because of multiple oxidation states.
These metals usually are transitions elements (see Periodic Table: Transition Metals) and post-transition elements (see Periodic Table: Representative Metals).
Alkali metals and alkanline earth metals (see Periodic Table: Representative Metals), like sodium and calcium, respectively, have only one type.
For example, FeCl2 contains Fe2+ ions while FeCl3 contains Fe3+ ions.
In such a case, the charge on the metal must be specified.
The names of the compounds would be named iron (II) chloride and iron (III) chloride, respectively, where the Roman numeral indicates the charge of hte cation.
An older system of naming these types of compounds name the ion with the higher charge with a name ending in -ic, and the one with the lower charge has a name ending in -ous.
In the older system, the compounds above would be named ferrous chloride and ferric chloride, respectively.
Click here for a table of ions and their oxidation states.
Some ionic compounds contain polyatomic ions.
For example, NH4NO3 contains the polyatomic ions NH4+ and NO3-.
They are assigned special names in order to name the compounds containing them.
Click here for a table of polyatomic ions and their oxidation states.
Many of the polyatomic anions contain oxygen atoms.
They are called oxyanions.
When there are two oxyanions that have the same element but a different number of oxygen atoms, the name of the one with fewer oxygen atoms ends in -ite while the one with more ends in -ate.
For example, sulfite (SO32-) and sulfate (SO42-).
When more than two oxyanions make up a series, hypo- and per are used as prefixes to name the members with the fewest and the most oxygen atoms, respectively.
Please see Atomic Structure and Bonding: Ionic Compounds for more information on ionic compounds.
Acids and Bases
Acids and bases are types of ionic compounds.
Acids have H+ cations; bases have OH- anions.
To name a base, use the same naming convention as naming other ionic compounds.
However, acids are named differently.
Please see Acids and Bases for more information on acids and bases.
If the anion does not contain oxygen, the acid is named with the prefix hydro- and the suffix -ic.
For example, HCl is called hydrochloric acid; HCN is called hydrocyanic acid.
If the anion name ends in -ate, the -ate is replaced by -ic.
For example, H2SO4 is called sulfuric acid; H3PO4 is called phosphoric acid.
If the anion name ends in -ite, the -ite is replaced by -ous.
For example, H2SO3 is called sulfurous acid.